Chapter 2: Anatomy and Roentgen Appearance of the Cisterns

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1 Acta Radiologica ISSN: (Print) (Online) Journal homepage: Chapter 2: Anatomy and Roentgen Appearance of the Cisterns To cite this article: (1959) Chapter 2: Anatomy and Roentgen Appearance of the Cisterns, Acta Radiologica, 51:sup185, , DOI: / To link to this article: Published online: 14 Dec Submit your article to this journal Article views: 180 Full Terms & Conditions of access and use can be found at

2 CHAPTER 2 ANATOMY AND ROENTGEN APPEARANCE OF THE CISTERNS INTRODUCTION AND HISTORICAL SURVEY The subarachnoid space lies between the thin arachnoidal membrane closely applied to the inner side of the dura mater and the pia mater which immediately invests the brain. Owing to the fact that, unlike the pia mater, the arachnoid bridges over the irregularities on the brain surf ace, these two membranes become separated from each other and correspondingly the subarachnoid space is widened over the sulci on the brain surface, over the large fissures between the lobes, and.over the base of the brain. Hence the subarachnoid space may be divided into three diff,erent parts, a narrow slit-like space over the gyri, extensions into the sulci, and wider intervals, called cisterns, situated mainly basally and around the brain stem. VIEUSSEN (1690) was the first investigator to demonstrate that the brain is enveloped by a thin membrane lying on the inner side of the dura mater. MAGENDIE (1825, 1842) found that the subarachnoid space contains a fluid, the cerebrospinal fluid. He described five expanded intervals in the subarachnoid space: (I) The first or posterior "confluens" situated below and behind the cerebellum. (2) The second or inferior "confluens" lying in front of the pons and between the peduncles. (3) The third or superior "confluens" located behind, above, and on each side of the pineal body. (4) The fourth or anterior "confluens" situated in front of the optic chiasm and below the gray matter forming the anterior-inferior wall of the third ventricle. (5) The fifth or lateral "confluens" lying in the fossa enclosing the Gasserian ganglion.

3 LUSCHKA (I ) described three paired and three unpaired enlargements of the subarachnoid space, giving them the same name as that used by BRUNS (I 854), subarachnoid sinuses: Unpaired (I) The first unpaired sinus lies between the posterior area of the medulla oblongata and the cerebellum. (2) The second corresponds to the triangular space bounded by the two peduncles, the margin of the pons, and the optic chiasm. (3) A third small sinus extends from the anterior surface of the optic chiasm over the gray lamina terminalis of the third ventricle to the genu of the corpus callosum. Paired (I) The first paired sinus lies below the cerebellum, between this structure and the lateral aspect of the medulla oblongata, extending as far as the anterior margin of the pons. (2) The other two paired sinuses correspond respectively with the transverse cerebral fissure and the fossa of Sylvius. In 1875, KEY & RETZIUS published their comprehensive work on the cisternal anatomy, suggesting a classification which has been in use almost without alteration ever since then. The following cisterns were described and named: Cisterna cerebellomedullaris. Cisterna pontis with a pars medialis and partes laterales. Cisterna intercruralis. Cisterna chiasmatis. Cisterna laminae cinereae terminalis. Cisterna corporis callosi or spatium subarachnoidale corporis callosi. Spatium subarachnoidale corporum quadrigeminorum. Cisterna ambiens. Cisterna fossae Sylvii. Velum triangulare. LOCKE & NAFFZIGER (1924) suggested an entirely new classification of the cisterns, based on information obtained from casts of the subarachnoid spaces: Cisterna magna cerebellomedullaris. Cisterna basalis with a pars pontis, pars interpeduncularis and pars chiasmatis. The paired cerebrocortical channels. The paired internal channels. The paired lateral cerebellocortical channels. The single sagittal channels: The cerebrosagittal and cerebellosagittal channels. 16

4 SPATZ & STROESCU (1934) mentioned the following six cisterns: Cisterna magna cerebellomedullaris. Cisterna pontocerebellaris. Cisterna basalis. Cisterna fissurae lateralis. Cisterna fissurae interhemispherica. Cisterna fissurae transversa. The classification advocated by LOCKE & NAFFZIGER differs from that 01 KEY & RETZIUS in that the former authors divided the cisterns into paired and unpaired channels. The cisterna magna cerebellomedullaris described by LOCKE & NAFFZIGER corresponds with the intertonsillar portion of the much larger cistern with the same name mentioned by KEY & RETZIUS. The paired lateral cerebellocortical channels consist of the lateral continuations of the cisterna magna along the posterior surface of the cerebellar hemispheres. The basal cisterns of LOCKE & NAFFZIGER include the subarachnoid space below the pons as far as the gyrus rectus, and have been divided into pontine, interpeduncular, and chiasmatic portions; these correspond with the cisterns of the same name described by KEY & RETZIUS. The paired cerebrocortical channels are the Sylvian fissures. The paired internal channels correspond with the two ambient cisterns in the KEY & RETZIUS classification. The single sagittal channels are the cisterna corporis callosi and the subarachnoid space above the cerebellar vermis. SPATZ & STROESCU kept fairly closely to the classification suggested by KEY & RETZIUS, except that they grouped the pontine and interpeduncular cisterns as well as the chiasmatic cistern as far anteriorly as the posterior surface of the chiasm under one heading, the cisterna basalis. The subarachnoid space in front of the chiasm as far as the margin of the falx is included together with the cisterna corporis callosi under the term cisterna fissurae interhemispherica. The cisterna fissurae transversa is the same as the ambient cistern. With the exception of the publications of DAVIDOFF & DYKE in the I~~o s, the studies reported by ROBERTSON in the I~~o s, and a book by RUGGIERO in 1958, the subarachnoid cisterns have not recently been the subject of detailed roentgenologic investigation. In addition to these comprehensive works, however, a large number of less detailed publications have appeared; these all contain fairly incomplete descriptions of the roentgenographic appearance of the cisterns and display considerable diversity with respect to terminology and classifications. The ambient cistern seems to be the most common subject for confusion. This space was originally described by KEY & RETZIUS as a paired cistern which encircles the brain stem. The spatium subarachnoidale corporum quadrige- 7

5 minorum, however, was also considered by these authors to be a part of the ambient cistern. In later publications the cisterna venae magnae Galeni above the collicular plate was separated from the ambient cistern; the latter cistern was considered to emerge from the former and invest the brain stem (ROBERTSON). DAVIDOFF & DYKE divided the ambient cistern into two portions, a superior one which is single and situated above the collicular plate and an inferior portion which is paired (cisterna intercommunicans) and situated laterally between the cerebral peduncles and the mesial aspect of the hippocampal gyri. Other investigators have described the cisterna venae magnae Galeni as a part of the ambient cistern. A common definition of the cisterna ambiens is that it corresponds with the subarachnoid space above the quadrigeminal bodies (FELD & CLBMENT). ANTOINE, CREUSOT & DUREUX referred to the ambient cistern as canaux pidonculaires ou lateropidonculaires, calling the central portion of the citerne de la grande veine de Galen the citerne ambiente. PENDERGRASS, SCHAEFFER & HODES described the space above the collicular plate under the name cisterna venae magnae Galeni, cisterna ambiens, or erroneously Bichat s canal. The term cisterna ambiens has often been applied to the two crescentshaped streaks of air which encircle the pulvinar of the thalamus and which were first demonstrated by GOETTE as lying within the choroid fissure. The reason for this variation in the definition of the ambient cistern would seem to be that the respective author has not formed a clear idea of its appearance. Thk pontine cistern is more easily interpreted, and has aroused little discussion except that some authors have preferred to include it together with the other cisterns at the base of the brain under the term cisterna basalis. The pontocerebellar cistern has not been adequately treated in the literature, despite the fact that it plays a highly important r6le in the diagnosis of extracerebral tumours in the posterior cranial fossa. It has been called the lateral recess of the pontine cistern by SHELDON, PENNYBAKER & WICKBOM, and has also been mentioned by MASY & GR~GOIRE, RUGGIERO, ROBERTSON, DAVIDOFF & DYKE, and LINDGREN. Another cistern which has caused controversy is the chiasmatic cistern. Changes in this space are an important factor in the concept opto-chiasmal arachnoiditis, and several investigators have therefore made a study of its roentgenographic appearance. According to some authors the cistern consists only of the space behind and below the optic chiasm as far as the hypophysial stalk, as well as the area between the two diverging optic nerves. The space in front of the optic chiasm is considered by these authors to be part of the cisterna laminae terminalis. In the opinion of other investigators (DILENGE) the cistern also extends lateral to the chiasm as far as the uncus of the hippocampal gyrus and up to the posterior surfaces of the frontal lobes, the gyrus rectus. I8

6 French investigators, like the Italians, often use their own nomenclature. RUGGIERO called the subarachnoid space above the roof of the third ventricle the "citerne sous-trigonale"; the ambient cistern was termed the "citerne periptdonculaire", and the subarachnoid space around the pulvinar of the thalamus (named by the present author the wing of the ambient cistern) the "citerne retropulvinarienne". DILENGE referred to the chiasmatic cistern as the "citerne opto-chiasmatique". Both MASY & GR~GOIRE and ANTOINE, CREUSOT & DUREUX called the subarachnoid space around the superior surface of the cerebellum the "citerne sous-tentorielle". This space is also called by the latter authors the "citerne ctrtbelleuse superieure" as opposed to the "citerne ctrtbelleuse inferieure" which corresponds with the cisterna magna. In addition to these more or less well-established terms the nomenclature suggested by LOCKE & NAFFZIGER is sometimes employed, often intermingled with the original terminology of KEY & RETZIUS. This practice may be seen in the work of PENDERGRASS, SCHAEFFER & HODES. In plastic casts the cisterns communicate freely with one another without sharply defined boundaries. Hence it might seem logical to accept LOCKE & NAFFZIGER'S division into paired and unpaired channels. In roentgen views, however, the cisterns have a more detailed appearance, having less the character of channels than of spaces more or less separated from one another. The encephalographic technique is partly based on this fact. For this reason, a small quantity of air injected into the subarachnoid space in the lumbar region can be made to pass into the different fluid spaces according to the position of the head. By manipulating the head, each individual part of the subarachnoid space may be examined separately. It is therefore more suitable to describe the cisterns by names taken from the anatomical structures determining the cisternal appearance. By adopting this system, stress is also laid on the fact that it is not only the actual cisterns that are under examination but also - and equally important - the adjoining surfaces of the cerebral structures forming the cisternal walls. In the following descriptions of the cisterns the nomenclature used by KEY & RETZIUS has been followed with very few exceptions. The reason for making exceptions was that, when filled with air, certain portions of the cisterns have such a characteristic roentgenographic appearance that they form one roentgenologic unit. Such portions were therefore given special names. The Sylvian fissure, on the other hand, is a transitional type of subarachnoid space between sulci on the brain surface and cisterns. It has been classed among the sulci by the majority of authors (cf. DAVIDOFF & DYKE), and the present author is of the same opinion. The Sylvian fissure will therefore not be discussed. The cisterns were classified into three different groups, a subdivision chosen partly for practical reasons connected with the examination technique and

7 partly because, in different pathologic processes, the cisterns undergo changes in groups. The three groups were as follows: (I) The cisterns in the posterior cranial fossa. (2) The suprasellar cisterns. (3) The cisterns in the tentorial notch including the cist,erna corporis callosi. CISTERNS IN THE POSTERIOR CRANIAL FOSSA The cisterns in the posterior cranial fossa comprise the cisterna magna, the cisterna medullaris, the cisterna pontis and the two pontocerebellar cisterns. Anat om y Cisterna mgna cerebellomedullaris The cisterna magna cerebellomedullaris, or cisterna magna, is a large subarachnoid space located dorsal to the medulla oblongata and the cerebellum. It is formed by the arachnoid membrane bridging over the deep transverse fissure, the fissura cerebellomedullaris (Fig. I). Inferiorly the cistern has no distinct boundary but narrows off at the level of the foramen magnum and inferior to the foramen becomes continuous with the subarachnoid space dorsal to the medulla spinalis. KEY & RETZIUS gave the width of the cervical subarachnoid space dorsal to the medulla oblongata at the level of the first cervical vertebra as 3 mm. They also stated that the cisterna magna begins at the foramen magnum in the form of an abrupt expansion of the subarachnoid space. In the preparations which I examined, the cisterna magna was seen to narrow at the foramen magnum, but before tapering in a downward direction it also had a fair sagittal depth below the foramen; thus there would seem to be some justification for including as part of the cisterna magna the uppermost portion of the cervical subarachnoid space dorsal to the medulla spinalis as far down as the arch of the atlas. Other authors (INZUNZA, LOWMAN & FINKELSTEIN) also considered this to be justifiable. Anteriorly the cisterna magna is bounded in the mid-line by the dorsal area of the upper portion of the medulla spinalis, by the lower part of the medulla oblongata, and by the tela chorioidea posterior. Above the foramen of Magendie the cistern is limited by the inferior vermis in the mid-line and by the inferior- 20

8 Fig. I. Cisterna magna seen from behind. posterior portions of the cerebellar hemispheres lateral to the mid-line. The cerebellar tonsils lie within the cisterna magna. Around their inferior and lateral surfaces the cistern communicates freely with the subarachnoid space ventral to the medulla oblongata. In the mid-line the cisterna magna continues forwards into the vallecula between the two cerebellar tonsils and the inferior vermis. This smaller anterior portion of the cistern may be distinguished from the larger posterior portion which expands sideways over the dorsal and inferior surfaces of the cerebellar hemispheres. The anterior portion, the vallecula, is bounded laterally by the medial surfaces of the tonsils, its bottom is formed by the medulla oblongata together with the tela chorioidea posterior and its roof by the inferior vermis with the uvula. At the anterior extremity of the vallecula lies the foramen of Magendie, which runs into the fourth ventricle as a narrow, canal-like continuation of the fluid space. Hence this part of the cisterna magna has the 21

9 Fig. z. Plastic cast of the subarachnoid cisterns. I. Fourth ventricle. 2. Foramen of Luschka. 3: Foramen of Magendie. 4. Vallecula. 3. Pontocerebellar cistern. 6. Ambient cistern. 7. Interpeduncular cistern. 8. Crural cistern. 9. Infundibulum and optic chiasm. 10. Cisrerni laminae terminalis. appearance of a space in the mid-line tapering off forwards and prolonged into the fourth ventricle by the foramen of Magendie (Fig. 2). Posteriorly the cisterna magna is bounded by the arachnoid membrane, which extends from the medulla spinalis to the dorsal and inferior surface of the cerebellum. The cistern extends for a varying distance along the dorsal surface of the cerebellum; in the mid-line it reaches as a rule to the level of the pyramid of the vermis, where it narrows abruptly. The border in the midline as well as over the dorsal surfaces of the cerebellar hemispheres is formed by the arachnoid becoming more firmly attached to the pia mater on the surfac: of the brain; this border, as KEY & RETZIUS demonstrated, is observable as a thickening of the arachnoid on the brain surface. 22

10 a Fig. 3. Extension of cisterna magna in the mid-line. (a) Small cistern. (b) Large cistern. b Posteriorly in the mid-line the two cerebellar hemispheres are separated by a fairly deep, narrow notch, the incisura cerebelli posterior, which penetrates to the vermis and is occupied by a sickle-shaped process of the dura mater, the falx cerebelli. Within the falx cerebelli, which tapers off downwards toward the foramen magnum as the incisura cerebelli posterior grows shallower, the occipital sinus runs from the foramen magnum to the straight sinus. The arachnoid accompanies the dura mater into the incisura cerebelli posterior, and hence the cisterna magna, instead of following the inner surface of the occipital bone to the tentorium, curves off forwards in the mid-line and then runs along the vermis to the pyramid where the cistern comes to an end. If the falx cerebelli is absent, or if it is abnormally under-developed, the arachnoid membrane will bridge across the incisura cerebelli posterior. The cisterna magna will then be very large, extending superiorly to the lower surface of the tentorium (Fig. 3). The lateral extensions of the cistern may vary, as KEY & RETZIUS have already pointed out. These portions have been described by LOCKE & NAFFZIGER as "the paired lateral cerebellocortical channels". They may extend for a long distance laterally over the surfaces of the cerebellar hemispheres, but may also end fairly close to the mid-line (Fig. 4). On the anterior aspect, lateral to the tonsils and below those structures as far as the flocculus, the cisterna magna communicates broadly with the cisterna medullaris (cf p. 34). The latter space was regarded by KEY & RETZIUS as belonging to the cisterna magna cerebellomedullaris.

11 a Fig. 4. Laternal extensions of cisterna magna. (a) Small cistern. (b) Large cistern. b There is still another communication anteriorly, leading from the cisterna magna to the medullary cistern, namely that between the medial surfaces of the tonsils and the lateral area of the medulla oblongata (Fig. 5). The tonsils lie in a depression in the cerebellar hemispheres, the nidus avis s. externus, which is formed as the biventral lobule develops and arches over the tonsils. Hence there is a slit-like space, the fissura posterolateralis, between the tonsils and uvula on the one side and the nodulus and flocculus on the other side, running to the :ela chorioidea posterior close to the posterior surface of the lateral recesses of the fourth ventricle. This narrow space is filled with a large number of connective tissue trabeculae and contains the inferior posterior cerebellar artery. These communications appear as more or less narrow slits on each side of the medulla oblongata connecting the bottom of the vallecula with the lateral portions of the medullary cistern. Ensephalographic appearances When the cisterna magna is to be examined encephalographically, the head of the patient should be placed so that the injected air collects in the cistern. This is best achieved at the beginning of the examination, while the patient is still in the sitting position. If the head is flexed forward the air will ascend on the dorsal side of the medulla spinalis and enter the cisterna magna. When filling has been achieved, roentgen views of the cistern are taken in different 24

12 Fig. 5. Transverse section of medulla oblongata and tonsils. Subarachnoid space filled with barium gelatine. Arrows (+) point to the space between the tonsils and medulla o blongata. projections and with varying angulation of the beam. Later in the examination, when the patient is placed in the supine position, the cistern becomes completely filled with cerebrospinal fluid. To a certain extent, however, it is possible to make a supplementary examination of the cistern even after the patient has lain down, if he is turned into the prone position. The head should then be placed so that the air passes to the cistern. The ventral portion of the cistern will always remain more or less filled with fluid, however, and hence only the posterior portion can be filled with air for an examination in this position. The cisterna magna has a characteristic roentgenographic appearance. It is best seen in the lateral view, in which it appears as a more or less thick layer of air between the inner table of the skull and the posterior cerebellar surface. It extends upwards for a varying distance on the posterior aspect of the cerebellum. The distance between the highest point of the cistern and the posterior edge of the foramen magnum was measured in 85 instances in the present material, the figures obtained ranging from 15 to 55 mm (Diagram 2). These measurements agree fairly well with the figures quoted by ROBERTSON for the size of the cisterna magna; according to his observations this cistern generally extends to a point situated midway between the inion and the foramen magnum. The appearance of the cistern is determined partly by the shape of thc posterior fossa and partly by its extension laterally over the cerebellar hemispheres. When the posterior fossa is deep - a feature which may be established from a lateral view - and the lateral extent of the cistern is considerable, a

13 NO. OF CASES Diagram 2. Variations in the height of the cisterna magna. Estimated mean value r8 mm. Estimated standard deviation mm mm. layer of air close to the internal occipital crest in the mid-line as well as another layer extending further back toward the occipital bone to the side of the mid-line may be seen in the roentgen view. According to my observations, the cisternal appearances in the lateral view, with the patient in the sitting posture, may be divided into five different. groups. In Fig. 6 are seen diagrammatic drawings of the 5 different groups and in Fig. 7 the corresponding encephalographic appearances. In the first group, comprising 26 cases in this material, the tonsils bulge into the cistern (Fig. 7 a). Above these, another rounded indentation in the cistern, caused by the posterior surface of the vermis, may be distinguished. At this site the cistern tapers abruptly to a point and terminates close to the internal occipital crest. The extension of the cistern lateral to the mid-line is limited in these cases. In the second group (Fig. 7 b), comprising 27 cases, the tonsils are not visible. The cistern is much smaller and narrower than in the preceding group; it does not extend so far up along the posterior cerebellar surface, and its lateral extent is not considerable. In the third group (Fig. 7 c), comprising 49 cases and hence the largest of the five, the tonsils may be identified as a rounded indentation. The cistern is wider than in groups I and 2, and it does not narrow in an upward dirxtion but terminates in an outline either more markedly rounded or wavy and irregular, at approximately the same level as in group I. Its lateral extensions over the cerebellar hemispheres are wider than in the first two groups. 26

14 b B \" Fig. 6. Diagrammatic drawings showing five different appearances of the cisterna magna at encephalography. (a) Group I. (b) Group 2. (c) Group 3. (d) Group 4. (e) Group 5. The fourth group comprised 5 cases (Fig. 7 d). Here the cistern narrows as it rounds the vermis, then curves forwards to terminate in an anteriorly directed peak. Its lateral extent is fairly considerable. The tonsils are to be seen in the cistern in this group also. The cistern acquires its appearance from the fact that it continues for a short distance along the vermis in the incisura cerebelli superior. The fifth group, finally, also comprised 5 cases (Fig. 7e). The cistern here is appreciably wider than in the other groups and reaches to the posterior portion of the inferior surface of the tentorium. Two patients in the material had a communication between the cisterna magna and the quadrigeminal cistern: the cisterna magna continued as a narrow streak of air around the vermis to the subarachnoid space above the quadrigeminal plate (Fig. 8). The tonsils, which lie within the cisterna magna, may be identified in the cistern as a more or less rounded indentation which does not reach as far as the inner table. Their lower pole is located at the level of, or slightly above a plane through the foramen magnum; the pole is often projected so that it coincides with the lateral margin of the foramen magnum. The distance between the posterior surface of the tonsils and the posterior edge of the foramen magnum was measured as being between z and 10 mm, with an average of 6 mm.

15 C d Fig. 7. Lateral views from five encephalographies, each of which corresponds to one of the groups indicated in Fig. 6. (a) Group I. (b) Group 2. (c) Group 3. (d) Group 4. (e) Group 5. e 28

16 Fig. 8. Encephalography. A narrow strip of air (-+) surrounds the vermis from the cisterna magna to the quadrigeminal cistern. That part of the cistern which lies below the foramen magnum normally has a characteristic appearance in the lateral view (Fig. 9). It narrows downwards, the average depth at the level of the'arch of the atlas being 6 to 7 mm. The anterior border of the cistern is formed by the medulla spinalis, which appears as a soft tissue structure curving gently forwards below the foramen magnum. The vallecula and foramen of Magendie are not as a rule distinguishable in the lateral view, in ordinary encephalographic films, owing to the fact that the temporal bones are superimposed. Tomography is of assistance here. The simplest measure is to ask the patient to make small rotating movements of the head with the dens of the second cervical vertebra as the axis. If the exposure time has been sufficiently long, i.e. 2 to 3 seconds, a distinct view will then be obtained of all air-filled fluid spaces in the vicinity of the pivot, and the foramen of Magendie will be visible. This procedure was advocated by ZIEDSES DES PLANTES. The foramen of Magendie is seen as a straight, narrow channel connecting the air in the vallecula with that in the fourth ventricle (Fig. 10). In the ordinary lateral view a streak of air may sometimes be seen connecting the fourth ventricle with the air in the cisterna magna, if the extension of the cellular system of the mastoid is small. As has already been

17 a b Fig. 9. Extracranial portion of the cisterna magna. (a) Cistern filled with barium gelatine. (b) Cistern filled with air. The medulla oblongata (+) swings forwards towards the foramen magnug. pointed out by DAVIDOFF & DYKE this streak of air corresponds to the location and appearance of the foramen of Magendie.. An examination of the cisterna magna with sagittally directed rays should take place at an early stage, while air is present only in this cistern and not in other parts of the subarachnoid space. As a rule, however, the layer of air is too thin, except in the vallecula, to be demonstrated by this means. In those cases in which the layer is thicker, in other words in those falling under group 5, the cistern may be demonstrated as a thin layer of air. Likewise, in cases belonging to groups 3 or 4 the layer of air may sometimes be thick enough for the cistern to be visible in a posteroanterior view. The cistern extends for a varying distance lateral to the mid-line and often has an irregular border on the lateral aspect. Superiorly the air narrows off to terminate in the mid-line as a rounded border with the convexity facing upward (Fig. 11). The vallecula is the part of the cisterna magna best seen in the posteroanterior view (Fig. 12). It appears as a narrow streak of air in the mid-line widening out inferiorly below the lower surface of the tonsils. As the vallecula is located far back in the skull, it is essential that the projection should be

18 b Fig. 10. (a) Foramen of Magendie seen in an ordinary lateral view (+). (b) When not seen in an ordinary lateral view the foramen of Magendie usually can be demonstrated by autotomography (+). (c) Specimen with barium gelatine. Arrows (+) point to foramen of Magendie. C straight, as even slight obliquity will cause it to be projected to the side of the mid-line. The vallecula may be displaced from the mid-line by an expansive process in the posterior fossa. Usually, howev'er, it is also tilted in these cases. The vallecula may be studied to the best advantage with the patient in the sitting position and with the rays directed so that it is projected in or immediately above the foramen magnum. If the beam direction coincides with 3'

19 b Fig. II. P-a drawings. views of the cisterna magna in 2 different cases supplemented by diagrammatic 32

20 Fig. 12. P-a view. The vallecula appears as a strip of air in the mid-line between the tonsils (+). There is a notch in the floor of the vallecula (*). Fig. 13. P-a view showing air (+) lateral to the internal occipital crest. the anterior part of the vallecula a small notch may be seen in its bottom in the mid-line (Fig. 12). Further posteriorly, the air encircles the inferior pole of the tonsils. These structures are symmetrical and of equal size, but if the projection is even slightly oblique they may appear to be of different sizes. If the internal occipital crest is broad and higher than usual - a feature observable in plain films of the skull - the shape of the cisterna magna will reflect this situation; it will be deeper sagittally on both sides of the crest. The layer of air consequently will be thick enough to be visible in the posteroanterior view as two straight streaks inclining toward each other and converging upwards (Fig. 13). In some instances air is shown on only one side and must not be misinterpreted as a displaced vallecula. From the lateral portion of the bottom of the vallecula narrow communications run to the medullary cistern between the medial surfaces of the tonsils and the lateral areas of the medulla oblongata (Fig. 14). These slit-like spaces occasionally become air-filled, appearing as two narrow, slightly curved streaks of air, one on each side of the mid-line, encircling the medulla oblongata. In order to demonstrate these air streaks in the posteroanterior view the beam direction must be practically parallel to the longitudinal axis of the medulla oblongata. 33

21 Fig. 14. Air in the medullary cistern and in extensions (+) from the cerebellomedullary cistern delimits the medulla oblongata. (Cf. Fig. I.) Anatomy Cisterna medullaris The relatively narrow subarachnoid space ventral to the medulla oblongata is a direct, although wider, continuation intracranially of tbe fluid space ventral to the medulla spinalis. It was considered by KEY & RETZIUS to be part of the cisterna magna, but will be treated in the present paper as a separate cistern. Like ROBERTSON, I have called it the medullary cistern. On each side of the medulla oblongata the cistern is bounded by the inferior surface of the cer,ebellar tonsils. Its anterior-superior boundary corresponds with the border between the medulla oblongata and the pons, which is marked by a transverse furrow, the sulcus caecum. Here the cistern communicates broadly with the pontine and the pontocerebellar cisterns (Fig. 15). As it is deeper on the lateral aspects of the medulla oblongata than ventrally, it forms two elongated enlargements laterally which correspond to the lateral recesses of the medullary cistern mentioned by ROBERTSON (Fig. I 6). These recesses terminate as small widened areas anteriorly in th'e transverse fissure between the pons and the medulla oblongata. In the mid-line also, there is a slight local enlargement of the cistern in the foramen caecum. In the lateral portions of the cistern lie the vertebral arteries, which unite to form the basilar artery. From the 34

22 Fig. 15. Tomography. Axial view of a preparation with barium gelatine in the cisterns. Arrows (+) point to the lateral recesses.of the medullary cistern and (+) to the passage anterior to the pontocerebellar cisterns. vertebral arterises the posterior inferior cerebellar arteries emerge; in the cistern they curve around the tonsils into the space,between these structures and the medulla oblongata. At its anterior extremity, close to the pons, the cistern is traversed by the ninth and tenth cranial nerves, and further back by the eleventh and twelfth pair of nerves. Encephalographic appearances To demonstrate the medullary cistern the central ray must be directed parallel to the longitudinal axis of the medulla oblongata, in other words the cistern must be project'ed in the foramen magnum. The films should be taken as early as possible in the examination, before the cisterns at a higher level have become filled with air and thus overlie tbe medullary cistern. 35

23 a b Fig. 16. (a) Transverse section of medulla oblongata with barium gelatine in the medullary cistern. Arrows (+) point to the lateral recesses. (b) Tomographic view. a b Fig. 17. Encephalography. P-a views. (a) Medulla oblongata surrounded by air in the medullary cistern. (b) When the beam is not completely in alignment with the long axies of the medulla oblongata only the lateral mcesses of the cistern can be seen. 36

24 The medulla oblongata appears as a rounded soft tissue structure encircled by air both ventrally and laterally (Fig. 17). The layer of air is deeper on the lateral aspects of the medulla oblongata than ventrally, especially anteriorly where the cistern becomes continuous with the pontine cistern. These are the widened areas referred to by ROBERTSON as the lateral recesses of the medullary cistern. These recesses are equally wide on both sides of the medulla oblongata, which normally lies in the mid-line. It is of importance to be aware of these features, as expansive processes in the posterior cranial fossa, especially those occurring in the pontine angle, displace the medulla oblongata and cause dilatation of the medullary cistern below the tumour. When the central ray coincides with the longitudinal axis of the vertebral artery this vessel is seen as a round soft tissue formation in the lateral recess of the cistern. In an ordinary lateral view the cistern cannot be identified as it is concealed by the petrous portion of the temporal bones. Anatomy Cisterna pontis In the anterior-superior direction the medullary cistern is in direct continuity with the pontine cistern, which lies between the ventral surface of the pons, and the clivus and dorsum sellae. On both sides of the pons the two pontocerebellar cisterns communicate through a wide opening with the pontine cistern (Fig. 18). Anteriorly and superiorly the subarachnoid space is continued in the mid-line by the interpeduncular cistern into the interpeduncular fossa. On each side of the pons the two ambient cisterns which encircle the brain stem also arise. The cisterna pontis is narrow'er than the medullary cistern and resembles a bowl which encircles the ventral surface of the pons. It is traversed by the basilar artery and the fifth to eighth cranial nerves. The sixth cranial nerve arises near the mid-line in the transverse furrow delimiting the pons from the medulla oblongata and continues anteriorly in the cistern parallel to and adjacent to the ventral surface of the pons. The seventh and eighth cranial nerves begin at the side of the pons in close proximity to its junction with the rnedulla oblongata, and run anterolaterally in the pontocerebellar cistern to the porus acusticus internus. Further to the front and at a higher level, the fifth nerve arises from the lateral surface of the pons and proceeds forwards in the cistern to Meckel's cavity on the anterior surface of the petrous bone. Also contained within the cistern is the basilar artery, which is situated in or near the mid-line. 37

25 Fig. 18. Barium gelatine in the pontine and pontocerebellar cisterns showing the broad passage between them. Emephalographic appearances The cisterna pontis is best seen in the lateral view, with the patient sitting. To achieve complete filling the head should be raised so that the air ascends ventral to the pons. If the space is deep, it may be necessary to inject large amounts of air and withdraw cerebrospinal fluid in order to avoid a fluid level in the partially fluid-filled cistern (Fig. 19). When the patient sits with the head flexed forward the 'brain sinks slightly downwards towards the clivus; this may cause slight flattening of the cistern, with the result that the roentgen appearance may be misinterpreted as a pathologic condition. When the patient is placed in the supine position the air leaves the cistern fairly slowly and the cistern can thus be studied in this position also, as was clearly demonstrated in 112 of the 120 cases in this material. In the prone position the cistern is always incompletely filled. Air may sometimes be present in front of the 38

26 Fig. 19. Encephalography. Incomplete air- Fig. 20. The dura mater (+) stretches from filling of the pontine cistern. A fluid level a large processus dorsi sellae to the clivus. is se.en at (-+). Arrow (+) points to the cortex of the clivus. ventral surface of the pons but as a rule there is a layer of fluid ventrally in the cistern and complete filling is not obtained. DAVIDOFP & DYKE stated that the depth of the pontine cistern may vary from 5 to 12 mm, with the average at 8.2 mm. Their measurements represented the distance from the dorsum sellae to the point on the ventral surface of the pons where the curvature is at its maximum. DAVIDOFF & EDSTEIN stated that the cisterna pontis is normally between 6 and 11 mm deep. I investigated the depth by measuring the distance between the ventral surface of the pons and the clivus, at the site where these two surfaces are wholly or nearly parallel; this point is usually found close to the base of the dorsum sellae. The distance varied from z to 9 mm in the supine as well as in the sitting position. It is apparent from Diagram 3, however, that the difference between the two value distributions is significant with the risk of error being I Yo. This is due to the fact that the brain sinks backwards, thus causing widening of the cistern. The cistern does not always extend as far as the cortex of the clivus; a more or less thick layer of soft tissue is always seen between the clivus and the cisternal air. This tissue layer, which consists of a network of veins on the clivus under the dura mater, is deepest anteriorly and tapers posteriorly along the clivus. This is due to the dura mater stretching from a large processus dorsi posterior to the clivus (Fig. 20). At this site in children, because of the presence 39

27 NO. OF CASES ' ' Diagram 3. Variations in the depth of the pontine cistern, (0-0) sitting position, (0---0) supine position. Estimated mean value in sitting position 5 mm. Estimated standard deviation 1.5 mm. Estimated mean value in supine position 6 mm. Estimated standard deviation ' 10 mm mm. The difference is significant. of a layer of cartilage which is not roentgenographically demonstrable, the pontine cistern never extends as far as the cortex (GREITZ). The cistern acquires its shape from the ventral surface of the pons with its characteristically rounded form. This surface is always smooth but its curvature may vary, being greatest at the level of the dorsum sellae after which it decreases abruptly in a posterior and inferior direction so that the surface finally r'uns parallel to the clivus. If, on the other hand, the pons is displaced ventrally by an expansive process in the posterior cranial fossa certain changes are apparent; whereas the distance between the pons and the dorsum may still appear to be normal the pontine and clival relations have altered so that the surface of the pons, instead of running parallel to that of the clivus, gradually approaches it (Fig. 21). The basilar artery appears in the cistern as a straight or tortuous band, a few millimeters in breadth, continuing upwards into the interpeduncular cistern (Fig. 22). The artery was demonstrated in I 12 (94 Glo) of my 120 cases; in 6 it was not recognised with certainty, and in 2 it could not be identified although the cistern was completely filled with air. The probable reason for this is that the artery was more firmly attached than usual to the ventral surface of the pons and hence the air could not penetrate between it and the pons. It is also conceivable that in some of these cases the artery was shorter than normal and bifurcated sooner, with the result that its distal portion did not project upwards into the anterior-superior part of the cistern. Other structures contained within the cistern cannot as a rule be identified. The posterior-inferior portion of the cistern is concealed in this projection by the pyramids and the mastoids.

28 a Fig. 11. (a) Normal encephalographic appearance of the pontine cistern. (b) Same case. Compression of the pontinc cistern after accidental subdural injection of air. b Fig. 22. The basilar artery in the pontine cistern. To obtain a posteroanterior view of the cisterna pontis the beam direction should coincide with the inferior surface of the pons and th,e clivus (Fig. 23). The pons is then seen to be enveloped by air. As the air in the other parts of the basal cisterns is superimposed upon the air in the central portions of the pontine cistern, the details in this area are not clearly demonstrated in the posteroanterior view. The lateral portions of the cistern, i.e. the areas where it becomes continuous with the pontocerebellar and ambient cisterns, may, however, always be studied in this view. 4'

29 b Fig. 23. (a) Transverse section through the mesencephalon to show the relationship between the pons and peduncles. (b) Encephalography. P--a view. Arrows (-+) point to the ventral surface of the pons.

30 Fig. 24. Plastic cast of the basal cisterns. The pontocercbellar cisterns are triangular in shape. Anat om y Cist eina pont ocerebel laris The pontocerebellar cistern,' one on each side of the pons, is continuous through a wide opening with the posterolateral portion of the pontine cistern. It resembles a triangle with the apex directed laterally, upwards, and backwards (Fig. 24) and is situated in the pontine angle, i. e. in the space between the posterior surface of. the temporal bone, the inferior surface of the cerebellum, the pons, and the tentorium. Its boundary is formed anteriorly and laterally by the posterior surface of the pyramids and posteriorly and superiorly by the cerebellar hemispheres, embracing in front close to the tentorium, the lobulus quadrangularis and at the r,ear the anterior portion of the posterior cerebellar lobule. These structures are separated anteriorly by the fissura horizontalis cerebelli, which extends to the lateral margin of the brachium pontis; between them lie the brachium pontis, and further back, the flocculus. Medially the cistern is bounded by the pons, the brachium pontis, and the anterolateral part of the medulla oblongata, i.e. the obex (Fig. 2 5 ). At the anterior extremity the boundary consists of the tentorial attachment on the pyramids. In the posteromedial portion of the cistern adjacent to its communication with the medullary cistern the flocculus pro- 43

31 Fig. 25. Basal surface of brain showing the pontine angles. jects downwards to form a roun.ded indentation into the cistern. In the immediate vicinity of th,e posterior portion of the flocculus is the foramen of Luschka, encircled by a small choroid plexus (cf. Fig. 2). The foramen curves in an upward and medial direction, one on each side of the restiform body, to emerge in the lateral recess of the fourth ventricle. In front of the flocculus the seventh and eighth cranial nerves run obliquely forward and laterally in the cistern to the porus acusticus internus. Around these nerves, a small div,erticulum passes from the subarachnoid space into the porus; this pocket is largest ventrolaterally, where it continues for a short distance into the meatus. Behind the flocculus the ninth and tenth cranial nerves leave the medulla oblongata to pass laterally OUT into the posterior portion of the cistern. Further toward the anterior and medial aspects the trigeminal nerve proceeds anteriorly and slightly laterally to the Gasserian ganglion in Meckel s cavity on 44

32 the anterior aspect of the pyramid. Topographically, the cranial nerves in the pontocerebellar cist,ern may be divided into three groups, an anterior group embracing the trigeminal nerve, an intermediate group consisting of the seventh and eighth cranial nerves, and a posterior group comprising the ninth and tenth nerves. The inferior anterior cerebellar artery also passes through the cistern, bending in a shallow curve around the flocculus and continuing to the brain stem. Laterally in the cistern, on the inferior aspect of the cerebellar hemispheres, a relatively large blood vessel, the petrosal vein, empties into the superior petrosal sinus. This vein receives blood from a vein from the temporal lobe and one from the pons, as well as from an inferior cerebellar vein running to the petrosal vein from the nidus through the sulcus paramedianus cerebelli and a superior cerebellar vein from the upper surface of the cerebellum. The petrosal vein, which is situated lateral to the trigeminal nerve and medial to the porus acusticus internus, passes [hrough the cistern from the cerebellum to the petrous portion of the temporal bone and flows into the superior petrosal sinus. Further laterally variable inferior and superior cerebellar veins also traverse the cistern to empty into the superior petrosal sinus. Encephalographic appearances The pontocerebellar cistern is best seen in posteroanterior views. The patient should be in the sitting posture and the central ray directed so that the upper margins of the pyramids are projected above the superior border of the orbits. Owing to superimposition by the pyramids and the mastoids the cistern is not visible in the lateral view. In those cases in which the cistern extends for a good distance along the petrous bone, however, it is occasionally recognisable in the lateral view as a collection of air above and in front of the pyramid; its shape is then triangular and may be misinterpreted as a displaced fourth ventricle (GREITZ). A comparison of the lateral view with a posteroanterior view will, however, make the situation clear when it can be established whether the fourth ventricle is air-filled or not. The superior boundary of the pontocerebellar cistern is formed by the inferior surface of the cerebellum, consisting of the brachium pontis, the flocculus, and the posterior lobule on the posterior aspect, and of the inferior surface of the lobulus quadrangularis in the anterior direction (Fig. 26). The flocculus dips down from above into the posterior portion of the cistern, producing a rounded indentation in the cisternal outline in close proximity to the site where the cistern communicates with the medullary cistern. Lateral to this, there is usually visible another small round indentation in the cistern which may have the appearance of a small knob hanging on a narrow string (Fig. 27). In other cases, this formation may be more irregular in shape, being sometimes stellate, sometimes angular with

33 Fig. 26. Specimen with barium gelatine in the cisterns. P-a view. Arrows (+) point to the inferior surface of the cerebellum. a diverticulum medially in front of the pons. The formation consists of the petrosal vein and its tributaries from the cerebellum and pons, or one of the variable superior cerebellar veins which, like the petrosal vein, traverses the cistern to empty into the sinus petrosus superior. The vein becomes contrast-filled at vertebral angiography and has a characteristic appearance in these films. In 20 cases in which encephalography and vertebral angiography were carried out in one and the same patient, the soft tissue structure in the cistern could be identified as the petrosal vein. After having studied the films from 50 angiographies of the vertebral artery I established that this vein may show considerable variations in size. If it is large, it may cause differential diagnostic difficulties. It may have the same appearance as a small expansive process at the porus which has not yet caused deformation of the cistern or adjacent parts of the brain. A round structure enveloped by air is often visible medially in the cistern. It is best seen when the direction of the rays brings the superior margin of the pyramids into alignment with the superior border of the orbits (Fig. 28). If the degree of angulation of the beam is increased, the structure becomes elongated and is no longer completely encircled by air. This structure consists of the trigemiml nerve, which passes through the cistern to the Gasserian ganglion on the front 46

34 ;I b Fig. 27. (a) Encephalography. A small soft tissue structure (+) is seen in the left pontocerebellar cistern. (b) Vertebral angiography in the same case. The soft tissue structiire is the petrosal vein (-+). of the pyramid. The trigeminal nerve is best seen if the films are taken with the cenrral ray directed so that the superior surface of the pyramids coincides approximately with the upper border of the orbits, whereas the petrosal vein can be demonstrated to the best advantage if the pyramids are projected above the orbits. Medially the cistern is in communication with both the pontine and ambient cisterns, as well as with the medullary cistern posteriorly; laterally it extends past the porus acusticus internus. In some instances it may terminate immediately adjacent to the porus, in others it continues for a good distance lateral to the porus. KARLEFORS studied the size and variations of the basal cisterns in IOO autopsies. In 94 cases the lateral boundary of the pontocerebcllar cistern was located close to and lateral to the vagus and glossopharyngeal nerves, and in 4 cases 10 to 20 mm lateral to these nerves. The exeension of the boundary lateral to the trigeminal and acoustic nerves showed considerable variation; in 86 cases the cistern extended respectively 5 to 22 and 3 to 15 mm lateral to the nerves and in 14 cases the corresponding figures were 20 to 40 and 16 to 39 mm. The pontocerebellar cistern may be divided according to its roentgen appearances into three groups (Fig. 29). In the present material it was air-filled bilater- 47

35 b C Fig. 28. (a) Plastic cast. The trigeminal nerve pierces the pontocercbellar cistern medially (+). (b) Specimen with barium gelatine in the cisterns. P-a view. Arrows (+) point to the trigemind nerves. (c) Encephalography. P-a view. Trigeminal nerve (+) visible medially in pontocerebellar cistern.

36 Fig. 29. Diagrammatic drawings illustrating three different appearances of the pontocerebellar cistern. (a) Group I. (b) Group 2. (c) Group 3. ally in all 120 cases; its shape and lateral extent could be estimated in roo of them. In the other 20 cases the roentgen views could not be used for these estimations (Fig. 30). Group I. 7 cases (7 70). The cistern is very short and terminates in a point. It extends for only a short distance lateral to the porus acusticus internus and the outlet of the ambient cistern. The petrosal vein is not seen in. the cistern. Group cases (78 70). The cistern terminates lat,eral to the porus acusticus internus in an enlargement which is sometimes rounded, or occasionally pointed. The petrosal vein is usually seen as a knob hanging down in the cistern. Group 3. ZJ cas,es (15 9'0). The cistern extends laterally for a long distance along the petrous temporal bone. In this portion of the cistern the layer of air is often so thin that it is not always possible to distinguish a lateral boundary with certainty. This observation agrees with that of KARLEFORS; in 14 olo of his autopsies the pontocerebellar cistern extended for a long distance lateral to the porus acusticus internus. When seen, the petrosal vein is surrounded by air in the cistern. 49

37 Fig. 30. Encephalography. P-a views of the pontocerebellar cistern. (Cf. Fig. 29.) C

38 Fig. 31. Incomplete air-filling of the pontocerebellar cisterns with a fluid level. The cistern usually has approximately the same appearance and extension on both sides. Small variations may be due to asymmetry of the skull, with the result that the two petrous bones may lie at different levels, or their posterior surfaces incline at different angles; the roentgen view of the cistern will then display slight differences. In 10 cases (10 Yo), however, the cistern was either wider on the one side, or extended further laterally on one side than on the other. If the cisterns are dilated it may be difficult to obtain complete air-filling (Fig. 31). In these cases it is not uncommon for the lower portion of the cistern to be filled with Gerebrospinal fluid, with a fluid level as the result. By tilting the patient s head laterally the cistern lying highest up can be completely filled with air. In order that enough air may collect in the cistern it may be necessary to inject more air with the head flexed so that the cistern lies at a higher level in relation to other parts of the subarachnoid space. It may also be necessary to withdraw more fluid than usual in order to empty such a wide cistern. If air enters the cistern on only one side the reason may be either that an expansive process is present on the non-filling side or that an incorrect technique has been applied. If no other changes suggesting a tumour are to be observed,

39 the explanation probably lies in inadequate technique. In thes,e cases the injection should be made with the patient's head in such a position that the air can travel up into the unfilled cistern, in other words the hsead must be inclined so that the pontocerebellar cistern on the unfilled side will be situated higher than the cistern on the other side. If the unfilled cistern is normal it can always be filled by this technique. SUPRASELLAR CISTERNS On the basal surface of the brain there is a large subarachnoid space bounded posteriorly by the pons, anteriorly by the posterior surfaces of the frontal lobes and on both sides by the anteromedial surfaces of the temporal lobes (Fig. 32). In plastic casts of the cisterns this space is divided by the optic chiasm, the optic tract and nerve, and the infundibulum into two compartments, a large posterior portion and a smaller anterior portion which are in communication only through the subarachnoid space lateral to the optic chiasm and nerve (Fig. 33). It has long been customary to divide this large subarachnoid space into two major cisterns, the cisterna interpeduncularis and the cisterna chiasmatis, and to regard the infundibulum and tuber cinereum as forming the boundary between them. Among the suprasellar cisterns are also counted the crural cisterns, the cisterna laminae terminalis, and the olfactory sulci. Anatomy Cist erna int erpeduncular is The interpeduncular cistern consists partly of a blind sac, the interpeduncular fossa, which terminates the. pontine cistern anterosuperiorly in the mid-line, and partly of the subarachnoid space below the peduncles down to the dorsum sellae. It is bounded behind by the anterior surface of the pons, above by the posterior perforated substance, on each side by the medial surfaces of the peduncles, and in front by the corpora mammillaria, the tuber cinereum, and the infundibulum. Laterally the cistern extends below the inferior surfaces of the peduncles and continues into the crural cistern. At the bottom of the interpeduncular fossa, in the sulcus nervi oculomotorii, arise the two oculomotor nerves which are situated in the latleral portions of the cistern. These nerves cross the cistern in an anteroinferior and lateral direction and make their exit medial to the posterior aspect of the uncus at the level of the dorsum sellae, passing between the posterior cerebral and superior cerebellar arteries. The arachnoid is closely adherent to the oculomotor nerve as well as to

40 Fig. 32. Specimen cut in the mid-line showing the cisterns filled with barium gelatine. a b Fig. 33. (a) Basilar surface of a brain showing the interpeduncular and chiasmatic cisterns. (b) Plastic cast showing the furrow made by the optic chiasm. 53

41 a Fig. 34. (a) The thin arachnoid membrane is secn covering the cisterns on the basilar aspect of the brain. A narrow white band runs between the two oculomotor nerves. (b) A hole has been made in the arachnoid. A thin membrane (-+) is seen stretching between the two oculomotor nerves. b the posterior communicating artery, the posterior surface of the infundibulum, and thle tuber cinereum (Fig. 34); from this site an arachnoidal membrane stretches backwards between these structures to the posterior cerebral and basilar arteries. It does not always reach is far as the basilar artery but instead may terminate in a free margin slightly to the front of it. The insertion of this membrane in the arachnoid proper appears on the outer side of the arachnoid as a white band one millimeter thick arching forward between the two oculomotor nerves. In the mid-line the membrane extends to the infundibulum. It is also adherent to th: lateral parts of the optic chiasm, with the result that a probe inserted into this space from behind always meets with resistance when a cautious attempt is being made to find a passage in front of and lateral to the chiasm. This membrane was described by KEY & RETZIUS, but since then seems to have been forgotten. These authors found the interpeduncular cistern to be divided by the membrane into a superficial and a deep portion. Together with the arachnoid, the membrane forms a more or less completely blind sac which opens up posteriorly into the 54

42 pontine cistern. In plastic casts of the cisterns the membrane itself is not seen, but at the site of its attachment to the arachnoid a narrow arched groove in the plastic material may be distinguished giving the impression of a division of the mass. The interpeduncular cistern contains several important structures which may be identified in the roentgen films. The point of division of the basilar artery lies within the cistern, as well as the first part of the posterior cerebral arteries before they bend around the brain stem in the ambient cistern. GREITZ & LOFSTBDT, in a roentgen study on the relations of the basilar artery to the interpeduncular cistern and the third ventricle, described variations in the shape and location of the artery in the cistern. Exrephalographic appearances Air ascends readily from the pontine to the interpeduncular cistern at the encephalographic examination. The cistern includes not only the subarachnoid space in the fossa interpeduncularis as far as the aforementioned membrane which forms the natural boundary with the chiasmatic cistern, but also the space below the inferior surface of the peduncles extending downwards to the dorsum sellae. As the interpeduncular fossa is a narrow space it is only visible as a thin layer of air in the lateral view (Fig. 35). Its shape is so charactseristic, however, that this part of the cistern can always be recognised if it is filled with air. Anteriorly, the mammillary bodies bulge into the cistern and cause narrowing of its inferior portion in the sagittal plane. These bodies were clearly demonstrated in 74 instances in the present material; in 21 cases they were faintly visible and in the other 25 cases they were not identified, probably because of a slightly unsuitable projection, or of incomplete filling of the cistern. The mammillary bodies occasionally bulge more or less prominently into the third ventricle, and this may be one of the reasons why these structures are not always identified in the interpeduncular cistern, even when it is well filled with air. The interpeduncular fossa is slightly wider at the bottom than at the level of the mammillary bodies. The greatest extent of the fossa in the sagittal plane, as measured in the roentgen view, was found to vary from 7 to 16 mm. The distance between the mammillary bodifs and the anterior surface of the pons measured from 5 to 10 mm (Diagram 4). The depth of the interpeduncular cistern was stated by ROBERTSON to be "0.6 to 1.2 cm above the dorsum sellae and 0.6 to 1.5 cm dorsal to the dorsum sellae". I measured the depth in 107 cases by measuring the distance from the tip of the dorsum sellae to the bottom of the interpeduncular fossa. The figures varied from 14 to 23 mm (Diagram 5); hence the distance was considerably greater than mentioned by ROBERTSON, probably owing to the fact that he had not observed the thin film of air in the interpeduncular fossa. 55

43 Fig. 35. Encephalography. The interpeduncular fossa is filled with air. (Cf. Fig. 32.) Below the inferior surfaces of the peduncles the cistern widens both inferiorly and laterally. This is reflected roentgenographically by a distinct straight line, corresponding to the lower peduncular surfaces, with a considerable amount of air collected below it (Fig. 35). These surfaces were seen in 92 instances in the present material. In 15 cases, however, no outline definitely recognisable as the peduncles was identified, although there was air in the interpeduncular fossa; this was due to the central ray not being parallel to the peduncular surfaces. The two oculomotor nerves emerge from the oculomotor sulcus in the posterior portion of the inserpeduncular fossa and pass obliquely forwards, downwards, and laterally in the cistern. The proximal portions are sometimes seen as a millimeter wide band surrounded by air in the interpeduncular fossa (Fig. 35). This feature was observed in 44 of my cases; in 16 the films could not be interpreted with certainty and in 60 the nerve could not be identified. This nerve is slightly broader at its point of emergence than further peripherally; this may also be observed in the roentgen films. Its peripheral part is not as a rule identifiable, partly owing to the fact that its course coincides with that of the posterior communicating arteries, and partly because this portion of the nerve is situated close to the lateral wall of the cistern and thus is not completely surrounded by air. In 35 Vo of the

44 NO. OF CASES NO. OF CASES mm mm a b Diagram 4. Extent of the interpeduncular fossa. (a) At the bottom, estimated mean value II mm. Estimated standard deviation 1.8 mm. (b) At the level of the mammillary bodies, estimated mean value 7 mm. Estimated standard deviation 1.4 mm. cases studied by DAVIDOFF & DYKE the oculomotor nerve appeared as a straight band, 2 mm wide and I cm long, running parallel with and behind the tuber ciner cum. Immediately in front of and below the mammillary bodies is seen the distal part of the optic tract. Together with the tuber cinereum and the optic chiasm this structure forms a straight band, usually about 3 mm wide, which continues anteroinferiorly through the chiasmatic cistern. The basilar artery and its bifurcation lies within the posterior portion of the interpeduncular cistern and is usually visible if the cistern is sufficiently airfilled. The shape and position of the artery in the cistern could be observed in 112 out of 120 cases. The first part of the posterior cerebral artery as well as the superior cerebellar artery at their points of origin from the basilar artery are also occasionally demonstrated, and in some instances the posterior communicating artery is seen as a narrow band running downwards and forwards from the basilar artery toward the internal carotid artery at the level of the dorsum sellac. It is well known that during the first stage of an encephalographic examination the flow of air is sometimes stopped by an obstrucrion at the level of the 57

45 NO. OF CASES mm Diagram 5. Depth of the interpeduncular fossa. Estimated mean value 19 mm. Estimated standard deviation 1.9 mm. dorsum sellae (Fig. 36). This occurred in 25 of the present cases. Not until more ai;,had been injected did it travel past this obstruction to fill the other parts of the subarachnoid space. In 8 of these cases the obstruction probably consisted of the aforementioned membrane between the two oculomotor nerves. Only when air has been injected in sufficient amounts either to burst the membrane or to pass between its margin and the surface of the pons, do the other parts of the cistern become filled. In the large majority of cases this membrane cannot be encephalographically demonstrated. Occasionally, however, it is distinguished in encephalographic views as a faintly outlined structure znveloped by air which, as regards both' appearance and shape, corresponds exactly with the barrier to the passage of the air. Like the barrier, this structure is convex in a forward direction and arises from the top of the dorsum sellae. From this site the membrane curves forward to the level of the mammillary bodies. In 4 cases such a membrane, surrounded by air, was identified without its having caused obstruction to the passage of the air in the early stage of the examination (Fig. 36). As no other suitable anatomical structure is present, this membrane may be regarded as the boundary between the interpeduncular and chiasmatic &terns. If it is not visible in the roentgen films the boundary may be taken as a line extending from the top of the dorsum sellae to the anterior-inferior margin of the mammillary bodies; this corresponds roughly with a straight line drawn

46 a Fig. 36. Encephalography. (a)showing the arachnoid membrane (-+) separating the interpeduncular and.chiasmari.c cisterns. (b) Air arrested against the membrane. b from the top of the dorsum sellae at right angles to the roof of the subarachnoid space. In 17 cases the air was arrested at the dorsum sellae at the beginning of the examination although the barrier did not have the characteristic shape of the membrane in question. The air travelled as far as the mammillary bodies but did not fill the interpeduncular fossa. In all probability a membrane was the cause of the stoppage in these cases also, but either it was incompletely developed or it was so closely attached to the chiasm and the infundibulum that the characteristic curve was not demonstrated in the roentgen film. In posteroanterior views thc air in the interpeduncular fossa is seen in the midline extending laterally around the lower surfaces of the peduncles into the crural cisterns. Anatomy Cisterlra cruralis The interpeduncular cistern is continued laterally by a narrow subarachnoid space which covers the inferior and lateral surfaces of the crura cerebri (Fig. 37). This space has not previously been described as an independent cistern but has been regarded as a part of either the ambient or the interpeduncular cistern. From the roentgenologic view-point it is, however, advisable to describe this portion of 59

47 Fig. 37. Transverse section through the mesencephalon. Arrow (+) points to the crural cistern between the uncus and peduncle. the subarachnoid spacc as a separate cistern as it is identifiable at encephalography as a relatively delimited space with a characteristic appearance. Furthermore, as this part of the cistern is of significance in pathologic conditions, a knowledge of its normal appearance is of assistance in avoiding diagnostic errors. The cisterna cruralis would seem to be a suitable name for this cistern (LILIEQUIST 1956). Anteriorly and superiorly the cistern is limited by the posterior margin of the optic tract. Laterally its boundary is formed by the uncus of the hippocampal gyrus. Its medial border consists of the lateral surfaces of the peduncles, below which it communicates through a wide opening with the interpeduncular cistern and further forward wirh the chiasmatic cistern. Posteriorly it is freely continuous with the ambient cistern latmeral to the brain stem. Errcephalographic appearances Together with the interpeduncular cistern the crural cistern appears in the posteroanterior view as "a crown with three peaks" (Fig. 38). The medial portion is formed by air in the interpeduncular cistern and the lateral portions by- the air in the subarachnoid space lateral to the peduncles. These lateral diverticula were called by RUGGIERO "les recessus latkraux de la citerne interpkdonculaire". In the present work the term crural cistern has been used. 60

48 a Fig. 38. (a) Specimen with barium gelatine in the cisterns. Tomographic view. The interpeduncular and crural cisterns together have the appearance of a "crown with three peaks" (+). (b) Encephalography. P-a view showing the same features. b From the roentgenologic standpoint there is reason for treating these subarachnoid spaces separately from both the interpeduncular and the ambient cisterns and for giving th'em a special name, the crural cisterns. They differ in shape and appearance from the ambient cistern. The crural cisterns diverge laterally and upwards to terminate in a peak. As these spaces are situated further forward and higher up than the ambient cisterns, they overlie the latter cisterns in the posteroanterior view, crossing them almost at right angles (Fig. 39). They also lie higher up and further anteriorly than the pontocerebellar cisterns. The position of these two pairs of cisterns is such that the crural cistern may be misinterpreted as an upward displaced pontocerebellar cistern, especially in those cases in which only one crural cistern is filled with air. They differ so appreciably from each other in appearance, however, that no confusion should arise, The crural cistern can be studied only in the posteroanterior view. The crural cistern extends to the optic tract and at its anterior extremity continues into the chiasmatic cistern. Its lateral border is formed by the uncus gyri hippocampi. The appearance of the cistern partly corresponds with the shape of the uncus, i. e. it describes a gentle curve with the convexity directed medially. This curvature is more marked at the anterior extr,emity near the optic 61

49 1 b Fig. 39. (a)specimen with barium gelatine in the cisterns. The crural cistern (+) is seen crossing over the ambient cistern (+). (b) Encephalography. 1'--a view showing the same relationships. tract. The cistern may appear differently in the posteroanterior projections according to how the central ray has been directed (Fig. 40). If the beam coincides with the anterior portion, the cistern is seen as a narrow streak of air curving gently upward and laterally and tapering to a peak; in these cases there is usually no sign that air has passed into the interpeduncular cistern. If, on the other hand, the ray coincides with the posterior portion, the cistern appears as a straight rod running upward and laterally; in these cases also, the cistern tapers to a point but air can nearly always be seen to have passed toward the mid-line and entered the interpeduncular cistern. The middle portion of the cistern will also lie closer to the mid-he than its anterior portion, owing to the fact that the uncus bulges into the cistern. Similarly, the anterior portion will be situated further away from the mid-line than the posterior portion since the peduncles diverge anteriorly, superiorly, and laterally. Anatomy Cirterna chiasmatis The chiasmatic cistern is formed by the subarachnoid space extending in front of the aforementioned membrane to the posterior surfaces of the frontal lobes. It has recdeived its name from the optic chiasm, which is situated within it (Fig. 62

50 C d Fig. 40. (a, b) Specimen with barium gelatine in the cisterns. The outline of the crural cisterns (+) changes according to the angulation of the beam. (c, d) Encephalography. P-a views. In views a and c the central ray was in alignment with the anterior portions of the cisterns and in b and d with the posterior portions. 63

51 Fig. 41. Plastic cast of the cisterns. I. Cisterna laminae terminalis. 2. Olfactory sulcus. 3. Crural cistern. 4. Interpeduncular cistern. 5. Furrow made by optic chiasm. 6. Ambient cistern. 7. Sylvian fissure. 41). The cistern extends laterally to the medial surface of the uncus, around which it is in communication with the crural cistern. Hence the medial portion of the uncus lies within the cistern and the tip of the uncus reaches medial to the free margin of the tentorium (Fig. 42). DAVIDOFF & DYKE stated that the cistern lies chiefly between tbe two diverging optic nerves and extends laterally to the mesial border of the carotid artery. DILENGE also, in a paper from 1957 on the anatomy and encephalographic appearances of the chiasmatic cistern, stated that laterally the cistern extends to the temporal lobe. According to his description, however,!he roof is "virtuel" and represented "par le point oh la citerne devient trts mince et se continue avec la citerne de la lame terminale"; the impression gained from his paper is that the cistern is very narrow. Actually, however, it is very broad, and its roof to the side of the chiasma and the anterior recesses of the third ventricle is formed by the anterior perforated substance (cf. Fig. 33). Below the anterior perforated substance on each side, in front of the uncus, tbe cistern communicates through a wide opening with the subarachnoid space in the Sylvian fissure. Anteriorly the cistern is limited by the gyrus rectus, the surface 64

52 Fig. 42. Spcimen with barium gelatine in the cisterns. The medial tip of the uncus (+) located medial to the tentorial margin (+). is of which is gently convex in a posterior direction. In the mid-line it connects freely with the subarachnoid space between the medial surfaces of the frontal lobes. From this site there is also a narrow continuation upward of the subarachnoid space in front of the anterior wall of the third ventricle, the cisterna laminae terminalis, Within the cistern are contained the optic nerve and chiasm, which together with the infundibulum, the tuber cinereum, and the anterior recesses of the third ventricle, form an obliquely placed plate, several millimeters in thickness, which inclines anteriorly and posteriorly in the direction of the sulcus chiasmatis. Latferal to and slightly posterior to these structures the internal carotid artery passes through the cistern. This artery pierces the dura mater at the level of the anterior clinoid process, whence it runs backwards to the posterior clinoid process, then swings forwards and laterally to divide slightly in front of the optic chiasm into its two branches, the anterior and middle cersebral arteries. The anterior cerebral artery runs in front of the optic chiasm and is connected with its fellow of the opposite side by the anterior communicating artery. From the internal carotid artery arise two branches at the level of the dorsum sellae, the posterior coni-

53 Fig. 43. Encephalography. Lateral view showing the quadrangular shape of the interpeduncular and chiasmatic cisterns. municating artery and slightly more superiorly, the anterior choroid artery. The basal vein arises below the anterior perforated substance, and runs close to the superior surfac.e of the uncus and then around the brain stem in the ambient cistern. Encephalographic appearances Viewed together with the air in the interpeduxular cistern, the chiasmatic cistern is quadrangular in shape in the lateral view (Fig. 43). Its anterior border, formed by the posterior surface of the gyrus rectus, runs in a shallow curve with the convexity directed backwards. This border is not always easy to identify, as air in the anterior portion of the Sylvian fissure overlies this part of the cistern. The cisternal roof, which is formed by the anterior perforated substance, is straight and extends to the side of the anterior portion of the third ventricle. DILENGE described this cistern as a right-angled triangle. He gave its largest vertical and horizontal measurements as 8 to 16 mm and 8 to 14 mm, respectively, but did not mention between which points his measurements had been made. As before mentioned, the cistern has received its name from the optic chiasm. This structure, together with the intracranial portions of the optic nerves, appears in the films as a straight band 2 to 3 millimeters in breadth, running obliquely 66

54 Fig. 44. Encephalography. Lateral view. (+) tuber cinereurn; (+) rnarnrnillary bodies. backwards and upwards across the cistern from the sulcus chiasmatis to the anterior recesses of the third ventricle. (Cf. Fig. 44.) This entire structure has been referred to in the roentgenologic literature as the chiasm or the optic chiasm. Its posterior surface is always straight and is usually seen extending past the infundibular recess to the anterior margin of the mammillary bodies. The optic chiasm forms the posterior wall of the recessus opticus of the third ventricle which tapers downwards as a cone a few millimeters in front of the chiasm. This part of the third ventricle is usually demonstrated, as its anterior wall is formed by the lamina terminalis which separates the cisterna laminae terminalis from the third ventricle. The posterior recess of the third ventricle, the infundibular recess, situated behind the chiasm, projects downwards as a cone-shaped bulge into the infundibulum. Its posterior wall, the tuber cinereum, usually appears, as DAVIDOFF & DYKE have already pointed out, as a straight, narrow, soft tissue structure between the air in the interpeduncular cistern and the third ventricle (Fig. 44). The tuber cinereurn appears on the films to be a direct continuation of the posterior surface of the optic chiasm extending to the anterior margin of the mammillary body. The optic nerve thickens slightly as it approaches the site of the chiasm, i. e. near the optic recess, but on my films no distinct local thickening was demonstrated, and DILENGE also was unable to distinguish this feature. The exact site of the optic chiasm may be established when both the optic and infundibular recesses are filled with air. BULL demonstrated that "the chiasmal point", i. e. the incisure between the optic and infundibular recesses, lies at a varying distance

55 NO. OF CASES Diagram 6. Length of.optic nerve. Estimated mean value 19 mm. Estimated standard deviation 2.9 mm. from a straight line drawn between th,e nasion and the tuberculum sellae. SCHAEFFER and DILENGE also studied the variations in the position of the chiasm in relation to the sulcus chiasmatis and the sella turcica, and GREITZ & LOFSTEDT mationed variations in the relations of the anterior recesses of the third ventricle to the sella turcica- The position of the chiasm may also be established in the air-filled cistern if the optic recess of the third ventricle is demonstrated. Thre posterior wall of the anterior recess is formed by the chiasm. The posterior surface of the chiasm may be followed to the level of the anterior recess and then traced to the mammillary bodies as the tuber cinereum, which appears as a direct continuation of the posterior surface of the chiasm. A fairly exact idea of the position of the chiasm may be gained by making a tracing of all these structures. ROBERTSON stated that the length of "the optic nerve", as seen at encephalography, varies from IZ to 17 mm. DILENGE gave the length of the optic chiasm together with the intracranial portion of the optic nerve as 13 to 18 mm, with the average at 15 mm. BULL found the distance from the "chiasmal point" to the tuberculum sellae to be 10 to 23 mm. Such a line does not, however, correspond to the course of the optic nerve, but is more in the nature of an oblique section through the optic nerve and chiasm, from the anterior-superior corner of the chiasm, the "chiasmal point", to the tuberculum sellae. ZANDER demonstrated in cadavers that a fully developed sulcus chiasmatis is present in only 34 olo of adults. In children, on the other hand, this sulcus is always well developed (GREITZ). As the optic foramina cannot be demonstrated in the lateral view 68

56 NO. OF CASES TILT OF SELLAR ENTRANCE Diagram 7. there is no definite bony landmark to which the length of the chiasm may be measured. As the optic nerve and chiasm may to a certain degree be directly demonstrated in a roentgen view its direction may also be exactly established. If the anterior surface of the optic nerve is prolonged by a straight line till it meets the planum sphenoidale, a more exact figure for the intracranial length of the nerve may be obtained. The combined length of the optic nerve and chiasm was measured by this method in the present material. The superior boundary of the chiasm was determined by the position of the optic recess or, in those cases in which this could not be identified, by the position of the anterior-superior boundary of the optic chiasm. The length measured on the films varied from 12 to 27 mm and was distributed as shown in Diagram 6. BULL in 1955 discussed the possible significance of the tilt of the sellar entrance in relation to a base line from the nasion to the tuberculum sellae, as well as of the relation of the chiasm to the sellar entrance, in suprasellar expansive processes with different symptomatology. There appears to be a fixed relationship between the length of the optic nterve and the tilt of the sellar entrance. In the present material the tilt varied from - IS O to O (Diagram 7). In Diagram 8 it is seen that there is a correlation between the length of the optic nerve and the tilt of the sellar entrance. In most instances, the steeper the tilt the longer was the 69

57 LENGTH OF OPTIC NERVE NO. OF CASES I TILT OF SELLAR ENTRANCE I I Diagram 8. Correlation between length of optic nerve and tilt of sellar entrance. Estimated correlation coefficient r = Diagram 9. Inclination of the optic nerve. optic nerve. This may provide an explanation of the different symptomatology occurring in intracranial tumours with suprasellar expansion. An investigation to study the significance of this correlation has been planned but has not yet been carried out. The slope of the optic nerve may be determined in the same way as the tilt of the sellar entrance. Diagram 9 shows the variations obtained. No correlation was found between the inclination and length of the nerve. A thin, soft tissue structure continuing downwards to the sellar entrance may occasionally be seen behind the optic chiasm. This is the infundibulum. As has already been mentioned, the tuber cinereum may be identified as a thin band of soft tissue between the air in the third ventricle and the interpeduncular cistern stretching downwards from the mammillary body to the infundibulum, or, if the latter is not seen, to the posterior surface of the optic chiasm (Fig. 44). In the anterior part of the cistern a soft tissue structure several inillimeters in width may usually be seen. As PENDERGRASS & PERRYMAN and DILENGE have already pointed out, this is the internal carotid artery, which appears as a 3 to 4 mm wide soft tissue structure arching in a forward direction from the level of

58 Fig. 45. Internal carotid artery (+) crossing over' the optic chiasm (-I+) in the chiasmatic cistern. the sellar entrance or the dorsum sellae and crossing over the anterior portion of the optic chiasm (Fig. 45). It may be distinguished as far as the upper corner of the chiasmatic cistern where it terminates in a round knob, which is formed by the middle cerebral artery or one of its large branches. This round structure was believed by PENDERGRASS to be the anterior communicating or middle cerebral artery, and by DILENGE to be the anterior cerebral artery. After comparing encephalographic with angiographic views taken in the same patient, I found that it consists of the middle cerebral artery or one of its larger branches which runs parallel to the central ray. As the two internal carotid arteries overlie each other in the roentgen view, their appearances are not uniform; hence it is not always possible to distinguish the entire course of the internal carotid artery, only parts of it being as a rule identified. Nor can the arteries of the two sides be distinguished from each other. If the posterior communicating artery is wide, it is occasionally seen as a band extending to the basilar artery. The chiasmatic cistern is bounded inferiorly by a fold of dura mater, the diaphragma sellae, which forms a roof over the sellar entrance. This boundary is always distinctly delineated, describing as a rule a gentle curve with a downward convexity and thus giving the impression that the air is bulging slightly into the sellar entrance. In none of my cases was air observed under this fold of dura mater in the sella turcica. It is of the greatest importance to have a thorough knowledge of the shape and contents of the chiasmatic cistern. Its appearance changes when expansive 71

59 NO. OF CASES ! mm Diagram 10. Distance from gyrus rectus to anterior surface of pons. Estimated mean value 21 mrn. Estimated standard deviation 2.3 mm. processes are present above the sella turcica, and it is also probable that alterations,may occur in frontal and other supratentorial tumours causing sphenoidal herniation. SDATZ & STROESCU demonstrated that the gyrus rectus is often displaced downwards into the anterior portion of the chiasmatic cistern, causing alteration of the optic chiasm. Such herniation affects the shape of the cistern by displacing its anterior boundary, the gyrus rectus, in a backward direction. These deformities or displacements of parts of the brain cannot always be referred to adjacent skeletal points, as these relations often normally show considerable variations. A more reliable basis for interpreration is the relationship between adjacent parts of the brain. For an assessment of the anterior boundary of the chiasmatic cistern only one such relation can be measured and estimated with certainty, namely, the distance from the gyrus rcctus to the anterior surface of the pons. In the present material the figures obtained for this distance were 17 to 28 mm (Diagram 10). The chiasmatic cistern can be studied in the posteroanterior view only with the aid of tomography. In ordinary views the air in this cistern cannot be distinguished from that in other superimposed air-filled cisterns nor from the air in the sphenoidal and ethmoidal cells. Tomography demonstrates the lateral extent of the cistern as well as its communications with the Sylvian fissure (Fig. 46). The internal carotid artery is occasionally recognised, as well as the middle cerebral artery far out in the Sylvian fissure. Medial to the carotid artery may

60 Fig. 46. P-a view. Tomography in a plane. through the chiasmatic cistern. Arrow (+) points to the Sylvian fissure. (Cf. Fig. 42.) be seen a soft tissue structure whose outlines are not always distinctly delineated but whose position in the cistern corresponds with that of the optic chiasm. Anatomy Cisterna laminae terminalis cinerea ventriculi tertii KEY & RETZIUS described this space as a narrow continuation of the chiasmatic cisttrn in the mid-line in front of the anterior wall of the third ventricle. The cistern extends to the inferior border of the anterior commissure, continuing from there to the genu of the corpus callosum. Anteriorly there is no boundary with the much larger subarachnoid space between the frontal lobes which extends to the border of the falx. SPATZ & STROESCU called this space, together with the cisterna corporis callosi and that portion of the chiasmatic cistern which is situated in front of the optic chiasm, the cisterna interhemispherica. In plastic casts and other preparations it is, however, possible to distinguish a separate space immediately in front of the third ventricle, formed by a widen- 73

61 Fig. 47. Specimen with barium gelatine in the cisterns and cut in the mid-line. (+) laminae tcrminalis; ( b) pericallosal artery. cisterna ing of the fissure slightly anterior to the lamina terminalis (Fig. 47). In the lateral view this has the characteristic appearance of an inverted comma. Since thi! cisternal part also appears as a distinct structure at encephalography it seems logical to retain the old name cisterna laminae terminalis. To the front of the true cisterna laminae terminalis the two pericallosal arteries run between the frontal lobes to the genu of the corpus callosum. Their course is variable, and they may describe a gentle curve with the concavity directed anteroinferiorly..!kephalographic appearances The cistern usually has the characteristic appearance of a small, anteriorly directed beak or inverted comma in the lateral encephalographic view (Fig. 48). As the cistern is often overlapped by the air in the anterior portion of the Sylvian fissure it may be difficult to identify by an unskilled observer. In the present material it was recognised in 76 patients examined in the supine position and in 62 examined in the sitting position. In some cases the air continues as a narrow streak curving forwards and upwards to the genu of the corpus callosum. If air is also present in the third ventricle, the lamina terminalis appears distinctly as a thin septum extending upward to the anterior commissure and downward to the optic recess. Below the anterior perforated substance the cistxn is in direct continuity laterally with the Sylvian fissure and the chiasmntic 74

62 a b Fig. 48. Encephalography. Lateral views of thc cisrerna laminae terminalis (+). (a) The cistern has the appe.arance of an invcrted comma. (b) The cistern exrends up to rhc genu of rhc corpiis callosum. cistern, and above the substance the lateral boundary is formed by the gyrus subcallosus. The first portion of the pericallosal artery lies in the subarachnoid space betwecn the frontal lobes (Fig. 49), crossing over the gyri and sulci on the medial aspect of the frontal lobe to the genu of the corpus callosum. It may be surrounded by a layer of air which' connects the anterior portion of the chiasmatic cistern with the beginning of the cisterna corporis callosi but which in some cases only continues upwards for a short distance between the frontal lobes. This air layer should not be misinterpreted as the cisterna laminae termin?- lis or an upward displaced, air-filled, subarachnoid space around a subfrontal tumour. Occasionally the artery may appear as a band-shaped structure, a few millimeters in width, surrounded by air; confirmation of this may be obtained by carotid angiography. Anatomy Sulcus olfastoriuj The olfactory sulcus is situated on the inferior surface of the frontal lobes between the gyrus rectus and the orbital gyri and extends backwards to the trigonum olfactorium. Within it lies the lobus olfactorius. The subarachnoid space in the sulcus olfactorius communicates medially below the gyrus rectiis with the subarachnoid space between the frontal lobes and posteriorly with the 75

63 a b Fig. 49. (a) Specimen with contrast medium surrounding the pericallosal artery, which passes across the gyri and sulci. (b) Encephalography. Air around the pericallosal artery. chiasmatic cistern. The sulcus olfactorius measures nearly I cni in depth. The two sulci lie one on each side of the mid-line, and diverge in a slightly upward direction (Fig. 50). Encephalographic appearances Air is visible in the olfactory sulci only in those projections in which the central ray coincides with the plane of the cistern, i. e. in anteroposterior views taken with the patient in the supine position. The air then collects in the sulci (Fig. 51) and appears as two narrow strips, nearly I cm in height, on both sides of the mid-line diverging in a slightly lateral direction but not to the same degree as the crural cisterns. They are also situated nearer the mid-line than the latter cisterns. When the ray coincides with the bottom of the anterior cranial fossa, air is also seen in the subarachnoid space below the inferior surfaces of the frontal lobes linking the two cisterns with each other and with the air in the space between the frontal lobes. CISTERNS IN THE TENTORIAL NOTCH In the third large group of cista-ns were placed those spaces which lie in the tentorial notch or are closely related to it. These comprise the cisterna corporis callosi, the cistern lying above the quadrigeminal plate, or quadrigeminal cistern, 76

64 Fig. so. Transverse section through the frontal lobes of the brain to demonstrate the sulcus olfactorius. commonly called the cisterna venae magnae Galeni, the ambient cistern which envelops the brain stem, the wings of the ambient cistern, the cistern above the roof of the third ventricle, or cisterna veli interpositi, and the subarachnoid space around the superior vermis, or cisterna cerebelli superior. Anatomy Cisterna corporis callosi The arachnoid membrane follows mainly the distribution of the dura mater, continuing from'the margin of the falx to the cerebral lobes and thus forming a subarachnoid space between the border of the falx, the cerebral hemispheres, and the corpus callosum. This cistern receives its shape from the superior surface of the corpus callosum, and may be divided into three compartments: an anterior portion around the genu, a middle portion enclosing the corpus, and a posterior part around the splenium. Laterally the cistern is bounded by the gyrus cinguli, but between this structure and the corpus callosum it broadens out sideways into the callosal sulcus. Anteriorly it communicates with the cisterna laminae terminalis and with the subarachnoid space between the two frontal lobes as far as the margin of the falx, and posteriorly it opens into the quadrigeminal cistern but also runs around the splenium as a narrow slit to become continuous with the cistern above the roof of the third ventricle, the cisterna veli interpositi. 77

65 a Fig. 51. Encephalography. The olfactory sulci (a) are situated closer to the mid-line than the crural cisterns (b). b Encephalographic appearances When filled with air, this cistern has such a characteristic appearance that there is no possibility of confusing it. In the lateral view it covers the free surface of the corpus callosurn. Posteriorly in front of the splenium the callosal surface often describes a more or less marked downward bulge (Fig. 52). Around the splenium the air layer may extend for a good distance anteriorly and taper in an upward direction to a peak before it flows into the cisterna veli interpositi. By examining this cistern a dilatation of the lateral ventricles can be demonstrated without air-filling of the ventricles. The cistern has approximately the same width in its anterior, middle, and posterior portions. In posterior fossa tumours, however, the posterior portion of the cistern widens in relation to the two anterior portions. In the posteroanterior view the cistern appears as a narrow strip of air in the mid-line giving off laterally at the base two diverticula which extend along the surface of the corpus callosum into the callosal sulcus. The cistern is always situated in the mid-line, and the two diverticula pass symmetrically around its medial portion. If the cistern is tilted or shifted laterally, an expansive process may bc suspected.

66 b Fig. 52. Encephalographic views. (a) The cisterna corporis callosi filled with air. (b) The cistern bulges downwards posteriorly. (c) The cistern terminates around the spleniuni in a peak. (Cf. Fig. 53.) C 79

67 Fig. 53. Specimen with barium gelatine in the cisterns. Mid-line tomography. (+) Quadrigerninal cistern. (*) Cisternn corporis callosi around the splenium. (w) Cisterna laminae termirnlis. Anatomy Cisterna quadrigeminalis (Cisterna venae magnae Galeni) Above the quadrigeminal plate lies a large subarachnoid space usually referred to as the cisterna venae magnae Galeni (Fig. 5 3). This space is bounded inferiorly by the quadrigeminal bodies, anteriorly by the splenium of the corpus callosum, posteriorly by the cerebellar vermis and superiorly by the tentorial margin and by the arachnoid which bridges over the transverse fissure between the cerebellum and the cerebrum. The cistern lies in the mid-lin'e, and on each side of the collicular plate it is in communication with the ambient cistern. Anteriorly in the cistern, between the superior colliculi, lies the pineal,body; below the pineal body the cistern extends to the posterior commissure and above it the suprapincal recess bulges to a greater or lesser degree into the cistern. The anterosuperior portion of the cistern contains the vena magna Galeni which passes 80

68 posteriorly around the splenium, after receiving the internal cerebral veins and the basal veins, to open into the straight sinus. Above the great vein of Galen the cistern is limited anteriorly by the splenium, while below the splenium and above and to the side of the pineal body, superior to the habenulae, it continues forwards in the velum interpositum above the roof of the third ventricle. Around the splenium, the cistern is also in continuity with the cisterna corporis callosi. Behind the colliculi a small diverticulum runs from the cistern as a narrow slit between the anterior medullary velum and the lingula on the one side and the inferior surface of the vermis superior on the other side. At this site the cistern is separated from the fourth ventricle only by the thin anterior medullary velum and by the lingula cerebelli, which consists of a narrow tongue-like continuation of the vermis superior in intimate contact with the anterior medullary velum. Posteriorly the cistern is bounded by the vermis where it continues around its upper surface in the form of a narrow space, the cisterna cerebelli superior, which was called by LOCKE & NAFFZIGER the "cerebella-sagittal channel. The cislern receives its characteristic shape from the quadrigeminal bodies and thus a suitable name for it is the quadrigeminal cistern. The name cisterna venae magnae Galeni is not satisfactory from the roentgenologic viewpoint since it is the colliculi, not the great vein of Galen, which give the cistern its characteristic roentgenographic appearance. Actually, this is a return to the nomenclature of KEY & RETZIUS, but in a shorter form. Encephalographic appearances This cistern is best seen in the lateral view, in which it is demonstrated free from overlying air. In this projection it appears as a triangle with the base facing the collicular plate and the apex toward the tentorial notch (Fig. 54), where the cistern is continuous with the subarachnoid space around the superior surface of the vermis, cisterna cerebelli superior. The quadrigeminal cistern and plate should always be examined. Owing to the central position occupied by these formations in the tentorial notch the cistern often becomes deformed or displaced when temporal lobe herniations are present. It is therefore of importance to have a knowledge of the normal appearance and location of these structures. Just as it is not possible to give exact measurements, or even figures suitable for practical use, for the position of the pineal body in the cranium, or for the curvature of the aqueduct and its relation to surrounding points on the vault, it is likewise not possible exactly to define the position of the quadrigeminal cistern by measuring its distance from different points on the vault or the base of the skull. No attempts were there- 81

69 a b Fig. 54. (a) Specimen with barium gelatine in the cisterns. Mid-line tomography. (-+) Collicular bodies. (-I+) Anterior medullary velum. (b) Encephalography. Lateral view. Quadrigeminal cistern is conical. (+) Colli.cular bodies. (+) Anterior medullary velum. fore made to obtain such figures in the present investigation. It is my conviction that only considerable experience with lumbar encephalography can give the examiner a feeling for what is normal and what is yathologic. The superior and inferior colliculi appear in the air-filled cistern as two rounded indentations from below. The inferior pair is slightly smaller and more pointed than the superior pair, which is larger and lower. DAVIDOFF & DYKE (1933) were the first to describe the roentgenographic appearance of the quadrigeminal bodies. In their material the appearance and size of the colliculi showed only slight variarions in individual instances. "They appeared shorter and thicker in some and longer and narrower in others". (Fig. 55.) The two pairs of colliculi, together with that part of the brain stem which lies between them and the aqueduct, form the quadrigeminal plate which, according to DAVIDOFF & DYKE, measures "approximately 13 mm in length and 5 mm in thickness". The posterior surface of the inferior colliculi may be readily idcntified at encephalography whereas no definite line of demarcation can be given for the superior pair. Although there is no suitable point for the anterior boundary of the quadrigeminal plate, the anterior surface of the posterior commissure may be used. The latter is furthermore easy to recognise encephalographically. In the present material the length of the collicular plate was therefore measured from 82

70 a b Fig. 55. Different appearances of the quadrigeminal plate and quadrigeminal bodies (a) when the aqueduct is evenly arched and (b) when it has n "physiologic kink". the posterior inferior margin of the inferior colliculi to the anterior surface of the posterior commissure. This figure varied from I 5 to 24 mm (Diagram I I). In the lateral view the position and shape of the quadrigeminal plate correspond fairly exactly with the course of the aqueduct. LINDGREN & DICHIRO in 1953 described the normal shape of the aqueduct, stating that it was evenly curved in only 37 Yo of their cases while in the other cases it was more or less markedly bent or even kinked to such a degree that it resembled the deformity of the aqueduct seen in tumour of the vermis. This "physiologic kink" was in some cases caused by a small local enlargement of the aqueduct, the ventriculus mesencephalicus, below the quadrigeminal plate, and was situated at the level of the space between the superior and inferior colliculi. These investigators also demonstrated that in cerebellar tumours the collicular plate shows deformity in proportion to the degree of kinking of the aqueduct. In the present material a "physiologic kink" in the aqueduct was observed in 56 instances while in 33 (36 Yo) it was smoothly arched. When the aqueduct describes a smooth curve the colliculi are lower and the quadrigeminal plate is longer (Diagram 12). The inferior pair usually slopes gradually downward to the anterior medullary velum. When the aqueduct displays a physiologic kink the inferior colliculi in particular are higher and the quadrigeminal plate shorter (Fig. 55). The posterior outline

71 NO. OF CASES mm Diagram 11. Length of the plate. Estimated mean value Estimated standard deviation collicular 18 mm. 1.7 mm. NO. OF CASES Diagram 12. Length of collicular plate. (04) aqueduct with a "physiologic kink", (0---0) aqueduct evenly arched. Estimated' mean values 18 and 20 mm. Estimated standard deviations 1.2 and 1.5 mrn.

72 Fig. 56. Encephalography. Lateral view with the patient in the supine position. (+) Air in the cisterna veli interpositi. (+ ) Narrow soft tissue structure representing the habenula. of the collicular plate was more marked in the latter cases and ascended more abruptly from the anterior medullary velum. The strip of air behind and below the colliculi, above the anterior medullary velum, is usually demonstrated in the lateral view (Fig. 54). If air is also present in the fourth ventricle the anterior medullary velum appears as a thin membrane of soft tissue. Its length varied in this material between 2 and 8 mm, measured from the transition between the inferior colliculi and the anterior medullary velum to the point where the air terminates at the transition of the lingula into the vermis. Hence, if this part of the cistern, but not the fourth ventricle, is filled with air, a supposition may be made as to the position of the upper portion of the fourth ventricle. Between the anterior medullary velum and the inferior colliculi there was a distinct angle in every instance in this material, and it was particularly pronounced when the aqueduct displayed a physiologic kink. This would suggest that expansive processes in the posterior cranial fossa causing a kink in the aqueduct also cause a characteristic deformation of the colliculi and the quadrigeminal cistern. It seems probable that when the quadrigeminal plate becomes compressed owing to forward displacement of the fourth ventricle the colliculi become higher and more deformed as the angle between the anterior medullary velum and the posterior surface of the inferior colliculi becomes obliterated.

73 Fig. 57. Encephalography. Lateril vicw. Arrow (+) points to a round soft tissue structure in the quadrigerninal cistern representing the basal vein before it enters the great vein of Galen. No air extended between the pineal gland and the superior quadrigeminal bodies. On the other hand, air was demonstrated lateral to and above the pineal body, continuing anteriorly in the subarachnoid space above the roof of the third ventricle. When the third ventricle was also filled with air a narrow soft tissue foi-mation was seen between the cisternal air and the third ventricle (habenula with trigonum habenulae) (Fig. 5 6). Superiorly the boundary of the cistern curves gently downward and backward immediately below and behind the splenium. This corresponds to the inferior surface of the vena magna Galeni, which swings around the splenium to.merge in the straight sinus. The suprapineal recess, which may extend for a good distance backward in the cistern, is also situated here. At the level of the inferior colliculi a rounded indentation may occasionally be seen in the cistern; this is probably the basal vein seen immediately before it terminates in the great vein of Galen (Fig. 57). In this material the quadrigeminal cistern was well filled with air in 96 cases (80 Yo). In the other 24 cases it was not satisfactorily filled and the two pairs of colliculi were therefore difficult to identify in the lateral view. This cistern should be studied as early as possible in the examination, with the patient in the sitting posture, as the air then remains collected in the cistern. When the patient is placed in the supine position at a later stage, all air leaves the cistern and it cannot be filled again unless more air is injected with the patient sitting. To ensure that the cistern will fill, the air should be injected so as to obtain filling of the pontine and pontocerebellar cisterns; thereupon the air passes upwards via 86

74 Fig. 18. Encephalography. 1 -a view. Arrows (+) point to the quadrigeminal bodies. the ambient cistern to the quadrigeminal cistern. If the patient sits with his chin lifted during the injection procedure and then flexes his head well forward, filling will usually be achieved. In the posteroanterior view the two pairs of colliculi are distinctly seen delimiting the air in the downward direction (Fig. 58). The upper portion of the cistern, on the other hand, is projected over the third ventricle, as well as over the air in the.cistern above that ventricle, and over the air around the vermis. In this view, however, the communications of the cistern with the cisterna ambiens on each side of the brain stem may be seen. The pineal gland and the vena magna Galeni are occasionally demonstrated together with the vermis as a round structure surrounded by air above the quadrigeminal bodies. The superior boundary of the cistern is not distinctly delineated in this view. Cisterna ambiens Anatomy The ambient cistern is a paired space whose two arms encircle the brain stem. LOCKE & NAPFZIGER named it the paired internal channel. It opens from the subarachnoid space above the quadrigeminal plate, fanning out anteroinferiorly

75 Fig. 59. Plastic cast of the subarachnoid cist-rns seen obliquely from the front and from above. I. Interpeduncular cistern. 2. Crural cistern. 3. Ambient cistern. 4. Wing of ambient cistern. 5. Cisterna veli interpositi. 6. Furrow made by optic chiasm. 7. Groove made by mammillary bodies. 8. Hole made by infundibulum. 9. Cisterna laminae terminalis. on both sides of the brain stem and finally becoming continuous with the basal cisterns (Fig. 59). The cistern may be divided into an anterosuperior portion above the tentorium, and a posteroinferior portion below the tentorium. The supratentorial portion lies between the peduncles and the medial surfaces of the temporal lobes, and the infratentorial portion between the pons and the medial surfaces of the cerebellum. The extension of the cistern is greatest in the sagittal plane. The infratentorial portion opens into the pontine cistern and the anterior part of the pontocerebellar 88

76 Fig. 60. Diagrammatic drawings of the ambient cistern. (a) Infratentorial portion. (b) Suprarentorial portion. (c) Composite drawing of a + b. cistern. At this site it is triangular in shape with the base at its transition into the pontine and. pontocerebellar cisterns. This part of the cistern lies lateral to the pons, extending from the brachium pontis to the origin of the peduncle. Hence it lies in a plane forming an angle with the sagittal plane, so that its anterior margin is nearer the mid-line than its posterior border. Lateral to the pons the triangle narrows in an upward direction, filling out the sulcus mesencephalicus and at the same time twisting slightly so that its divergence from the sagittal plane lessens. The cistern then becomes continuous with the space above the quadrigeminal plate. The posterosuperior part of the cistern is formed by the interval between the lobulus quadrangularis and the brachium pontis and is continuous with the subarachnoid space behind the collicular plate above the anterior medullary velum. The supratentorial portion of the ambient cistern is situated between the lateral surface of the peduncle on the one side and the medial surface of the temporal

77 C d Fig. 61. Specimens with barium gelatine in the cisterns. P-a views. Tomography in different planes. (a, b) Supratentorial portion of the ambient cistern. (c, d) Infratentorial portion.

78 Fig. 62. Encephalography. 1'--a view. Arrows (+) point to the stilcus mesencephalicus. (Cf. Fig. 61 c.) lobe on the other side. Below the peduncle it is in communication with the interpeduncular cistern, through the crural cistern, and further forward it connects with the chiasmatic cistern. The.supratentorial portion terminates anterosuycriorly against the optic tract and thus is continuous with the crural cistern. Suyeriorly and posteriorly it opens, like the rest of the ambient cistern, into the cistern above the colliculi. The shape of this part of the cistern is determined by the peduncles, which diverge forwards and upwards. The supratentorial portion of :he cistern consequently forms an angle with the sagittal plane opposite to that formed by the infratentorial portion. Thus, the anterior and posterior porrions of the ambient cistern differ in apfearance (Fig. 60). The communications of the cistern with the crural and pontine cisterns lie in different planes, with the result that, in posteroanterior views, the two portions are superimposed, crossing each other almost at right angles. The crural cistern diverges superiorly and laterally while the infratentorial part of the ambient cistern converges superiorly and posteriorly in the direction of its communication with the quadrigeminal cistern. The ambient cistern thus has fairly complicated anatomical ramifications (Fig. 61); it is of the greatest importancc, however, to have a thorough knowledge of its appearance, since its various parts may be changed by pathologic processes. 9'

79 1 b Fig. 63. Encephalography. P-a views. Different appearances of the ambient cistern when the pyramids are projected (a) at the upper border of the orbit, and (b) above the orbit. Several important structures lie within the ambient cistern. At the anterior extremity the basal vein passes from the region of the anterior perforated substance, bends around the brain stem and empties into the great vein of Galen. Slightly posterior to this vein runs the posterior cerebral artery or one of its branches, as well as th,e trochlear nerve which swings around the brain stem from the collicular plate to the basal cisterns. The superior cerebellar artery is situated in the infratentorial portion of the cistern. Encephalographic appearances As already mentioned, the ambient cistern is a paired space encircling the brain stem, with its greatest extension in a sagittal plane. It is therefore best seen in the posteroanterior view, in which it appears as a relatively thin layer of air lying on each of the brain stem. The shape of the ambient cistern is determined partly by the peduncles and partly by the lateral surfaces of the pons. The sulcus mesencephalicus thus may usually be identified as a small enlargement in the air outlines medially, slightly below the middle of the brain stem (Fig. 62). As has already been pointed out, the ambient cistern does not lie in one plane: its posterior portion around the pons, as well as its communicating apertures with

80 a b Fig. 64. Encephalography. P-a views. (a) The infratentorial portion of the ambient cistern is filled with air. (b) Both the infra- and supratentorial portions are filled. the pontine and pontocerebellar cisterns, lie at an oblique angle in relation to the sagittal plane. The anterior portion lateral to the peduncles, situated supratentorially, also extends obliquely, but in the opposite direction. Thus, these two portions of the cistern form with each other an angle open laterally, with the apex at the level of the tentorium. Hence, in the posteroanterior view, the ambient cistern appears to be broader than, it is in reality, a feature applying to both the infratentorial and the supratentorial portions. As a result, the cistern will also show differences in appearance according to the beam direction used, since different parts of the air layer will be in alignment with the rays (Fig. 63). For the same reason, the roentgen appearances will differ when different portions of the cistern are filled with air. If only the posterior portion is filled, for instance, the cistern appears to be straighter and longer; the layer of air is also narrower. If air is also present in the supratentorial portion the cistern will appear broader and will extend further laterally, and the brain stem will have a different shape; the air on the lateral aspect of the brain stem then tapers off abruptly at the level of the uncus, becoming continuous here with the crural cistern (Fig. 64). The air in the quadrigeminal, ambient, pontine, interpeduncular, and crural cisterns encircles the brain stem, which is normally always symmetrical and situ- 93

81 Fig. 65. Encephalography. Lateral view. Arrow (+) points to air in the posterior portion of the arnbient cistern. ated in the mid-line. As a rule, the ambient cistern has the same shape and appearance on both sides. Laterally, the supratentorial portion of the ambient cistern is. bounded by the temporal lobes and the infratentorial portion by the cerebellar hemispheres. These structures are difficult to distinguish with certainty in the roentgen views. Usually, the layer of air in the ambient cistern is too thin to be demonstrated in a lateral view. As ROBERTSON has already pointed out, however, a thin layer of air is occasionally seen stretching downwards from the colliculi toward the pontine cistern on the lateral aspect of the pons. This layer of air must not be misinterpreted as a dislocation backward of the ambicnt wing or as a layer of air beneath a temporal lobe herniation. In most.cases this air is not continuous but appears on the films as irregular, elongated collections of air at the level of the posterior portion of the pons (Fig. 65). Anatomy Alae cisterna ambientium and cisterna veli interpositi At a given stage in the ontogeny of the brain the cerebral lobes increase in volume. The anlage of the corpus callosum, or commissura dorsalis, which connects the two cerebral adages with each other close to the rostra1 portion of the roof plate, also begins to expand. The commissure arches dorsally over 94

82 Fig. 66. Cisterns filled with barium gelatine. Cisterna veli interpositi (-+) is filled to the foramen of Monro. the roof plate and gives rise both to the corpus callosum and, in its hindmost part, to the TWO fornices. The corpus callosum is an unpaired structure which units the cerebral lobes whereas the fornices develop as a paired structure continuing forwards to the rostra1 portion of the roof plate, where they fuse and become continuous with the antcrior wall of the third ventricle. Posteriorly the fornices diverge and bend around the thalamus to form at the uncus the dentate gyri, which in their turn swing backwards to become the fornicate gyri. As the cerebral lobes expand they arch over the fornices and the third ventricle, forming here a deep transverse cleft, the fissura transversa cerebri, which runs to the site of the foramen of Monro. This fissure forms part of the general subarachnoid space and is filled by a fold of pia mater, the tela chorioidea ventriculi tertii, or tela chorioidea prosencephali. Together with a thin layer of ependyma this fold of pia mater also forms the chorioid plexuses of the third ventricle and the lateral ventricles. This structure is also called the velum triangulare or velum interpositum, and is situated below the fornices and corpus callosum but above the roof of the third ventricle and the pulvinar of the thalamus. It contains a narrow subarachnoid space which tapers anteriorly to terminate in a point at the foramen of Monro (Fig. 66), and posteriorly continues around the pulvinar of the thalamus, following the fornices to terminate against the posterior surface ot the uncus. Laterally this space extends to the medial wall of the temporal horn,

83 a Fig. 67. Wing of the ambient cistern. A transverse section cut as in (a) shows the wing of the ambient cistern (b). The wing extends along the free surface of the thalamus laterally to the temporal horn (c). b C 96

84 being separated from it by only a thin 1ay:r of ependyma (Fig. 67). Above the uncus this slit-like space opens medially into the anterior portion of the ambient cistern and thus lies at right angles to this cistern. GOETTE, who examined this slit by pouring barium into the chiasmatic cistern in cadavers, named it the chorioidal fissure. He found that it follows the free margin of the thalamus and that it corresponds to the crescent-shaped streak of air which may be seen in lateral encephalographic views and which is generally referred to in the roentgenologic literature as the cisterna ambiens. He did not describe the appearance of this slit in posteroanterior views. By the choriodal fissure is meant, anatomically speaking, the slit arising between the lateral ventricles and the brain surface when the chorioid plexus is removed. The name is therefore not a suitable one for the aforementioned part of the subarachnoid space, since this portion does not open into the lateral ventricles. The extensions of the ambient cistern (wings) around the posterior surface of the thalamus terminate at the anterior extremity against the posterior margin of the uncus. Here the slit widens slightly and encircles the posterior surface of the uncus as two small pockets (Fig. 68). Seen from the side, these two pockets describe an even curve with the convexity directed anteroinfetiorly. Both KEY & RETZIUS and LOCKE & NAFFZIGER described these structures. According to the former investigators the glue they used filled the whole velum iuterpositum in several cases. In the illustrations published by LOCKE & NAFF- ZIGER, showing casts of the cisterns, the structures surrounding the thalamus may be readily identified by their characteristic appearance. The relations of the structures to the thalamus and temporal lobes have also been described. Viewed directly from the front, the space surrounding the thalamus has the appearance of two wings broadening out laterally from the anterior portion of the ambient cistern (cf. Fig. 61). The wings taper off as they bend around the posterosuperior surface of the thalamus to continue in the velum triangulare below the fornices on the upper aspect of the thalamus to the foramen of Monro. Viewed directly from above the velum triangulare is triangular in shape with the base continuing into the cistern above the quadrigeminal plate and the apex directed anteriorly. Encephalographic appearances In the roentgenologic literature, the term cisterna ambiens is often used to designate the two crescent-shaped strips of air surrounding the pulvinar of the thalamus. I called them in a previous article the wings of the ambient cistern. RUGGIERO used the name citerne retropulvinarienne. This portion of the subarachnoid space is a narrow slit, but when it is in alignment with the direction of the rays the air layer within it is dense enough to be roentgenographically de- 97

85 C Fig. 68. (a) Plastic cast seen from the side. The wing ends against the uncus. (b) Tomographic view through the wing (+). (c) Parasagittal section through a brain with barium gelatine in the ambient cistern showing the relation of the wing to the uncus.

86 a b Fig. 69. The ambient wing ends against the uncus (+). (a) Contrast medium in the wing. (b) Air in the wing. monstrated. As the wings lie one on each side of the mid-line they appear on the films as two independent strips of air (cf. Fig. 65). The wings are usually I to 2 mm deep, and deepest in the middle. They are seen in the lateral view as two smoothly rounded arches, which are convex in a backward direction and projected either over the superior colliculi or between this pair and the inflerior colliculi, never further down. They are also situated in a plane in front of the anterior limits of the trigonum of the lateral ventricle. The wings terminate inferoanteriorly against the uncus, usually as a point. Occasionally, however, the anteroinferior termination appears as a widening of the inferior portion of the wing, with a small bowl-shaped impression caused by the posterior surface of the uncus (Fig. 69). Since the wings surround the free surface of the thalamus they reflect the shape and position of this structure in a lateral view. Thus, the wings of the ambient cistern do not represent the cisterna ambiens in the lateral view, but lie in a plane at right angles to this cistern. If the wings are wider than usual they may also be recognised in a posterointerior view, in which they appear as two thin layers of air lateral to the upper and middle portions of the brain stem. In some instawes, instead of a continuous air layer there are seen at the site of the wings only thin, irregular streaks of air through which may run fine, tortuous bands corresponding to the lateral choroid artery (GALLOWAY & GREITZ) (Fig. 70). On the superior aspect of the thalamus the two wings communicate with the cisterna veli interpositi which extends to the foramen of Monro. This portion of the subarachnoid space has also been called the cistern above the roof of the third ventricle and by RUGGIERO the citerne sous-trigonale. In the postero- 99

87 2 b Fig. 70. Appearance of ambient wings in 1 -a views, filled (a) with air, and (b) with contrast medium. a Fig. 71. Cisterna veli interpositi filled (a) with air, and (b) with contrast medium. b anterior view this layer of air is in alignment with the beam direction and the cistern is demonstrated as two narrow streaks of air one on each side of the midline and curving gently in a lateral and upward direction (Fig. 71). They cannot always be traced as far as the mid-line. If the lateral ventricles are also air-filled the fornices appear as a narrow soft tissue formation between the two air layers. I00

88 Fig. 72. Encephalography. Lateral views in two different cases showing the cisterna veli interpositi supplemented by diagrammatic drawings. Arrow (+) points to the internal cerebral veins. In the lateral view the air in the cistcrna veli interpositi shows characteristic outlines. Roentgenographically, it may be divided into a medial unpairfed portion lying above the roof of the third ventricle, and two lateral narrow strips of air situated close to and below the fornices. The medial portion is a direct continuation forwards in the mid-line of the air above the collicular plate. The narrow lateral extensions, on the other hand, form a continuation of the wings of the ambient cistern below the fornices to the foramen of Monro (Fig. 72). These three portions may be filled with air independently of one another. The medial portion above the roof of the third ventricle is often well filled with air when the patient is in the supine position. It is then more distinctly delineated as the suprapineal recess is usually fi1le.d with cerebrospinal fluid. With I01

89 Fig. 73. Enccphalography. Lateral views showing different appearances of the cisrerna cerebelli superior. the patient in the prone position, the cistern usually contains no air. The lateral portions, on the other hand, are often air-filled in the prone position also. With the exception of its anterior extension above the roof of the third ventricle, the medial portion of the cistern shows little variation in appearance. In the present material, the cistern either extended as far as the foramen of Monro as a narrow, relatively high layer of air, or reached to about halfway between this foramen and the collicular plate, or appeared only as a short, pointed continuation of the quadrigeminal cistern. I02

90 Anat om y Cisterna cerebelli superior Anterior and lateral to the vermis lies a narrow subarachnoid space communicating broadly on its inferior aspect with the quadrigeminal cistern and the posterior portion of the ambient cistern. This space encircles the anterosuperior and lateral surfaces of the vermis, extending as a rule I to 2 cm behind the site of the tentorial notch on the superior aspect of the cerebellum. Laterally the cistern is continuous not only with the subarachnoid space between the brachium conjunctivum and the cerebellar hemispheres, forming a narrow upward continuation of the posterior portion of the ambient cistern past the quadrigeminal cistern, but also with the space in front of the two cerebellar hemispheres extending between the latter and the inferior surface of the tentorium as far down as the pontocerebellar cistern. Encephalographic appearances In a lateral view the air around the vermis is seen as a narrow layer between the cerebellar surface with its lobulus on the one hand and the smooth surface of the arachnoid covering this area on the other hand (Fig. 73). The cistern is usually angular in shape owing to the fact that the air is bounded anteriorly and superiorly by structures with straight surfaces, in other words the arachnoid and the inferior surface of the tentorium. The boundary towards the vermis, on the other hand, is more rounded and irregular and the air between the narrow lobulus renders the cistern easy to identify in a lateral view. In the present material, the cistern was filled with air in IOI cases. In 70 cases it was angular, in 2 slightly pointed, and in 29 flat. The anterior surface of the vermis was never concave but was either convex or straight. The cistern emerges into the posterior portion of the quadrigeminal cistern and is directly continuous here with the air above the anterior medullary velum and the lingula. Occasionally the air may be traced past the anterior medullary velum for a short distance down towards the pontine cistern; this air lies in the posterior part of the ambient cistern between the lobulus quadrangularis and the brachium conjunctivum. As a rule the air terminates against the brachium pontis, which at this site passes into the cerebellum and bulges out laterally. In some instances the air may be followed down toward the pontine cistern to a site roughly corresponding to the free margin of the tentorium or for a short distance below it; it may curve gently downwards and backwards, and is situated between the lobulus quadrangularis and the pons, forming a part of the infratentorial portion of the ambient cistern. In the posteroanterior view the inferior surface of the tentorium and the shape of the vermis give the air outlines around the vermis a characteristic appearance

91 a Fig. 74. Encephalogr-aphy. 1 --a views. (a) Arrows (+) point to air in the cisterna ccrcbelli superior. (b) Subdural air (+) below the tcnrorium. b (Fig. 74). The cistern appears as two, or more, narrow streaks of air converging to meet in the mid-line. At first sight, the air has the same appearance as air in a subdural position, but it never reaches so far in the lateral direction; it envelops the irregular surface of the vermis as well as portions of the cerebellum near the vermis, and terminates laterally in a slightly uneven outline. Laterally the cistern continues into the subarachnoid space around the cerebellar lobes. Subdural air, on the other hand, has an even outline and lextends below the whole length of the tentorial surface. A comparison of posteroanterior and lateral views will, however, dispel all uncertainty as to where the air is situated.