of the intercostal muscles and a suggested nomenclature for the

Transcription

1 15 J. Phy8iol. (1964), 172, pp With 2 pltes nd 4 text-figures Printed in Gret Britin THE FIBRE CALIBRE SPECTRA OF SENSORY AND MOTOR FIBRES IN THE INTERCOSTAL NERVES OF THE CAT By T. A. SEARS* From the Deprtment of Physiology, The Austrlin Ntionl University, Cnberr, Austrli (Received 27 Jnury 1964) There pper to be no published fibre-clibre spectr of sensory nd motor fibres in the intercostl nerves of the ct. Such dt were needed for correltion with the results of recent physiologicl investigtions on the thorcic spinl cord (Eccles, Sers & Shely, 1962; Sers, 1963). Accordingly, ct intercostl nerves hve been de-fferented by gnglionectomy nd de-efferented by sectioning the ventrl roots so tht the fibre-clibre spectr of motor nd sensory fibres could be determined. The results of these investigtions together with brief ccount of the innervtion of the intercostl muscles nd suggested nomenclture for the relevnt nerves, re described in the present pper. METHODS Opertions. The opertions to de-fferent nd to de-efferent the intercostl nerves were performed septiclly on four cts under sodium pentobrbitl nesthesi. After the vertebrl column ws exposed, hemilminectomy of the relevnt vertebre ws performed; ll subsequent dissections on the spinl roots were mde with the ssistnce of binoculr stereo microscope. The de-fferenttions were difficult to chieve. This is becuse distl to the gnglion some of the dorsl root fibres immeditely interdigitte with the fibres of the ventrl root mking it virtully impossible to remove the gnglion completely without dmging some of the ventrl root fibres. The technique employed ws first to divide the dorsl roots 2-3 mm centrl to the gnglion. The dur round the peripherl stump of the dorsl root nd the gnglion ws opened widely so tht the gnglion could be lifted upwrds with toothed forceps. A fine glss hook ws then used gently to tese the dorsl root distl to the gnglion wy from the ventrl root so tht the entire gnglion could be removed. Some dmge to ventrl root fibres must hve resulted (presumbly in fibres of ll sizes), but this procedure ws preferred to the lterntive possibility of leving some gnglion cells intct (cf. Eccles & Sherrington, 193). The completeness of the de-fferenttion ws checked histologiclly s described below. De-efferenttion of the intercostl nerves ws chieved by removing 2-3 mm of the ventrl root nd this ws done between ligtures in order to prevent the troublesome bleeding tht otherwise occurred. Since the intercostl nerves hd to be removed for histology reltively close to the spinl cord, the possibility * Visiting Wellcome Reserch Fellow, on leve of bsence from the Institute of Neurology, The Ntionl Hospitl, Queen Squre, London, W.C. 1.

2 DIAMETERS OF INTERCOSTAL NERVE FIBRES 151 tht nerve fibres injured t the time of the first opertion might regenerte into the intercostl nerves hd to be considered. Regenertion from dorsl root gnglion cells ws not problem since no gnglion cells were left in the preprtion from which the fibre-clibre spectr for motor fibres were prepred. However, s mentioned bove, some injury to ventrl root fibres ws inevitble. In their work on limb muscle nerves Eccles & Sherrington (193) nd Boyd & Dvey (1961) used survivl times rnging from 35 to 58 dys. In the present investigtion the survivl time ws 28 dys which ws compromise between the possible inclusion of smll regenerting fibres nd the need to wit long enough for ll the myelin debris to dispper. As further precution the intercostl nerves were removed for histology s fr distl s possible s ws comptible with the informtion required. Histology. The cts were nesthetized nd the nerves removed nd ttched under light tension to frmes prepred from crd. The nerves were stored in freshly prepred Flemming's fluid for 24 hr nd stined by the modified Weigertfs stin (the Flemming- Wolter technique) ccording to the detiled method described by Willims & Wendell- Smith (196). In order to check if de-fferenttion ws complete, the ventrl spinl rmus, the ventrl root nd distl stump of dorsl root were removed nd seril, 2 y prffin sections prepred for stining with Cresyl Violet. Every section ws exmined nd counts mde of the number of gnglion cell bodies present. In one segment only of ech of two nimls ws de-fferenttion found to be complete. The nerves were well stined nd nondistorted, nd sections of them (from the 8th thorcic segment) provide the photogrphs nd histogrms shown. Unfortuntely, the sections of the contrlterl nerves, which were to be used s controls, proved unsuitble for the construction of fibre-clibre spectr, lthough sufficient evidence ws obtined from them to show tht there ws nothing bnorml in the fibre complement of the nerves of these nimls. The control nerves were therefore tken from the sme segment in nother niml. The mesurements of fibre dimeter were mde from enlrgements t 5 dimeters from photogrphs tken with Leitz Pnphot pprtus. All the photogrphs nd prints together with enlrgements of micrometer scle were mde in one btch. Mesurements of the dimeters of the lrgest fibres mde from the photogrphs were checked ginst their direct visul imge in the microscope with n occulr micrometer. The externl dimeters of the fibres were mesured in groups of 2 / by mens of thin Perspex sheet on which were engrved series of circles of dimeter 5 x 2 t, 5 x 4 p, etc. RESULTS Antomy Text-figure 1 illustrtes the principl ntomicl fetures of lower thorcic segment (T. 6-T. 11) including the innervtion of the bdominl wll. The innervtion of the intercostl muscles of the upper thorcic segments is similr. The ventrl spinl rmus divides into two brnches. The lrger of these (), known generlly s 'the intercostl nerve', is here described s 'the internl intercostl nerve'. It gives off regulrly throughout its length fine filments ech of which innervtes sector of the internl intercostl muscle (b). The sme filments lso innervte n extremely thin lyer of muscle which often forms discontinuous sheet deep to the internl intercostl muscle. This muscle (not illustrted) ppers to be homologous with the 'intrcostlis' muscle described by Wlmsley (1916); it is referred to in some textbooks s 'the inner lyer of the internl intercostl muscle' (e.g. Grnt, 1947). A lterl brnch of the internl intercostl nerve (c) innervtes the bdominl oblique muscle (d) through one

3 152 T. A. SEARS division nd through two others, the lterl nd ventro-lterl skin (e). Ventrlly, the internl intercostl nerve provides brnches which innervte the rectus bdominis muscle (f) nd the ventrl skin (g). Becuse the internl intercostl, the bdominl oblique nd rectus bdominis muscles re expirtory muscles, the internl intercostl nerve thus crries Text-fig. 1. Schemtic digrm of the ntomy of the intercostl nerves reproduced from the rticle in Nture by Eccles, Sers & Shely (1962)., internl intercostl nerve; b, internl intercostl muscle; c, lterl intercostl nerve (note the brnch innervting the bdominl oblique muscle); d, bdominl oblique muscle; e, cutneous brnches of lterl intercostl nerve; f, rectus bdominis muscle; g, brnches of the internl intercostl nerve innervting the ventrl skin; h, externl intercostl nerve; i, externl intercostl muscle; j, musculr brnch of posterior spinl rmus; k, cutneous brnches of posterior spinl rmus. the xons of ll the expirtory motoneurones whose outflow is in the ventrl spinl rmus. Close to its origin, the smller brnch of the ventrl rmus (h), described here s the 'externl intercostl nerve', penetrtes the internl intercostl muscle nd psses lterlly in the intercostl spce in position immeditely cudl to the rib giving off filments, ech of which innervtes sector of the externl intercostl muscle (i). Distl to the ngle of the rib

4 DIAMETERS OF INTERCOSTAL NERVE FIBRES 153 the externl intercostl nerve crosses the lterl intercostl nerve nd completes its distl course in the middle of the intercostl spce to terminte just short of the ventrl limit of the externl intercostl muscle. As the externl intercostl muscle is inspirtory in function, the externl intercostl nerve t its origin crries the xons of ll the inspirtory motoneurones whose outflow is in the ventrl spinl rmus. Histology A few explntory comments re necessry. Owing to the cutneous fibres entering in its lterl nd ventrl brnches, the internl intercostl nerve is mixed throughout its entire length. The lterl brnch is lso mixed until it divides to provide the brnch innervting the bdominl oblique muscle (Text-fig. 2, A.O.) nd usully two cutneous brnches (Text-fig. 2, C). No cutneous fibres re present in the filments of the internl intercostl nerve which innervte the internl intercostl muscle, or in the externl intercostl nerve nd its filments which innervte the externl intercostl muscle, so these nerves re nlogous to the muscle nerves of the limbs. Norml nerves. The norml internl nd externl intercostl nerves contin myelinted fibres rnging from 2 to 2,u in dimeter with usully few still lrger fibres being present s in Text-fig. 2A, B nd D. Photomicrogrphs of the nerves re given in P1. 1 (norml). These vlues cover the entire rnge of dimeters described for motor (Eccles & Sherrington, 193; Rexed & Thermn, 1948; Boyd & Dvey, 1961) nd sensory fibres (Lloyd & Chng, 1948; Rexed & Thermn, 1948) of vrious muscle nerves of the ct hind-limb, nd of limb cutneous fibres (e.g. Gsser & Grundfest, 1939; see lso the dimeters predicted from conduction velocity mesurements on cutneous fibres of the ct by Hunt & McIntyre, 196). The distribution is bi-modl but less mrkedly thn tht of the limb muscle nerves due principlly to the presence of the cutneous fibres. When the internl intercostl nerve divides, the lterl brnch (Text-fig. 2, B 2; P1. 1, norml, B 2) tkes the greter shre of the smll, nd the internl intercostl nerve (Text-fig. 2, B 1), tkes the greter shre of the lrge dimeter fibres. The externl intercostl nerve is lso bi-modl but the lrge fibre pek occurs t 1-12 p, compred with the 12-14,u pek of the internl intercostl nerve. De-fferented nerves. The distribution of motor fibres in the internl intercostl nerve (T. 8) is mrkedly bi-modl (Text-fig. 3A). The lrge dimeter fibres rnge from 1 to 2, (2 fibres re greter thn 2 p) of which 21 % re over 16, nd in which the 16-18, group re s numerous s ny others. The other, completely de-fferented internl intercostl nerve (T. 7), referred to on p. 151, similrly shows fibres rnging from 1

5 154 T. A. SEARS to 2, nd there is substntil number in the rnge 16-2,u. The size nd distribution of these lrge dimeter motor fibres re prlleled in limb muscle nerves only by the fibres of the semitendonosus nerve (Eccles & Sherrington, 193) nd of the popliteus nerve (Boyd & Dvey, 1961), nd by the fibres of the ventrl roots described by both uthors. The section of the de-fferented intercostl nerve (A) shown in P1. 1, A M.1. V. E z D E.I. 5 'R.A Text-fig. 2. On the right, histogrms of the exteml dimeters of myelinted fibres in norml intercostl nerves (T. 8) stined by the Flemming-Wolter method. Abscisse, externl dimeter of fibres in groups of -2 Is, 2-4 p, etc.; ordintes, number of fibres (totl counts). The dotted lines A, B, C nd D in the digrm on the left indicte the pproximte levels of the trnsverse histologicl sections from which the mesurements of fibre dimeter were mde. Two histogrms re shown t the B level. The full line (B 1) is the histogrm of the inteml intercostl nerve distl to the emergence of the lterl intercostl nerve; the interrupted line (B 2) is the histogrm of the lterl intercostl nerve proximl to its division into one musculr nd two cutneous divisions.

6 DIAMETERS OF INTERCOSTAL NERVE FIBRES illustrtes the segregtion of the motor fibres which occurs in the internl intercostl nerve before the formtion of the lterl intercostl nerve. This lterl intercostl nerve (P1. 1, de-fferented B2) contins pproximtely 7 fibres, lso segregted, which more distlly mke up the nerve lo rfa 7n De-fferented z Text-fig. 3. Sme s in Text-fig. 2 but from different ct. The nerves (T. 8) were de-fferented by section of the dorsl roots distl to the dorsl root gnglion 28 dys previously. C 1, musculr brnch of the lterl intercostl nerve innervting the bdominl oblique muscle. 155 which innervtes the bdominl oblique muscle (P1. 1, de-fferented C1). The two cutneous divisions of the lterl intercostl nerve (P1. 1 defferented C2 nd C3) show complete bsence of fibres thus providing proof dditionl to tht described under Methods, of the completeness of the de-fferenttion. The few, drkly stined rings re blood vessels, which re more numerous in these de-fferented nerves.

7 156 T. A. SEARS The lrge dimeter motor fibres of the externl intercostl nerve lso show wide rnge of dimeters with well-defined pek t 1-12 p, nd subsidiry pek t 14-16,u (Text-fig. 3D). These vlues my be compred to those of the internl nd lterl intercostl nerves which show peks t 16-18, nd p respectively. Both the externl nd internl intercostl nerves contin smll dimeter motor fibres with well-defined pek t 4-6,u. However, the presence of fibres of intermedite size, especilly in the externl intercostl nerve, mkes the division between the groups of lrge nd smll dimeter fibres less prominent thn in the limb muscle nerves. The group of smll dimeter fibres is mde up from thickly nd thinly myelinted fibres, the dimeter of the ltter tending to be greter, s ws described for limb muscle nerves by Boyd & Dvey (1961). However, this difference ws not ssocited with ny conspicuous grouping within the fibre-clibre spectrum of the smll dimeter fibres. It my be noted tht not ll the hind limb nerves displyed two distinct groups of smll dimeter fibres even though the grouping into thinly nd thickly myelinted fibres ws clerly pprent (Boyd & Dvey, 1961). De-efferented nerves. The histogrms of Text-fig. 4A nd D, re from T. 8 of the sme niml, nd tht of Text-fig. 4C is from T. 9 of different niml. Proximlly, the fferent fibres of the internl intercostl nerve show prominent pek of 4-6 p, nd brod distribution of the lrger fibres rnging from 8 to 2 It (Text-fig. 4A). There is no tri-modl distribution comprble to tht of the limb muscle nerves (Lloyd & Chng, 1948), difference which could be due to the presence of cutneous fibres in the intercostl nerves. Text-figure 4C shows the distribution of fferent fibres in the cutneous (C2 + C 3) nd musculr (C 1; note the different ordinte scle) divisions of the lterl intercostl nerve. Wheres the cutneous divisions contin comprtively few fibres over 14,u the musculr division hs prominent u pek, nd of its fibres greter thn 12, in dimeter, 25 % re over 16,u. The externl intercostl nerve shows mny fferent fibres of intermedite size with pek t 8-1,u (Text-fig. 4D). However, the lrgest fibres extend into the sme upper rnge of dimeters, s re present in the musculr division of the lterl intercostl nerve. Intercostl nerve filments. Exmples of the filments which innervte the intercostl muscles re shown in P1. 1 (designted by the letter f), nd t higher mgnifiction in P1. 2. Also shown in P1. 2 is norml filment (Fig. A) nd de-fferented filment (Fig. D) from T. 7 of the other preprtion referred to on p. 153 in which de-fferenttion ws complete. The norml filments contin 3-6 myelinted fibres whose dimeters

8 DIAMETERS OF INTERCOSTAL NERVE FIBRES 157 extend over the full rnge of 2-2, chrcteristic of the prent nerves. Figure A of P1. 2 is of interest, since this filment contins n unusully lrge number of fibres whose dimeters re 2 it or bove, feture which is shred by the prent nerve, sector of which is present in the upper , 15 U 1 Z Text-fig. 4. Sme s in Text-fig. 2. The nerves were de-efferented by section of the ventrl roots 28 dys previously. C l, musculr division, C 2 nd C 3, cutneous divisions of the lterl intercostl nerve. The histogrms shown in A nd D re from T. 8 of the sme niml, nd those of C re from T. 9 of different niml. prt of the photogrph. The dimeters of the motor fibres in the defferented filment from the sme niml lso ttin high vlue (2,u) but none is quite so lrge s the lrgest fibres (fferent fibres) present in the norml filment.

9 158 T. A. SEARS The de-fferented filments, like their prent nerves, lso contin smll dimeter fibres. These re best seen in P1. 2 F where they re prticulrly numerous. DISCUSSION The min points of comprison between the fibre-clibre spectr of intercostl nd limb nerves hve lredy been stted, so tht only few comments re mde below. Any trnsverse section of the externl or internl intercostl nerves will include some fibres bout to leve the nerve in filment which innervtes the djcent sector of intercostl muscle, nd fibres pssing to prts of the sme muscle which could be up to 8-1 cm distlly. This proximl versus distl distribution of the fibres my be ssocited with tendency for the distlly directed fibres to be of smller dimeter s described for limb muscle nerves by Fernnd & Young (1951). Nerve fibres bout to leve the prent trunk my lso undergo splitting before tking n intrmusculr course not more thn 15 cm wy (cf. Eccles & Sherrington, 193). Both these fctors, nd the fct tht the internl intercostl nerve contins, in ddition, fibres innervting the lterl nd ventrl bdominl wll, re probbly responsible for the brod rnge of fibre dimeters present in the intercostl nerves. However, the ntomicl complexities of the innervtion mke it difficult to mke quntittive estimtion of the reltive importnce of ech of these fctors. A comprison of the totl number of motor nd sensory fibres in the de-fferented nd de-efferented nerves to the totl number of fibres in the norml internl intercostl nerve, mesured t comprble proximl levels, shows tht pproximtely 3 % of the fibres re motor nd 45 % re sensory. Although the deficiency of 25 % could represent individul differences, prticulrly s these nerves re from different nimls, it lmost certinly reflects the dmge to motor fibres which occurred t the time of de-fferenttion (see Methods). This proportion of sensory to motor fibres should be contrsted to the 1 % of sensory fibres in the ct phrenic nerve, of which only 1 % re bove 12,u ccording to Hinsey, Hre & Phillips (1939). In the cutneous brnches of the lterl intercostl nerve only two fibres exceeded 16, in dimeter, wheres the prent internl intercostl nerve nd its musculr brnches contined 56 fibres rnging from 16 to 2 P. These lrge dimeter fibres must include primry fferent fibres innervting the muscle spindles known to be present in intercostl muscles (Huber, 192). In the norml intercostl nerve it is probbly sfe to ssume tht fferent fibres over 16, in dimeter re not cutneous fibres. But these lrge dimeter sensory fibres cnnot t present be distinguished histo-

10 DIAMETERS OF INTERCOSTAL NERVE FIBRES 159 logiclly from the numerous motor fibres of comprble dimeter. Be this s it my, their electricl thresholds differ to the extent tht, on stimultion of the intercostl nerves, ner-mximl excittory post-synptic potentil (EPSP, Brock, Coombs & Eccles, 1952) is evoked in thorcic motoneurones, due to primry fferent fibre excittion, (Eccles et l. 1962) t stimulus intensity which excites few motor fibres. The dimeters of the motor fibres in the internl nd externl intercostl nerves show bi-modl distribution. This distribution into lrge nd smll dimeter fibres persists in the filments innervting tie internl nd externl intercostl muscles, nd lso in the musculr brnch innervting the bdominl oblique muscle. Histologiclly the filments re uniform in tht they lwys contin lrge nd smll dimeter fibres, lthough the reltive numbers of ech vry considerbly. This vrition must be relted to the number of muscle spindles present in the sector of intercostl muscle, nd lso to the complexity of the muscle spindles with regrd both to their efferent nd fferent innervtion (Boyd, 1961, 1962). Huber (192) reported tht the upper six intercostl spces of the ct contin 6-1 muscle spindles lthough his figure for 'sensory endings' my include the multiple secondry endings of complex spindles. Confirmtion tht muscle spindles re indeed numerous in the intercostl muscles hs been obtined. In seril, trnsverse sections of the internl nd externl intercostl muscles (tken seprtely from the 4th intercostl spce) I hve found tht ech block of tissue pproximtely 1 cm wide contins t lest one or s mny s four independent muscle spindles. This bundnce of intercostl muscle spindles nd their ssocited smll dimeter fibres ccounts for the regulrity with which fusimotor neurone dischrges my be recorded from intercostl nerve filments (Sers, 1962, 1963). In conclusion it is emphsized tht there re likely to be differences in the fibre-clibre spectr of the intercostl nerves t different levels of the chest, ccording to the reltive mounts of intercostl nd bdominl muscle innervted t ech segmentl level, nd ccording to the regionl distribution of the muscle spindles. A knowledge of these fctors is clerly essentil to wider understnding of the proprioceptive regultion of respirtory movements. SUMMARY 1. The fibre-clibre spectr of motor nd sensory fibres in norml, de-fferented nd de-efferented intercostl nerves hve been determined in the ct. 2. The internl nd externl intercostl nerves both contin smll nd lrge dimeter motor fibres rnging from 2 to 2,. Although the fibreclibre spectrum of these fibres is bi-modl, the division between the groups

11 16 T. A. SEARS is less prominent thn in limb muscle nerves. The grouping into lrge nd smll fibres is lso present in the nerve filments innervting the internl nd externl intercostl muscles. 3. The lrgest dimeter sensory fibres of the intercostl nerves re contined in their musculr brnches. 4. A brief ccount of the ntomicl distribution of the ct intercostl nerves is given nd nomenclture for them is introduced. I m indebted to Sir John Eccles, F.R.S. for his encourgement nd dvice. I wish to thnk Mrs. M. Oshim for the histologicl preprtions nd Mr V. Prl for photogrphy. The publishers of Nture re thnked for their permission to reproduce Text-fig. 1. This work ws done during the tenure of Weilcome Reserch Fellowship. REFERENCES BOYD, I. A. (1961). The motor innervtion of mmmlin muscle spindles. J. Physiol. 159, 7-9 P. BOYD, I. A. (1962). The structure nd innervtion of the nucler bg muscle fibre system nd the nucler chin muscle fibre system in mmmlin muscle spindles. Phil. Trns. B, 245, BOYD, I. A. & DAVEY, M. R. (1961). The groups of origin in the nerves to skeletl muscle of the V1 nd V2 fusimotor fibres present close to nd within, mmmlin muscle spindles. In Symposium on Muscle Receptors, ed. BARKER, D., pp Hong Kong: University Press. BROCK, L. G., CooMBs, J. S. & EccLEs, J. C. (1952). The recording of potentils from motoneurones with n intr-cellulr electrode. J. Physiol. 117, Eccixs, J. C. & SHERRINGTON, C. S. (193). Numbers nd contrction vlues of individul motor units in some muscles of the limb. Proc. Roy. Soc. B, 16, ECCLES, R. M., SEARS, T. A. & SHEALY, C. N. (1962). Intr-cellulr recording from respirtory motoneurones of the thorcic spinl cord of the ct. Nture, Lond., 193, FERNAND, V. S. V. & YOUNG, J. Z. (1951). The sizes of the nerve fibres of muscle nerves. Proc. Roy. Soc. B, 139, GASsER, H. S. & GRUNDFEST, H. (1939). Axon dimeters in reltion to the spike dimensions nd the conduction velocity in mmmlin A fibres. Amer. J. Physiol. 127, GRANT, J. C. B. (1947). An Atls of Antomy, Fig. 365, p London: Billiere, Tindll nd Cox. HINSEY, J. C., HARE, K. & PHILLIPS, R. A. (1939). Sensory components of the phrenic nerve of the ct. Proc. Soc. exp. Biol., N.Y., 41, HUBER, G. C. (192). Neuro-musculr spindles in the intercostl muscles of the ct. Amer. J. Ant. 1, HuNT, C. C. & MCINTYRE, A. K. (196). An nlysis of fibre dimeter nd receptor chrcteristics of myelinted cutneous fferents in cts. J. Physiol. 153, LLOYD, D. P. C. & CHANG, H.-T. (1948). Afferent fibres in muscle nerves. J. Neurophysiol. 11, REXED, B. & THERMAN, P.. (1948). Clibre spectr of motor nd sensory nerve fibres to flexor nd extensor muscles. J. Neurophysiol. 11, SEARS, T. A. (1962). The ctivity of the smll motor fibre system innervting respirtory muscles of the ct. Aust. J. Sci. 25, 12. SEARS, T. A. (1963). Activity of fusimotor fibres innervting muscle spindles in the intercostl muscles of the ct. Nture, Lond., 197, WALMSLEY, T. (1916). The costl musculture. J. Ant., Lond., 5, WILLiAMs, P. L. & WENDELL-SMITH, C. P. (196). The use of fixed nd stined sections in quntittive studies of peripherl nerve. Qurt. J. micr. Sci. 11,

12 The Journl of Phy8iology, Vol. 172, No. I Plte 1 ~~~~~~~~~~4!.,:I w _;I:_:i '"S..t_...s. _i. l _ -~~~~~~~~~~~~~~~~~~~~~~~~~ Norml w'.. d-t- D.- De(Fieng..16I T. A. SEARS (Flciilg p. 16)

13 The Journl of Physiology, Vol. 172, No. 1 Plte 2 BR' >4', o - -S.) -S - lob'- T. A. SEARS (Fcitng p. 161)

14 DIAMETERS OF INTERCOSTAL NERVE FIBRES 161 EXPLANATION OF PLATES PLATE 1 Photomicrogrphs of norml, de-fferented nd de-efferented intercostl nerves stined by the Flemming-Wolter method. A, B, C nd D refer to the levels indicted in Text-fig. 2. B1, B2, C1, C2 nd C3 re s in Text-figs. 2, 3 nd 4. Clibrtion scle 1 I&. PLATE 2 Photomicrogrphs of intercostl nerve filments stined by the Flemming-Wolter method. Figs. A, B nd C, norml filments. Figs. D, E nd F, de-fferented filments.