London, N. W. 1. concluded that cephalic blood flow in these species is controlled by

Transcription

1 J. Phy8iol. (1963), 167, pp With 8 text-figure8 Printed in Great Britain THE EFFECT ON BLOOD PRESSURE IN THE SHEEP AND CALF OF CLAMPING SOME OF THE ARTERIES CONTRIBUTING TO THE CEPHALIC CIRCULATION BY B. A. BALDWIN* AD F. R. BELL Frorm the Department of Physiology, Royal Veterinary College, London, N. W. 1 (Received 2 July 1962) In other studies utilizing a number of different techniques we have reported on the distribution of blood to the brain of the ox and sheep and concluded that cephalic blood flow in these species is controlled by physical haemodynamic equilibrium mechanisms (Baldwin, 196; Baldwin & Bell, 1963 a, b). The work described here gives an account of the pressure changes which occur in the cranial circulation of the ox and sheep during disruption of blood flow to this area when occlusion of various combinations of the carotid, vertebral and occipital arteries is effected. Previous investigations reporting on pressure variations occurring in the cephalic vessels after clamping the common carotid arteries have usually been incidental to reflex responses following alteration of pressure within the carotid sinus (see Heymans & Neil, 1958). METHODS The experimental and surgical procedures used on the twelve sheep and nine calves of these experiments were the same as those described in the preceding paper (Baldwin & Bell, 1963b). Cephalic blood pressure was measured bilaterally by cannulation of the proximal ends of the divided lingual arteries, and systemic blood pressure was recorded from the femoral artery. Mercury manometers were used, with 5 % sodium citrate as an anticoagulant in the manometer systems throughout the experiments. Respiration was recorded by a pneumograph and tambour. RESULTS The sheep Effect of clamping one common carotid artery. Occlusion of one common carotid in the mid-cervical region produced an average fall of 31 % (range 22-4 %) in the pressure of the homolateral lingual artery. The fall * Present address: Department of Physiology, University of California, Los Angeles, California, U.S.A.

2 464 B. A. BALDWIN AND F. R. BELL in pressure was seen immediately occlusion of the artery had been effected, but some recovery of the pressure occurred during the next few seconds, so that the average fall became 21 % (range 15-3 %) (Fig. 1). In some experiments the occluding clamp was left in position for more than a minute without any further restoration of lingual pressure after the initial Rt-miritinnn \- M- LIV 1 l OEF E E LU_ - 1 E PCIL Cd I -j CL: 6 - CIO m r atw C. E.= E 1 I A B C D E Fig. 1. Sheep. In this and subsequent figures the upper record shows respiratory movements. The second record is systemic blood pressure recorded from the femoral artery. The third record shows pressure in the proximal end of the left lingual artery and the fourth record blood pressure in the proximal end of the right lingual artery. Time marker, 5 sec intervals. A downward-pointing arrow indicates when an artery was clamped and an upward-pointing arrow when an arterial clamp was removed. A, clamp right carotid artery; B, unclamp right carotid artery; C, clamp right carotid artery; D, clamp left carotid artery; E, unclamp both carotid arteries.

3 CGEjPHALIC BLOOD PRESSURE IN RUMINANTS 465 responses. On release of the carotid clamp the lingual pressure returned immediately to the level of the systemic pressure. Blood pressure in the contralateral lingual artery showed no change nor, usually, did the systemic pressure. Effect of clamping both common carotid arteries. When one carotid was clamped the results described above occurred, but when the other carotid was also occluded the lingual pressure fell further to an average of 54 % (range 45-7 %) below its original value. This fall occurred in both lingual arteries and there was no secondary stabilization of lingual pressure, as happens when only one carotid artery is occluded. Frequently the systemic pressure showed a small rise during the period of bilateral occlusion (Fig. 1). When one carotid was released the pressure in the homolateral lingual artery was restored immediately to that of the systemic pressure, but the pressure in the lingual artery on the side where the carotid remained clamped rose only to a level about 2 % below the resting value. Simultaneous release of both clamps resulted in an immediate return of both lingual pressures to the systemic level. Effect of clamping vertebral arteries. The occlusion of one or both vertebral arteries produced no demonstrable effect on the cephalic or systemic pressure. Effect of occluding the common carotid, the occipital artery and the external carotid artery of one side. This series of occlusions was performed in an attempt to determine the origins of the pressure which maintains lingual pressure when the carotid artery is clamped (Fig. 2). The initial clamping of the common carotid resulted in a fall of 31 % in the pressure in the homolateral lingual artery. When the homolateral occipital artery was then also occluded the pressure in the lingual artery fell further to a level 54 % below the resting value. Finally, on clamping the homolateral external carotid the lingual pressure was further reduced to 79 % below its initial level. Effect of occluding both common carotids when both occipitals have been clamped previously. When one carotid was clamped, following the occlusion of both occipitals, the pressure in the homolateral lingual artery fell 51 % (range %), without any effect on the pressure in the contralateral lingual artery or significant effect on systemic blood pressure. When in addition the second carotid was also occluded, the pressure in both lingul arteries fell to 88 % of the initial pressure (range %), and within 15 sec the systemic pressure showed a marked and rapid rise, accompanied by inhibition of respiration often leading to apnoea. On release of the carotid clamps both lingual pressures were restored immediately to the resting level and the systemic hypertension was rapidly 3 Physiol. 167

4 466 B. A. BALDWIN AND F. R. BELL reduced (Fig. 3). Release of both occipital arteries while the carotids remain clamped resulted in incomplete restoration of lingual pressure, but systemic hypertension was completely alleviated. Respiration V C1- - b4 oi 1 E E a _ '~ r 1 -r I E C6 -E a- m t4 1 C: Qo A CL Vt t Fig. 2. Sheep. A, clamp right carotid artery; B, clamp right occipital artery; C, clamp right external carotid artery; D, unclamp right external carotid artery; E, unclamp right occipital artery; F, unclamp right carotid artery. Effect of clamping both common carotids with both vertebrals previowsly clamped. On occluding one carotid after both vertebrals have been occluded, the pressure in the homolateral lingual artery fell to an average of 32 % of its initial level (range %). There was no observable effect on the pressure in the contralateral lingual artery and little effect on systemic pressure. When the other common carotid was also occluded the pressure in both

5 CEPHALIC BLOOD PRESSURE IN RUMINANTS 467 lingual arteries fell to 88 % of-the initthal pressure (range 87-9 %). With this manoeuvre the systemic blood pressure again showed a rapid and marked rise, accompanied by apnoea or intermittent hyperpnoea within 15 sec. On releasing the vertebral arteries the lingual pressure failed to Respiration, E E E U - I E E 1 CL Co -E I_ 11 1 _ XIL C = Cr_ A B C Fig. 3. Sheep. In this animal both occipital arteries had been clamped previously. A, clamp right carotid artery; B, clamp left carotid artery; C, unclamp both carotid arteries. return immediately to its resting value, but the systemic hypertension promptly returned to a normal level. Release of the carotid arteries, with the vertebrals remaining clamped, caused restoration of both lingual and systemic pressures to resting values (Fig. 4). 3-2

6 468 B. A. BALDWIN AND F. R. BELL Respiration 1 I E E 1 ab C-J i 1 " E.E E cc A b C D Fig. 4. Sheep. In this animal both vertebral arteries had been clamped previously. A, clamp right carotid artery; B, clamp left carotid artery; C, unclamp both carotid arteries; D, unclamp both vertebral arteries. Effect of clamping both external carotid arteries. The occlusion of one external carotid artery, the clamp being placed proximal to the origini of the lingual artery, did not affect lingual nor systemic pressure. When another clamp was similarly applied to the opposite external carotid, the systemic pressure rose precipitously within 1 sec and there was a temporary interruption in respiration (Fig. 5). This rise in systemic pressure was reflected faithfully in both lingual arteries, so that at this time carotid intrasinusal pressure was high. These experiments indicate clearly that the origin of the systemic hypertension is central and that it is probably

7 CEPHALIC BLOOD PRESSURE IN RUMINANTS 469 caused directly by the cerebral ischaemia. Release of the clamps from the external carotids quickly restored the systemic blood pressure to normal. Additional corroborative evidence that the systemic hypertension is caused by cerebral ischaemia and not by carotid body or sinus receptors was obtained by infiltrating the reflexogenic areas with local anaesthetic. Respiration E LL 7- I E P 1 I E cl C6 ^ 1 oo.e E ti L Fig. 5. Sheep. A, clamp right external carotid artery; B, clamp left external carotid artery; C, unclamp both external carotid arteries. Clamping the carotids after previous occlusion of the occipitals produced identical hypertensive effects on the systemic circulation after the application of the local anaesthesia as before.

8 47 B. A. BALDWIN AND F. R. BELL The calf Effect of clamping one common carotid artery. This manoeuvre caused a fall in pressure in the homolateral lingual artery of 34 % (range 22-4 %) but had no effect on the contralateral lingual pressure. Some degree of recovery of pressure occurred within a few seconds from the initial fall of 34 % to 2 % of the resting pressure (Fig. 6). On release of the carotid clamp the lingual pressure always returned immediately to the resting level. Respiration C_ I b 1 M. cr_ : E 1 X. K C, -Ji _ to r Ir oef 4EE 1 _- O _ A B C D E F Fig. 6. Calf. A, clamp left carotid artery. B, clamp right carotid artery. C, unclamp right carotid artery. D, unclamp left carotid artery. E, clamp right vertebral artery. F, clamp left vertebral artery.

9 CEPHALIC BLOOD PRESSURE IN RUMINANTS 471 Effect of clamping both common carotw arteries. When one carotid artery was clamped, only the homolateral lingual artery was affected, but on clamping the second carotid a fall of 54 % occurred in the pressure of both lingual arteries (range 5-56 %). A small rise in systemic pressure sometimes occurred on occluding the second carotid but there was no recovery in lingual pressure. On release of one carotid clamp, lingual pressure on the same side was restored immediately to the systemic level but the opposite lingual pressure rose only to the level observed when only one carotid was clamped, and returned to the resting level when the remaining clamp was removed (Fig. 6). Effect of occluding the vertebral arteries. Clamping one or both vertebrals had no effect on lingual or systemic blood pressure. The small effects seen in Fig. 6 are probably reflex in origin for manipulation of the vertebral vessels at this level could well stimulate receptors near the heart. Effect of occluding the common carotid, the occipital and external carotid arteries on one side. This series of occlusions was carried out in an attempt to investigate the sources of blood which assist in maintaining pressure in the lingual artery, and therefore in the circle of Willis also, when the homolateral common carotid is occluded. Occlusion of the common carotid in the upper cervical region resulted in a fall of 24 % in the pressure in the homolateral lingual artery (Fig. 7). When the occipital artery was also occluded the pressure fell immediately to 45 %, but recovered to 33 % of the initial level within a few seconds. Clamping the homolateral external carotid caused a further reduction in the lingual pressure to 58 % of the original resting level. Effect of ctmaping both common carotid arteries with both occipital8 previously clamped. When the one carotid was occluded the pressure in the ipsilateral lingual artery fell on average by 52 % (range %), but the contralateral lingual pressure was not affected. On now occluding the second carotid pressure fell in both lingual arteries to an average of 65 (range %) of the resting value. The systemic pressure was not affected. Release of both carotid clamps simultaneously restored the lingual pressures immediately to the pre-occlusion levels. Release of one carotid resulted in restoration of the homolateral lingual pressure only, the contralateral lingual artery showing an imcomplete rise in pressure. Effect of clamping both common carotid arteries with both vertebrals clamped. On occluding one carotid, when both vertebrals had already been clamped, the pressure in the homolateral lingual artery fell by 34 % (range %). There was no effect on the pressure in the contralateral lingual nor on systemic blood pressure. When the second carotid artery was occluded, pressure in both linguals

10 472 B. A. BALDWIN AND F. R. BELL fell by 8 % (range %). This change was accompanied by a large rise in systemic pressure and an increase in the depth of respiration (Fig. 8). In contrast to the sheep, in the calf this manoeuvre never produced apnoea nor was the systemic hypertension so marked. Respiration e w6 1 oi E u C: _ 1 C 1 c -J c -J t 4 1 C-.:4 E CC A B C D E FG H Fig. 7. Calf. A, clamp right carotid artery; B, clamp right occipital artery; C, clamp right external carotid artery; D, unclamp right occipital artery; E, clamp right occipital artery; F, unclamp right occipital artery; G, unclamp right external carotid artery; H, unclamp right carotid artery. Simultaneous release of both carotid clamps caused a rapid fall in systemic hypertension and recovery of pressure in both linguals. Retention of the carotid clamps with release of those on the vertebral arteries also caused complete reduction of the systemic hypertension.

11 CEPHALIC BLOOD PRESSURE IN RUMINANTS 473 Effect of occluding both external carotid arteries. The occlusion of one or both external carotid arteries in the ox, in contrast to the effect in the sheep, produced no effect on lingual or systemic pressures. Respiration l. ' 1 ZZO EL Co li 1 CO C -J cc,,1 _: A B C D E F G H Fig. 8. Calf. A, clamp right vertebral artery; B, clamp left vertebral artery; Cl, clamp right carotid artery; D, clamp left carotid artery; E, unclamp left vertebral artery; F, unclamp right vertebral artery; G, unclamp right carotid artery; H, unclamp left carotid artery. Experiments with prolonged cerebral ischaemia In this series of experiments occlusion of the arteries supplying blood to the brain was maintained for long periods in order to study the effects of prolonged cerebral ischaemia on blood pressure and respiration.

12 474 B. A. BALDWIN AND F. R. BELL The sheep Effect of occluding both common carotid and both vertebral arteries. Within 15 sec of applying the clamps the systemic pressure rose from 7 to 17 mm Hg and periodic breathing developed. After an interval the blood pressure developed an oscillatory wave form, the peaks of blood pressure coinciding with periods of respiratory activity. When in addition the left occipital artery was ligated, so that blood from the left vertebral was prevented from passing to the circle of Willis via the left external carotid, a further effect on respiration developed. This was seen as a well developed muscular spasm accompanied by an extension of the limbs and gasping respiratory movements. When finally the right occipital artery was ligated, systemic blood pressure fell gradually, and death occurred 7 min later, that is 43 min after the initial arterial occlusion. In two other animals death occurred in 6 min and 7 min after the initial clamping. Effect of clamping both common carotid arteries and both occipital arteries. This manoeuvre was performed in six animals. Systemic pressure rose to 25 mm Hg and respiration became intermittent. The palpebral reflex persisted for only 2x5 min but the corneal reflex was present much longer. Death was preceded by a further rise in blood pressure and by struggling indicative of an asphyxial spasm. Death occurred from 8 to 38 min after occlusion of the arteries. The calf Effect of clamping both carotid and both vertebral arteries. Six experiments were performed in this series. On clamping the common carotid and vertebral arteries the systemic blood pressure rose from a resting level of about 13 mm Hg to about 16 mm Hg, and respiration showed some increase in depth, without signs of distress. This rather minor deviation from normal could be readily maintained for 4 min, thus providing strong evidence that in the calf, in contrast to the sheep, the brain at this time was receiving an adequate supply of blood for normal metabolic purposes. In one animal the central spinal artery was occluded at the level of the atlanto-occipital joint without any effect on blood pressure or respiration. When in addition clamps were placed on both carotid and vertebral arteries there was no effect on systemic blood pressure or on respiration. The inferior cervical muscular branch of the left subclavian was then ligated without producing any change. Finally, when the right inferior cervical muscular branch was ligated the systemic blood pressure rose rapidly to 19 mm Hg from a resting level of 12 mm Hg, respiration became irregular and the animal died.

13 CEPHALIC BLOOD PRESSURE IN RUMINANTS 475 DISCUSSION In the sheep and the ox the initial fall in lingual pressure resulting from occlusion of the ipsilateral common carotid artery is less than that recorded under similar circumstances in the cat, rabbit (Chungcharoen, Daly, Neil & Schweitzer, 1952), or goat (Pyper, 1938) but is greater than that seen in the dog (von Euler & Liljestrand, 1936). The values obtained by von Euler & Liljestrand (1936) are probably not strictly comparable with the other results, because in their experiments sufficient time had elapsed before recording to enable blood to flow in from other sources and thus to allow the pressure to recover towards normal levels. The commonly seen rise in pressure which occurs after the initial fall is apparent in the sheep as in other animals, but in the ox the rise is almost absent. The relatively small fall in lingual pressure seen in ruminants is most probably accounted for by the immediate and direct provision of blood fromr the vertebral arteries through the anastomotic vessels (Baldwin, 196; Baldwin & Bell, 1963a). Occlusion of both carotids in the sheep and ox evoked falls in pressure in both lingual arteries of equal magnitude and of a similar order to those reported in other species, dog and cat (Schmidt, 1932), dog, cat and rabbit (Chungcharoen et al. 1952), cat (Holmes & Wolstencroft, 1959), and corroborates results obtained recently in the sheep (Waites, 196). In the sheep and ox pressure in the carotid sinus region can be reduced stepwise almost to zero by successive occlusion of the common carotid, the occipital arteries and the external carotid. This manoeuvre is without effect on the pressure of the contralateral lingual artery. During these procedures, however, the blood flow in the contralateral carotid and in both vertebral arteries would show considerable increase, sufficient in fact to prevent any marked reduction in the cephalic pressure or rise in the level of the systemic pressure (see Baldwin & Bell, 1963b). When both carotids and both occipitals are occluded in the sheep, the cephalic pressure as reflected in the lingual pressure falls to some 9 % of the resting pressure, but within 15sec of occlusion the systemic blood pressure shows a dramatic and rapid increase. When the electroencephalogram is recorded during such an experiment, electrical activity of the brain is completely abolished from the time the systemic hypertension develops, due to the cerebral ischaemia (Baldwin & Bell, 1963c). In this regard the sheep differs completely from other species, the physiological phenomenon being a reflexion of the anatomical differences between this species and other animals, for in the sheep the vertebral arteries do not form part of the cephalic vascular supply (Baldwin, 196). In the dog, cat, man and horse cerebral ischaemia could not occur on clamping the carotid and occipital

14 476 B. A. BALDWIN AND F. R. BELL arteries, because blood would flow directly to the brain from the vertebrals via the basilar artery. The ox is like the sheep in that the vertebral arteries do not communicate directly with the basilar artery, but differs radically from the sheep because the vertebrals communicate directly with the rete mirabile and then with the circle of Willis by means of the basi-occipital plexus (Baldwin, 196; Baldwin & Bell, 1963a). The goat has been shown to have a similar cephalic blood supply as the sheep (Anderson & Jewell, 1956) and presumably would react similarly to the sheep on induction of cerebral ischaemia. That the systemic hypertension occurring in the sheep on occlusion of the carotids and occipitals results from cerebral ischaemia and not from reflexes initiated by baroreceptors situated in the arterial tree is shown by the results of other experiments reported here. When the external carotids in the sheep are occluded distal to the lingual arteries a marked systemic hypertension develops quickly, but in this instance the pressure in the lingual arteries rises as it follows the rise of pressure which develops in the systemic pressure. Since the systemic hypertension occurs whether the intrasinusal pressure is raised or lowered it is unlikely that the baroreceptors of this region are the immediate cause. Furthermore, the systemic hypertension can be evoked after local anaesthesia of the reflexogenic areas. Occlusion of the occipitals and carotid arteries in the ox does not cause systemic hypertension as it does in the sheep. With this manoeuvre in the ox cerebral anoxia does not develop, because a marked increase in blood flow occurs in the vertebral arteries and passes to the circle of Willis through the basi-occipital plexus and thence to the cerebral hemispheres (Baldwin & Bell, 1963a). When, however, the vertebrals as well as the carotids are occluded in the ox then, as in the sheep, cerebral anoxia develops and becomes manifest as a considerable rise in systemic pressure. The rise in systemic pressure is, however, usually less in the ox than in the sheep and is more marked in some individuals than others, so that reaction of any individual animal probably depends upon the degree of efficiency of the central spinal artery in the ox in allowing blood to pass to the posterior brain stem (Baldwin & Bell, 1963b). The experimental results in the sheep and ox support the view that the cerebral circulation is not greatly influenced by changes in vasomotor activity (Bouckaert & Jourdan, 1949; Schmidt, 195). In these ruminants the correlation of cephalic pressure with blood flow strongly suggests that the increase in flow is the direct result of a fall in cephalic pressure which must occur when the area is rendered temporarily ischaemic. Our results lend support, therefore, to the view expressed by Bouckaert & Heymans (1935) that the haemodynamics of the cerebral circulation are governed by physical mechanisms and not by baroreceptor reflexes. Within the

15 CEPHALIC BLOOD PRESSURE IN RUMINANTS 477 cephalic circulation itself, however, it is possible that local areas, such as the hypothalamus and parietal cortex, may show local control of blood flow by vasoconstriction mediated by the sympathetic fibres, known to be present (Schmidt & Hendrix, 1938). De Boissezon (1941) concluded on anatomical grounds that because ruminants do not have a morphological carotid sinus therefore carotid sinus reflexes must be absent. Our results, however, confirm the experimental findings of Waites (196) that carotid sinus reflexes are demonstrable in the sheep, especially when both carotid arteries are occluded. In the ox, however, carotid sinus reflexes are much more difficult to elicit, probably because the potential diminution in intrasinusal pressure on carotid occlusion is counteracted by the increased blood flow which occurs in the large vertebral arteries and which passes to the sinus region, retrogradely, by the occipital arteries. Prolonged occlusion of the carotid and vertebral arteries in the sheep will cause death in an average period of 2 min. During this interval the sheep show periodic breathing associated with large fluctuations in systemic blood pressure. Linzell & Waites (1957) reported that sheep with both carotid and vertebral arteries ligated survived at least an hour and that medullary function failed only after additional ligation of one occipital artery. The much shorter survival time of our animals may be explained either by the depressant effect of anaesthesia or possibly by anatomical variations in the arterial anastomosis around the atlanto-occipital joint. In the calf occlusion of both carotids and both vertebral arteries resulted in only a minor degree of systemic hypertension and increased depth of respiration. To produce cerebral ischaemia in the calf, in addition to occlusion of the carotids and vertebral arteries the many large muscular branches of the subclavian arteries must be ligated as well as the ventral spinal artery. We have shown in numerous experiments that the calf can survive for many hours when the brain receives blood only from the ventral spinal artery or from the cervical branches of the subelavian arteries. On the other hand, the marked difference in resistance shown by the sheep and calf to prolonged ischaemia may be due to the relative immaturity of the calves used in these experiments, for it has been shown that the medullary centres of neonatal animals are more resistant to the effects of anoxia than adult animals (Kabat, 191). SUMMARY 1. The changes which occur in cephalic and systemic blood pressure on clamping some of the arteries concerned in cephalic blood supply have been examined in the sheep and the ox.

16 478 B. A. BALDWIN AND F. R. BELL 2. In both species clamping one carotid artery causes a fall of blood pressure only in the ipsilateral lingual artery, which is a reflexion of a similar fall in the cephalic pressure. 3. Clamping both carotid arteries at the same time results in a more severe fall in cephalic pressure than when one artery only is occluded and is seen as an identical reduction in both lingual arteries. 4. In both species bilateral clamping of the vertebral arteries has no effect on the cephalic or systemic blood pressure. 5. Because the vertebral arteries in the sheep do not provide blood to the circle of Willis, clamping the occipital and carotid arteries produces complete cerebral ischaemia. The induction of cerebral ischaemia by this manoeuvre in sheep results in a profound fall in cephalic blood pressure but a marked systemic hypertension. Death occurs if the clamps are left in position. 6. Bilateral clamping of the external carotid arteries or combined clamping of the carotid and vertebral arteries also causes a profound fall in cephalic blood pressure but a marked and sustained rise in systemic pressure. 7. By contrast, in the calf the presence of an arterial connexion between the vertebral arteries and the rete mirabile, namely the basi-occipital plexus, prevents the development of cerebral ischaemia when the external carotids are clamped or when the carotids and occipitals are clamped at the same time. 8. In the calf in order to induce cerebral ischaemia the vertebrals, carotids, the inferior cervical branches of the subclavian and the ventral spinal artery must all be ligated. Under these circumstances the extreme systemic hypertension indicative of cerebral ischaemia will develop and death will ensue. 9. The experimental results in the sheep and calf support the view of Bouckaert & Heymans (1935) that the haemodynamics of the cerebral circulation are governed by physical mechanisms rather than by baroreceptor reflexes. 1. When the brain of the sheep is partially deprived of blood periodic breathing develops associated with large fluctuations in systemic blood system. Limited experiments suggest that the periodicity is induced by ischaemia of the medulla rather than anoxaemia of peripheral receptors. We are grateful to the Animal Health Trust for generous financial support. REFERENCES ANDER5SON, B. & JEWELL, P. (1956). The distribution of carotid and vertebral blood in the brain and spinal cord of the goat. Quart. J. exp. Physiol. 41, BALDWIN, B. A. (196). The correlation between the vascular supply of the brain and cerebral function in ruminants. Ph.D. thesis, University of London.

17 CEPHALIC BLOOD PRESSURE IN RUMINANTS 479 BALDWIN, B. A. & BELL, F. R. (1963a). The anatomy of the cerebral circulation of the sheep and goat. The dynamic distribution of the blood supplied by the carotid and vertebral arteries to the cranial regions. J. Anat., Lond., (in the Press). BALDWIN, B. A. & BELL., F. R. (1963b) Blood flow in the carotid and vertebral arteries of the sheep and calf. J. Physiol. 167, BALDWIN, B. A. & BELL, F. R. (1963c). The effect of temporary reduction in cephalic blood flow on the spontaneous electrical activity of the cerebral cortex in the sheep and calf. Electroenceph. clin. Neurophy8iol. (in the Press). DE BOISSEZON, P. (1941). Le r6seau admirable arteriel intracranien de I'agneau. Bull. Soc. d'hist. nat. Toulouse, 76, BOUCKAERT, J. J. & HEYMANS, C. (1935). On the reflex regulation of the cerebral blood flow and the cerebral vaso-motor tone. J. Physiol. 84, BOUCKAERT, J. J. & JOURDAN, F. (1949). La circulation cerebrale. J. Physiol. Path. ge'n. 41, 69A-114A. CHUNGCHAROEN, D., DALY, M. DE B., NEIL, E. & SCHWEITZER, A. (1952). The effect of carotid occlusion upon the intra-sinusal pressure with special reference to vascular communications between the carotid and vertebral circulations in the dog, cat and rabbit. J. Physiol. 117, HEYMANS, C. & NEIL, E. (1958). Reflexogenic Areas of the Cardiovascular System. Churchill: London. HOLMES, R. L. & WOLSTENCROFT, J. H. (1959). Accessory sources of blood supply to the brain of the cat. J. Physiol. 148, KABAT, H. (194). The greater resistance of very young animals to arrest of the brain circulation. Amer. J. Physiol. 13, LINZELL, J. & WAITEs, G. (1957). Effects of occluding the carotid and vertebral arteries in sheep and goats. J. Physiol. 138, 2P. PYPER, J. G. (1938). The 'carotid sinus' region in relation to the respiration of the goat. M.D. thesis: The Queen's University of Belfast. SCHMIDT, C. (1932). Carotid sinus reflexes to the respiratory center. I. Indentification. Amer. J. Physiol. 12, SCHMIDT, C. (195). The Cerebral Circulation in Health and Disease. Illinois: C. C. Thomas. SCHMIDT, C. F. & HENDRIX, J. P. (1938). The action of chemical substances on cerebral blood vessels. Res. Publ. Ass. nerv. ment. Dis. 18, VON EULER, U. S. & LILJESTRAND, C. (1936). Chemical stimulation of the carotid sinus and the regulation of respiration. Skand. Arch. Physiol. 74, WAITES, G. M. H. (196). The influence of the occipito-vertebral anastomosis on the carotid sinus reflex of the sheep. Quart. J. exp. Physiol. 45,