AUTHOR ABSTRACT OF THIS PAPER ISSUED F? THE DIDLIOGRAPEOC SERVICE, JANIIARY 29 Reprinted from TeE JOURNAL OP COMPARATIVE NED/tOLOG I", Vol 35, No, 2, February, 1923 ORIGIN AND DISTRIBUTION OF THE TRACTUS SOLITARIUS IN THE GUINEA PIG WILLIAM F. ALLEN Department of Anatomy of the University of Oregon Medical School, Portland, Oregon :NINETEEN FIGURES CONTENTS Introductory 171 Review of the literature 172 Material and methods 173 Operations 174 Microscopical technique 176 Tractus solitarius (Pars nervi facialis) 177 Tractus solitarius (Pars nervi glossopharyngei) 184 Tractus solitarius (Pars nervi vagi) 188 Tractus solitarius proper 192 Summary and general considerations 198 Literature cited 202 INTRODUCTORY Several years ago the writer began a series of experiments and studies on the guinea pig brain to determine if perchance the central connections of the general visceral and gustatory systems in mammals were comparable to those found by Herrick and Johnston in fishes. In brief, Herrick description of the gustatory system of the carp, catfish and Amblystoma may be summarized as follows: Both gustatory and general visceral sensory impulses are con- ducted by the VII, IX and X nerves to facial and vagal lobes of the medulla. From these primary centers two descending secondary gustatory tracts follow the spinal V tract to the region of the commissural nuclei and two ascending secondary gustatory tracts accompany the spinal V tracts to two superior secondary gustatory nuclei, situated in the isthmus, ventrally to the line 171 172 WILLIAM F. ALLEN of fusion of the valvula cerebelli and the cerebellum. The principal tertiary fibers arising from the superior nuclei pass to the inferior lobes. Herrick notes also the possibility of a relay to lower centers by way of the tractus lobo-bulbaris. Johnston in a number of papers and in his book describes a tractus lobo- epistriaticus or tractus palli connecting the hypothalamus (inferior lobes) with the telencephalon. On p. 174 of his book Johnston figures visceral fibers from the spinal cord entering the secondary visceral tract. My own studies on this system in mammals, based largely on Marchi preparations, will be reported in three parts. The results of the first phase of the subject will be incorporated in this paper and will include a description of the distribution of the primary gustatory and general visceral fibers in the medulla of the guinea pig. This will be followed by a description of the course of the secondary visceral fibers taking origin from the primary visceral nucleus, namely, the nucleus tractus solitarii, and thirdly by a study of the basal cerebellar nuclei, emboliformis and globosus with a view of determining their relationships. REVIEW OF LITERATURE Complete summaries of the early work on the tractus solitarius and its components have been given by Koelliker and Van Gehuchten. As a result of these studies, dating from Stilling to Koelliker the tractus solitarius was found to be a continuous tract extending from the entrance of the VII nerve to the decussa- tion of the pyramids, and to be made up of sensory fibers from the VII, IX and X nerves. Koelliker noted the close proximity of the radix sensibilis nervi facialis to the radix decendens nervi vestibularis. Also he observed the nucleus tractus solitarii extend- ing the entire length of the tractus solitarius from the radix sensibilis nervi facialis to the decussatio pyramidum. As a result of a study of some Marchi sections of a brain from a patient who had a lesion causing the loss of taste and tactile sensations on the left half of the tongue, Wallenberg traced a degenerated fasciculus, cephalad, from the tractus solitarius at the level of the sensory root of the glossopharyngeal nerve, to the TRACTUS SOLITAIIIUS IN THE GUINEA PIG 173 bend of the motor root of the facial nerve. Since only normal fibers were found in the sensory roots of the facial and glosso- pharyngeal nerves, he concluded that these degenerated fibers came from the trigeminal nerve. In some surgical cases involving the removal of the ganglion semilunare, Cushing noted that there was no loss of taste in the distal end of the tongue, and concluded therefrom, that the chorda, tympani must carry gustatory sensations from the region of the tongue through the ganglion geniculi. Furthermore some experi- mental work by Hunt, demonstrated that tactile sensations may be present in the tip of the tongue after severing the nervus lingualis, centrally, to the exit of the chorda tympani. Bruce in 1898, had an opportunity of studying a Marchi series of a human medulla in which a tumor, after having destroyed the trigeminal nerve, completely severed the glossopharyngeal nerve in the jugular foramen, without injuring the vagus. His series demonstrated degenerated glossopharyngeal fibers entering the upper end of the tractus solitarius, which disappeared, caudally, by terminating in the gelatinous substance belonging to the vagus nerve; not a single fiber was traced to the dorsal nucleus or the nucleus ambiguus. Bruce concluded that the upper part of the tractus solitarius belongs to the glossopharyngeal nerve and the lower two-thirds to the vagus nerve. His figure 2, which is from the spinal medulla region. demonstrates a few glossopharyn- geal fibers in the tractus solitarius. Van Gehuchten has given us a most comprehensive work on the nerve components of the tractus solitarius in the rabbit, and my own investigation on this phase of the problem is largely a repeti- tion of this work extended a little further along some lines. A separate review of Van Gehuchten work will not be included for the reason thatrequent reference will be made to it through- out the text. MATERIAL AND METHODS The reasons for selecting the guinea pig for these investigations were essentially the same as those set forth in a previous paper (Jour. Comp. Neur., vol. 30, p. 176). For this study it was 174 WILLIAM F. ALLEN advisable to have at least three Marchi series of the medulla, showing in one series, on one side, the radix sensibilis nervi facialis in degeneration; a second series, in which the radix sen- sibilis nervi glossopharyngei of the same side was in degeneration; and a third series having the radix sensibilis nervi vagi also of the same side in degeneration. In fact, I had a number of Marchi series of each of the three components of the left tractus solitarius, one series of a rabbit medulla where the fibers of the left radix sensibilis nervi glossopharyngei et vagi were degenerated, and a number of series where the tractus solitarius had been cut at different levels. There were also available many Marchi series, which demonstrated the neighboring tracts, such as, the radix descendens nervi vestibularis, radix spinalis nervi trigemini, tractus spino-cerebellaris dorsalis et ventralis, lemniscus medialis, corpus restirforme, brachium conjunctivum, fasciculus longi- tudinalis medialis, and various motor roots in dengeneration. Then for comparison, the writer had a number of Weigert, Nissl, Golgi, and Cajal series of normal guinea pig brains. Operations: The general method of conducting the operations was the same as was described in detail in the paper cited above. For the sake of uniformity, all of the lesions were made on the left side. In the experiments designed to produce degeneration of the facialis portion of the tractus solitarius (no. 136 and others) an incision was made through the skin, immediately behind and below the left ear. The nervus facialis was carefully dissected to its entrance or exit from the foramen stylomastoideum; from whence-it was jerked out with a hemostat, after the technique employed by Van Gehuchten; this carried out about an inch of the nerve from the foramen, including the ganglion geniculi. In other operations, equally good results were obtained, though with more difficulty, by dissecting out the nervus facialis to and including its ganglion, with the aid of a small pair of bone forceps. The operation intended to bring about the degeneration of the glossopharyngeal component of the tractus solitarius was by far the most difficult of all the operations to perform, due partly to the close proximity of the nervus glossopharyngeus to the vagus TEACTUS SOLITARIUS IN THE GUINEA PIG 175 and its ganglia and partly to the general inaccessibility of the nervus glossopharyngeus in curving around behind the tympanic bulla. The skin incision was made a little median to the angle of the left mandibula. After reflecting the skin, the salivary and lymph glands were separated from the musculus masseter, bringing into view the posterior belly of the in. digastricus and the nervus hypoglossus traversing its median surface. Carefully separating the nerve from the m. digastricus and expanding the opening exposes the nervus glossopharyngeus crossing the outer surface of the m. stylopharyngeus, which can be followed with some difficulty along the median ventral surface of the tympanic bulla in front of the nervus vagus and ganglion nodosum to the foramen jugulare. After carefully separating the nervus glosso- pharyngeus from the vagus, the nervus glossopharyngeus and ganglia were jerked from the foramen without injuring the vagus at or above its ganglia. The general technique employed in destroying the sensory root fibers of the nervus vagus was identical to that used for destroying the glossopharyngeal sensory root fibers. It was, however, much easier to accomplish for the reason that it was not necessary to go in so deep. All that was necessary was to follow the nervus vagus to the ganglion nodosum and to sever the root fibers centrally to the ganglion, without injuring the glossopharyn- geal root fibers or its ganglia. In order to sever the tractus solitarius I made a median inci- sion through the skin of the occipital and neck region. After turning the skin to either side, the median incision was con- tinued through the muscles of the neck to the depth of the median occipital crest and dorsal arch of the atlas. Next the capitus muscles were severed close to the base of the skull and turned to either side, which exposes the membrane covering the foramen occipitale magnum. With a small pair of bone forceps some of the squamus portion of the os occipitale was clipped off in front of the occipital foramen on the right or opposite side from which the lesion is to be made, after which the free cephalic border of the membrane covering the foramen occipitale magnum was seized with a small pair of curved forceps, and 176 WILLIAM F. ALLEN turned backward at the same time cutting it free from the skull and atlas with a very small scalpel. This brings into view the entire dorsal surface of the medulla up to the cerebellum. Finally the cerebellum was gently lifted up from the medulla and a shallow lesion was made through the left tractus solitarius, being careful not to injure any of the dorsal tracts or nuclei approximating it. For making this lesion I filed down a piece of A in. steel wire in the form of a chisel and bent it at an angle of about 45 parallel to, and about one inch from the blade. Microscopical technique. Ordinarily the animal was killed on the 14th day after the operation. According to my experience with the Marchi method, the maximum degeneration appears in mammals about the 11th day, and it is best not to let the animal live more than 14 days after the operation, for the reason that retrograde degeneration will take place from the central motor nuclei, as a result of the chromatolysis of their cells, which will assume considerable proportions by the 18th day. Upon removal the brain was placed in a dish of water, where the cerebral hemispheres were severed and discarded, but leaving the corpus striatum intact with the brain stem. This portion of the brain was then placed in a 3 per cent solution of potassium bichromate for 4 days, from whence it was removed after a superficial hardening, to strip off the dorsal part of the cerebellar cortex and to cut up into transverse blocks of about 5 or 6 mm. in thickness. (If this procedure, which was necessary for the complete infiltration of osmic acid, was delayed much longer, there was danger of the brain stem becoming brittle and crumbling in making these sections.) These blocks were returned to a 3 per cent solution of potassium bichromate for a period of about 2 weeks to complete the infiltration; from which they were removed to a solution composed of 30 cc. of 3 per cent potassium bichromate plus 10 cc. of 1 per cent osmic acid, for about 3 weeks. Finally these blocks were thoroughly washed in water, dehydrated, thoroughly infil- trated with celloidin, cut 50 microns in thickness, and mounted serially. TRACTUS SOLITAR/US IN THE GUINEA PIG 177 TRACTUS SOLITARITIS (PARS NFRVI FACIALIS) A comparison of figure 3 with Van Gehuchten figure discloses a close resemblance in the guinea pig and the rabbit of the passage of the radix sensibilis nervi facialis or nerve of Wrisberg through the medulla to form the tractus solitarius. About the only difference is that the VIII root of the guinea pig separates into four instead of three bundles or rootlets in traversing dorso- ventrally, through the dorsal portion of the radix spinalis nervi trigemini (Sp.V.) and its sensory nucleus (Sp.V.N .) to reunite medially of the dorso-medial corner of the radix spinalis nervi trigemini in forming the tractus solitarius (T.S.). From the dorsal reconstruction, figure, 1A, it is obvious that the four EXPLANATION OF FIGURES Figures 1 and 2 are from graphic reconstructions, the remainder are from portions of transverse sections, or in a few instances, from a combination of several adjacent sections. The graphic reconstructions were made after a method given in an earlier paper (Quart. Journ. Micro. Sri., vol. 59, pp. 352-3). In each case the median sagittal plane of the medulla was chosen for the projection line (P). The. marginal scale is in terms of centimeters, each division equalling 1 cm. In these figures the degenerated components of the tractus solitarius are represented by parallel lines and motor nuclei as irregular coarse dots. The limits of sensory nuclei may be indicated by outlines or their cells ma y be shown as circles. The boundaries of normal nerve roots entering the tractus solitarius, the radix spinalis nervi trigemini and tract Z are drawn in outline; while the borders of the radix descendens nervi vestibuli are shown by very fine dotted lines. All of the drawings from the transverse sections were made with the aid of a projection drawing apparatus. In these figures degenerated fibers appear as black dots; other structures were drawn in sepia or in case of the sensory nuclei of the IX and X nerves were left in white. ABBREVIATIONS B., In fig. 1A, most cephalic nervus facialis fibers and collaterals to the nucleus tractus solitarii. In fig. 1B, most cephalic nervus glossopharyn- geus fibers and collaterals to the nucleus tractus solitarii. In fig. 1C, most cephalic nervus vagus fibers and collaterals to the nucleus tractus solitarii C. C., canalis centralis C. F., commissural fibers Corn. N., nucleus commissuralis or ganglion commissurale Cor. Sp. D., tractus cortico-spinalis dorsalis or dorsal pyramidal tract C. Trap., corpus trapezoideum Gun. N., nucleus fasciculi cuneati or nucleus of Burdach D. Car. N., nucleus nervi cochlearis dorsalis or dorsal cochlear nucleus Dec. Per., clecussatio pyramidum or decussation of the pyramids 178 WILLIAM F. ALLEN D. I. Arc., fibrae arcuate dorsales or dorsal internal arcuate fibers D.N., nucleus nervi vestibularis later- ails or Deiters nucleus E. Arc., fibrae arcuatae externae or external arcuate fibers F. R., formatio reticularis F. R. L., formatio reticularis lateralis F. R, M., formatio reticularis medialis Grad!. N., nucleus faseiculi gracilis or nucleus of Coil I. rc,, fibrae arcuata,e internae or internal arcuate fibers Int. S. N., nucleus intercalatus Staderini Lon. M., lemniscus medialis or median lemniscus Mon. N., nucleus Monakow of Winkler descriptions Of. I., nucleus olivaris inferior or inferior olive P-L., projection line in reconstructions 1 and 2 P. S. N., nucleus parasolitarius Pyr., tractus cortico-spinalis or pyramis Res. B., corpus restiforme Res. N., nucleus proprius corporis restiformis S., tractus solitarius fibers to nucleus motorius dorsalis nervi vagi Sp. V., tractus spinalis nervi trigemini or radix spinalis nervi V Sp. V. N., nucleus tractus spinalis nervi trigemini or substantia gela- tinosa trigeminal root St. Ac., striae acusticae or medullares T., In fig. 1A, termination of facialis fibers in the tractus solitarius. In fig. 113, termination of glossopharyn- geal fibers in the tractus solitarius T. S., tractus or fasciculus solitarius T. S. VII., tractus solitarius (pars nervi facialis) T. S. IX., tractus solitarius (pars nervi glossopharyngei) T. S. X., tractus solitarius (pars nervi vagi) T. S. N., nucleus tractus solitarii Tub. Ac. tubercultun acusticum V. Coc. N., nucleus nervi cochlear ventralis or ventral cochlear nuclei Ves. Crb., direct fibrae vestibulae-cert bello Ves. Sp. N., nucleus nervi vestibulari spinalis Z., tractus Z VII, radix sensibilis nervi facialis o nerve of Wrisberg VIIa-d., rootlets of radix sensibili nervi facialis VII M., radix motorius nervi (pars secunda) VII M. N., nucleus motorius nervi facialis VII T. S., In fig. 113, tractus solitarius (pars nervi facialis) VIII Ves., radix nervi vestibularis VIII Ves, D., radix descendens nervi vestibularis and tractus arcuatus Russell IX, radix sensibilis nervi glosso- pharyngei IXa-d, rootlets of radix sensibilis nervi glossopharyngei IXd. N., nucleus motorius dorsalis nervi glossopharyngei IX M., radix motorius nervi glosso- pharyngei IX S., nucleus sensibilis nervi glosso- pharyngei Xd. N., nucleus motorius dorsalis nervi vagi X M., radix motorius nervi vagi Xv. N., nucleus ventralis nervi vagi or nucleus ambiguus X 1 rootlets of the first radix sensibilis nervi vagi X 2 Xy a-b, rootlets of the second radix sensibilis nervi vagi XY, third radix sensibilis nervi vagi X4, fourth radix sensibilis nervi vagi XI N., nucleus nervi accessorii or caudal continuation of the nucleus motorius dorsalis nervi vagi XII, radix nervi hypoglossi XII N., nucleus nervi hypoglossi 9 11 la 15 11 is is 17 14 19 20 ,e 22 ZS 2 7.5 .215 17 23 29 30 51 32 33 39 S 17 3a 31 41 12 13 P L 2 Fig. 1 Graphic reconstruction showing the distribution of the components of the left trac- tus solitarius of a guinea pig viewed from above. X 10. A. Shows the distribution of degener- ated fibers resulting from the extirpation of the left nervus facialis. centrally to the ganglion geniculi, without injuring the nervus acusticus (Ex. 136). B. Illustrates the course of degenerated fibers resulting from pulling out the left nervus glossopharyngeus centrally to its ganglia, without injury to the vagus (Ex. 156). C. Demonstrates the distribution of degenerated fibers caused from severing the left nervus vagus centrally to the ganglion nodosum, without injury to the glossopharyngeal roots or ganglia (Ex. 121). Fig. 2. Graphic reconstruction showing the arrangement of the sensory roots of the vagus in a different guine . 2) from the one from which figure 1C was drawn; introduced to show individual variation of the vagus roots. X 6.6. 1 80 WILLIAM F. ALLEN rootlets (a-d) are situated one above the other and that they assume a slightly caudal direction in penetrating the medulla. In agreement with Koelliker and Van Gehuchten these rootlets pursue a parallel but ventral course to a number of radix nervi vestibularis bundles, and might readily be confused with them if the two were not differentiated by the Marchi stain. Especial note should be made of the close association of the facial and vestibular roots and to the many direct fibrae vestibu- lae-cerebello (fig. 3, Ves.Crb.) entering the cerebellum, and to the fact that no degenerated radix sensibilis nervi facialis fibers are seen going to the cerebellum in this series or in any other series where the facialis fibers were in degeneration. In fact, no degen- erated fibers were observed going to the cerebellum when all the components of the tractus solitarius (VII, IX and X sensory roots) were in degeneration. There is no branching of the facialis sensory fibers into superior and inferior branches as is charac- teristic of the sensory fibers of the trigeminus. All make a sharp caudal bend and continue in a general caudal direction. The degenerated particles of myelin are very minute, which is prob- ably indicative of finely medullated sheaths. Figures 3 to and including 6 are from sections of the same guinea pig series as reconstruction lA (Ex. 136) taken at various intervals from higher to lower levels intended to illustrate the distribution and relationships of the left radix sensibilis nervi facialis. Fig. 3 Left half of a transverse section at the level of the facial and vestibular nerve roots, on which were projected the degenerated fibers of the left radix sensibilis nervi facialis shown in the previous three sections and the following four sections. X 4. Fig. 4 Rather highly magnified drawing of the left tractus solitarius from a section of the same series as figure 3, taken a short distance from the origin of the tractus solitarius. In reconstruction IA this section would be situated in the 7th cm. plane. X 45. Fig. 5 Left half of a transverse section from the 9th cm. plane in reconstruction IA. X 4. Fig. 6 Left tractus solitarius region from the same series as figure 3 at the level of the radix sensibilis nervi glossopharyngei. X 18. Fig. 7 Reconstruction and projection of the four rootlets of the left radix sensibilis nervi glossopharyngei, taken from five sections of the same series as figure 1B and drawn on the most median section. In this series (no. 156) the left nervus glossopharyngeus was jerked out of its foramen so as to include both of its ganglia. X 4. .rilE J01_71:NAL 1OL. 35, xo. 2 TS.Yrr 182 WILLIAM F. ALLEN The general course of the facialis portion of the tractus soli- tarius of the guinea pig conforms very closely to Van Gehuchten description of the rabbit, namely, it is in no sense a solitary tract, being at first closely associated with the radix spinalis nervi trigemini and later with the radix descendens nervi vestibularis. Reconstruction IA demonstrates the tractus solitarius as taking origin at the 6th cm. plane; then for 3 cm. it passes rapidly mesad, and maintains this general position for some distance caudad. A transverse section through the 9th cm. plane (fig. 5) portrays the tractus solitarius (T.S.) situated ventrally in the ventro- median corner of the radix descendens nervi vestibularis. To acquire this median position the tractus solitarius gradually crossed the ventral border of the vestibular root. At its origin (fig. 4) the tractus solitarius is oval in section, but in the 9th cm. plane (fig. 5) it has become greatly flattened horizontally. Differing from Van Gehuchten description of the rabbit, the tractus solitarius of the guinea pig is accompanied from its very beginning by a terminal sensory nucleus, which in figure 4 is a small light area (T .S.N .), situated medially and ventrally to the tractus solitarius. At the level of the 9th cm. plane in figure lA this nucleus (fig. 5) broadens out laterally under the tractus solitarius, and gradually increases in size caudally until at the level of the entrance of the IX sensory root (fig. 6) it has assumed enormous proportions. Here it is more or less continuous with a larger ventral nucleus, the nucleus parasolitarius (P.S.N .). At its origin (figs. lAand 4) the nucleus tractus solitarii receives a few degenerated facialis fibers. These are probably collaterals rather than terminal fibers. On a level with the 9th cm. plane in reconstruction lA (fig. 5) there are many degenerated fibers in the nucleus tractus solitarii. The maximum number of degenerated facialis fibers found in this nucleus occurs in the region of the entrance of the IX sensory root (fig. 6). From this level caudal, the number gradually decreases until very few are present at the level of the entrance of the first root of the X nerve. Before the IX root is received, the tractus solitarius gives off at least two bundles of fibers for the nucleus tractus solitarii. The first of these bundles is a dorsal fasciculus, which leaves the tractus TRACTUS SOLITARIUS IN TIIE GUINEA PIG 183 solitarius at the 7.5th cm. plane in figure lA and extends about 2 cm. in this reconstruction before ending in the nucleus tractus solitarii. This bundle is shown in transverse section in figure 5, where it would be undifferentiated from the bundles of the radix descendens nervi vestibularis, but for the Marchi stain. The second bundle is given off laterally and ventrally from the tractus solitarius at the level of the 9th cm. plane in figure 1A. Two places on the facialis portion of the tractus solitarius are of especial interest, namely, the entrance of the IX and the first X sensory roots. Inasmuch as their mode of entrance is almost identical, a description of the IX will suffice for the X. Figure 6 demonstrates the normal glossopharyngeal fibers crossing under the degenerated facialis fibers of the tractus solitarius, in part they intermingle with the facialis fibers, but chiefly they add to the width and height of the tractus solitarius, making it crescent- shaped at the ventro-median corner of the radix descendens nervi vestibularis. Contrarily, Van Gehuchten on page 9 of his second paper and in figure 20 of the same paper, represents the IX sensory root as forming a fasciculus, lateral to the nerve of Wrisberg. No more degenerated facialis fibers appear in the tractus solitarius or its nucleus at the level of the entrance of the second radix sensibilis nervi vagi than are present on the opposite or normal side. The writer would place their ending at the point marked (T) in figure 1A; which will be seen to be some distance cephalad of the termination of these fibers in the tractus soli- tarius of the rabbit as determined by Van Gehuchten. The dif- ference in levels of the ending of the facialis fibers in the tractus solitarius in the guinea pig and the rabbit may be accounted for by the fact that, since the nucleus tractus solitarii began much further cephalad in the guinea pig, it consequently might be expected to terminate further cephalad, thereby bringing about an earlier ending of the tractus solitarius fibers. 184 WILLIAM F. ALLEN TRACTUS SOLITARIUS (PARS NERVI GLOSSOPITARYNCIEI) It is apparent from a superficial examination of the radix sensibilis nervi glossopharyngei in reconstructions 1B and 7 that there is a close resemblance to the radix sensibilis nervi facialis, but many differences from Van Gehuchten figure 4 of the IX sensory root of the rabbit. Like the facialis root, the degenerated particles of myelin are very small, indicative of minute medullary sheaths. Transverse reconstruction 7 shows the radix sensibilis nervi glossopharyngei of the guinea pig breaking up into three small bundles upon entering the medulla. Before penetrating the tractus spinalis nervi trigemini, the most dorsal bundle separates into two; so that there are at least four distinct bundles or root- lets of degenerated radix sensibilis nervi glossopharyngei fibers IXa-d) traversing the dorsal portion of the tractus spinalis nervi trigemini (Sp.V .) and its sensory nucleus (Sp.V.N .). The most dorsal of these bundles, rootlet (a), crosses the ventral surface of the radix descenders nervi vestibularis (VIII Ves. P.). Root- lets (b) and Cc) unite and their common stem joins dorsally with (a), forming a common bundle that soon receives rootlet (d) from below. This common fasciculus curves around the ventral and median surfaces of the normal facialis fibers of the tractus solitaries, in part to intermingle with them, but mainly to con- tinue caudad, median and dorsal to them. Since Van Gehuchten figure 4 shows such a different arrange- ment of the rootlets of the IX sensory root within the medulla, it seemed advisable to make a reconstruction of this root from a rabbit series where the IX and X nerves were in degeneration. his reconstruction (fig. 8) demonstrates an arrangement of four rootlets (a-d) more closely resembling the disposition shown in reconstruction (fig. 7) than Van Gehuchten figure 4. The chief point of difference in the rabbit, as shown in figure 8, consists of an earlier union of rootlets (a-c) and the consequent elongation of the combined bundle. It should be recorded that a small portion of the delicate rootlet (d) appears in a number of sections, so that it is highly probable that this rootlet would have been over- looked if a reconstruction had not been made. TRACTUS SOUTAR - 185 Throughout the entire course of the glossopharyngeal portion of the tractus solitarius, the tractus solitarius continues to occupy the ventro-median corner of the radix descendens nervi vesti- bularis. Caudally, it gradually acquires a more dorsal position, to become located medially, in the dorso-ventral corner of the vestibular root. A highly magnified drawing (fig. 9) of the left tractus solitarius from a section immediately behind the entrance of the radix sensibilis nervi glossopharyngei, demonstrates degenerated glossopharyngeal fibers (IX T .8 .) median to the normal facialis fibers (VII T.S.), ventrally, and median to the radix descendens nervi vestibularis fibers, dorsally. It is im- possible in this section to discriminate between facialis and descending vestibular bundles, but after examining a similar section from a series where the facialis portion of the tractus solitarius was in degeneration, it is safe to affirm that those bundles designated as (VII T.S.) in figure 9 are facialis bundles of the tractus solitarius. At the level of the entrance of the first radix sensibilis nervi vagi (fig. 10) the normal fibers of this root (X i) are clearly shown below and median to the degenerated glossopharyngeal fibers (T.S. IX). Some of the vagus fibers apparently intermingle with the glossopharyngeal fibers but the majority retain their ventral and median positions. A section taken from the level of the entrance of the third radix sensibilis nervi vagi demonstrates but few degenerated glosso- pharyngeal fibers in the tractus solitarius or in its nucleus. A section taken immediately caudal of the entrance of the fourth. radix sensibilis nervi vagi exhibits no more degenerated glosso- pharyngeal fibers in the left tractus solitarius or its nucleus than are present on the normal or right side. In the guinea pig the- writer would place the caudal ending of the glossopharyngeal fibers, both in the tractus solitarius and its sensory nucleus, opposite the entrance of the third radix sensibilis nervi vagi in. the tractus solitarius (fig. 1B, T.). This is considerably cephalad of Van Gehuchten level for the termination of these fibers in the rabbit. In addition to the previous notes on the termination of the glossopharyngeal fibers in the nucleus tractus solitarii it should 186 W ILLIAM F. ALLEN be recorded that a few degenerated glossopharyngeal fibers were found in this nucleus opposite the entrance of this root (figs. I B and 7, T.S.N.), and at the level of the entrance of the first vagus root (fig. 10, T.S.N.) they were very abundant. In another Marchi series (No. 161) where a lesion had severed the IX sensory root in its course through the tractus spinalis nervi trigemini, without injuring the tractus solitarius or the X sensory roots, the distribution of the IX sensory fibers in the medulla, is identical to those series in which the IX nerve was severed centrally to the ganglia. It is clear from all sections taken from the region of the entrance of the various sensory roots of the X nerve (fig. 10) that the gray area (white in the figures), situated below and median to the tractus solitarius, and known generally as the dorsal nucleus of the glossopharyngeal and vagus nerves, has assumed such pro- portions and complications that it merits separate consideration. Van Gehuchten described it on p. 186 as a clear oval spot, traversed by fasciculi belonging to the acoustic nuclei. These Fig. 8 Similar projection to figure 7, but from a rabbit series (no. 143), where the nervus glossopharyngeus had been severed centrally to its ganglia. To obtain this reconstruction of the radix sensibilis nervi glossopharyngei it was necessary to utilize eleven sections and project them on a drawing of the most median section. X 4. Fig. 9 From the same guinea pig series as figures 113 and 7 taken through the left tractus solitarius a short distance after the receipt of the left radix sensibilis nervi glossopharyngei. X 45. Fig. 10 Portion of a transverse section from the same series as figure 7, includ- ing the union of the first radix sensibilis nervi vagi with the tractus solitarius. X 18. Figs. 11-19 are from transverse sections of a guinea pig medulla (Ex. 121), where the vagus had been severed centrally to the ganglion nodosum without injuring the glossopharyngeal roots or ganglia. Fig. 11 A reconstruction of the rootlets of the first left radix sensibilis nervi vagi taken from eleven dif f median section. X 4. Fig. 12 Reconstruction and projection of the second left radix sensibilis nervi vagi taken from seven transverse sections of the same series as figures in and 11 and projected on a drawing of the most median section. X 4. Fig. I2A More highly magnified view of the left tractus solitarius taken from a section of the same series as above, directly below the level of the second left radix sensibilis nervi vagi. X 45. 8 1- 7 RaPhe s- Len, Ft b 5p.VM1/ xI 12 T.S.N. 12A sr. Y. NJ..., SITS( Ve s. D. - 10 187 188 WILLIAM F. ALLEN fibers were said to divide the gray mass into two parts: an internal mass of small cells, the motor nucleus; and an external part, the terminal cells of the tractus solitarius. Van Gehuchten sen- sory nucleus can readily be subdivided into central and pe- riphereal areas. The former is composed only of small cells possessing little or no Nissl substance and receives few or no degenerated fibers from the tractus solitarius. The latter or more lateral area is situated mainly median and ventral to the tractus solitarius. It includes a few medium-sized cells and numerous degenerated fibers from the tractus solitarius in addition to the small cells described for the more median area. According to von Monakow these larger cells, which contain Nissl substance, are undoubtedly the cells of origin of the secondary visceral tract to the thalamus. It is possible, then, to divide the so-called klorsal glossopharyngeal and vagus nucleus into three parts: A median area, the nucleus motorius dorsalis nervi vagi a lateral area approximating the tractus solitarius, the nucleus tractus solitarii; and an intermediate area composed only of small cells, which may function as a visceral reticular nucleus. TRACTUS SOLITARIUS (PARS NERVI VAGI) As was noted by Van Gehuchten, the passage of the vagus sensory fibers through the medulla to the tractus solitarius is very different from either the glossopharyngeal or the facial. Instead of being composed of a single fasciculus or root, more or less into several bundles or rootlets, the vagus fibers enter the tractus solitarius through several distinct fasciculi or roots. The relative number of sensory fibers of the X nerve, which contains few, if any, taste fibers is much greater than the com- bined VII and IX fibers. Also the degenerated particles of myelin are much coarser, indicating thicker medullated sheaths. According to my interpretation of the vagus sensory complex, there are at least four distinct fasciculi or roots entering the tractus solitarius in the guinea pig. In series 121, the first radix That this nucleus was in reality a motor nucleus was determined by section- ing the IX and X nerves peripherally to their ganglia and studying this nucleus for chromatolysis and also by tracing retrograde motor fibers to it. TRACTUS SOLITARIUS IN THE GUINEA PIG 189 sensibilis nervi vagi (reconstructions 1C and 11) consists of four bundles or rootlets (X ia-X id). The first three rootlets (fig. 1C, X ia-X,c) are very closely associated and are situated in about the same vertical plane. In figure 11 rootlets (Xia-Xic) pass dorso- ventrally, one above the other, through the fibrae arcuatae externae (E. Arc.), to penetrate the dorsal portion of the tractus spinalis nervi trigemini (Sp.V.) and its sensory nucleus The most.dorsal of these roots (X ia) continues across the ventral border of the radix descendens nervi vestibularis (VIII Ves.D.). Rootlets (X ib and X i() unite in the dorsal part of the nucleus tractus spinalis nervi trigemini and the common stem continues dorso-medially, to unite dorsally with rootlet (X ia) a short distance from the tractus solitarius. Rootlet (X id), which will later be demonstrated to be a somewhat irregular bundle, is situated in figure 1C midway between rootlets (X ia-c) and rootlet (X 2 and when about to reach the tractus solitarius, it bends cephalad and dorsad to join the combined trunk of rootlets (X ia-Xic). This common fasciculus is shown in figure 11 passing under the normal glossopharyngeal fibers of the tractus solitarius (T.S.) to constitute the beginning of the vagus portion of the tractus solitarius, obviously situated median and ventrally to the glossopharyngeal portion of the tractus solitarius. The second radix sensibilis nervi vagi appears as two medium- sized bundles (X2 X 2 in the dorsal reconstruction, figure 1C, situated one behind the other and pursuing an almost median course. Transverse reconstruction (fig. 12) demonstrates root- let (X 2 considerably ventrad as well as caudad of rootlet (X2a). It occupies almost the same horizontal plane as rootlet (Xid). Reconstructions 1C and 12 show these two rootlets uniting in a common fasciculus close to the tractus solitarius, which in turn crosses under the tractus solitarius. It is apparent from figure 12a that the tractus solitarius is composed mainly of degenerated vagus fibers from the first two roots (T.S. X), immediately behind the entrance of the second vagus root and that the few remaining glossopharyngeal fibers (T.S. IX) are situated laterally to the vagus fibers. At the level of figure 12 there are but few degener- ated vagus fibers in the nucleus tractus solitarii (T.S.N.). A motor root (X. M.) will be seen ventrally to rootlet (X2b). 190 WILLIAM F. ALLEN The third radix sensibilis nervi vagi is a large fasciculus in this series (figs. 1C and 13, X:,) situated caudad to (X 2 course is directly median, it can be followed in one section (fig. 13) traversing the fibrae arcuatae externae (E.Arc.), the dorsal part of the tractus spinalis nervi trigemini (Sp. V.) and its sensory nucleus (Sp.V.N.); thence across the ventral surface of the radix descendens nervi vestibularis (VIII Ves.D.) and the tractus solitarius (T.S.), to acquire a position in the tractus solitarius median and ventral to the degenerated fibers of the first two sensory roots of the vagus. It is obvious that there are but few degenerated vagus fibers in the nucleus tractus solitarii (T.S.N.) at this level. The fourth and last radix sensibilis nervi vagi (figs. 1C and 14, X 4 differs from the preceding root in being notably smaller and assuming a more caudal direction in its median course to join the tractus solitarius. As a result only a part of it appeared in any one section and it was necessary to utilize three sections to obtain figure 14. It will be seen that (X4) pursues a more dorsal course than root (X 5 but terminates in the tractus solitarius in like manner. It is reasonable to suppose that there would be some, if not con- siderable, variation in the arrangement and number of the roots and rootlets of the sensory part of the vagus. To determine the amount of individual variation, a dorsal reconstruction was made of the sensory roots of the vagus from another guinea pig Fig. 13 Left half of a transverse section from the same series as figure 11 at the level of the entrance of the third radix sensibilis nervi vagi. X 4. Fig. 14 Reconstruction of the fourth left radix sensibilis nervi vagi from three sections of the same series as figures IC and 11 projected on a drawing of the middle section. X 4. Fig. 15 Portion of the left half of a transverse section from the same series as figure 11, situated in the 30th cm. plane in reconstruction X 12. Fig. 16 Left tractus solitarius and adjacent structures from the same series as figure 11, taken from the 32.5 cm. plane in figure IC or about four sections cephalad of the passage of the ventriculus quartos into the census centralis. X 12. Fig. 17 Left half of a transverse section through the spinal portion of the medulla. In reconstruction 1C this section would lie in the same plane that tractus (Z) crosses the tractus solitarius. X 4. lot T5N ILI Arc h.r1 Mon h Gr N T 5 N f) T. TFU-. X..101 n oPheF.km, Xv-N P.S.N ffir VesU Res .N Ros.k=. F. Arc Mon. N P N 16 192 WILLIAM F. ALLEN series (no. 132), in which the nervus vagus had likewise been severed centrally to the ganglion nodosum. This reconstruction (fig. 2) when compared with figure 1C discloses surprisingly little variation. The same roots and rootlets are present and a similar arrangement is maintained; some irregularity, however, occurs in connection with rootlet (X id). This rootlet will be seen in figure 2 to be closely associated with the other rootlets of this root during the first part of its course through the medulla, but upon approaching the tractus solitarius, it bends caudally to enter the tractus solitarius directly in front of the second radix sensibilis nervi vagi. Future investigation may reveal it best to consider this rootlet as a separate root of the vagus sensory complex. In figure 2 the two rootlets of the second radix sensi- bilis nervi vagi (X 2 X 2b) are in about the same vertical plane, in place of (X2 1C. Also in figure 2 the roots (X,) and (X 4 size, instead of (X 3 Throughout the general area of the passage of the sensory vagus roots through the tractus spinalis nervi trigemini there are possibly a few more degenerated fibers in the left tract than in the right, but no degenerated fibers were observed leaving the sensory roots of the vagus to enter the tractus spinalis nervi trigemini. Possibly if the nervus vagus had been severed centrally to the ganglion jugulare, some degenerated general cutaneous fibers would have appeared in the tractus spinalis nervi trigemini. The nucleus parasolitarius is of considerable size in this region, but no degenerated tractus solitarius fibers were distributed to it. There was no bunching of the motor roots in this area, as is charac- teristic of the sensory roots. Tractus solitarius proper: Caudally to the entrance of the fourth and last radix sensibilis nervi vagi, the tractus solitarius (pars nervi vagi) as it is from this point caudad, is a true solitary tract. This tract gradually assumes a more median position, is constantly giving off fibers and collaterals until its supply is exhausted in the neighborhood of the decussatio pyramidum, and can conveniently be divided into three parts for descriptive purposes. TRACTUS SOLITARIUS IN THE GUINEA PIG 193 The first of these areas is shown in figure 1C to extend from the 29th to the 32d planes. In this region, the tractus solitarius (T.S.), its sensory nucleus (T.S.N.) and the nucleus motorius dorsalis nervi vagi (Xd.N.) are gradually acquiring a more median position in passing caudad. it is a locality where many degenerated fibers are given off to the nucleus tractus solitarii (T.S.N.), and at the center of this area, extending between the 29.5th and the 31st cm. planes, some degenerated tractus soli- tarius fibers or collaterals continue medially into the nucleus motorius dorsalis nervi vagi (S). A transverse section taken through the center of this area (fig. 15) discloses the tractus, solitarius to be a compact cylindrical bundle, excepting the extreme lateral portion, which is beginning to display a tendency to break up into smaller bundles. It is obviously well separated from the ventro-median corner of the radix descendens nervi vestibularis (VIII Ves.D.) by a portion of the nucleus tractus solitarii. The descending vestibular tract has become greatly reduced in size. A number of fibrae arcuatae dorsales (D.I.Arc.), cross above, below and even through the tractus solitarius. Many degenerated tractus solitarius fibers and collaterals are present in the nucleus tractus solitarii (T.S.N.), which at this level not only surrounds the tractus solitarius, but extends some little distance medially and dorso-medially. Practically all of the area between the tractus solitarius and the nucleus motorius dorsalis nervi vagi contains degenerated fibers, and a string of degenerated fibers extends into the motor nucleus (Xd.N.). There are also some degenerated fibers in a radix motorius nervi vagi (X M.) and in the ventro-median part of its nucleus, caused from the chromatolysis of the motor cells, resulting from severing the nervus vagus. No degenerated tractus solitarius fibers were seen in the nucleus parasolitarius (P.S.N.) or in the nucleus nervi vestibularis spinalis (Ves.Sp.N.). The second or middle area of the true tractus solitarius is short, extending from the 32d to the 33d cm. planes in figure 1C. It is characterized by its abrupt median turn, and by its breaking up laterally into many small bundles. Since the 33d cm. plane in figure 1C marks the level of the termination of the ventriculus 194 WILLIAM F. ALLEN quartus into the canalis spinalis, this marked median turn of the tractus solitarius simply conforms to the general contour of the medulla rapidly tapering down into the medulla spinalis. Figure 16, which is from the 32.5 cm. plane in figure 1C, at the level where the medulla is about to enter the medulla spinalis, dem- onstrates very clearly the migration of all structures medially. The nucleus rnotorius dorsalis nervi vagi (Xd.N.) is near the median sagittal plane, dorsad of the nucleus nervi hypoglossi (XL/ N.). The radix descendens nervi vestibularis (VIII Ves.D.) has been reduced to a very small tract, which from a Marchi series after a cerebellar lesion, is shown to be composed almost entirely of tractus arcuatus Russell or fasciculus cerebello- bulbarius fibers at this level. The tractus solitarius has been separated still further from the radix descendens nervi vesti- bularis by the increase in size of the nucleus tractus solitarii, laterally. It is broken up laterally into many small bundles, which branch and rebranch to terminate, dorsally, ventrally and medially, but especially dorsally in the nucleus tractus solitarii (T.S.N.). This nucleus has assumed considerable pro- portions about the tractus solitarius, dorsally and medially. No degenerated tractus solitarius fibers were found in the nucleus rnotorius dorsalis nervi vagi (Xd.N.), but a few retrograde degenerated motor fibers were present in this nucleus and in a radix motorius nervi vagi (X.M.). The third and last area of the true tractus solitarius is an area of considerable importance that is confined solely to the medulla spinalis. Here the tractus solitarius pursues only a slightly median course; it is rapidly breaking up into small bundles, which soon exhaust themselves in the nucleus tractus solitarii and neighboring nuclei. A level of especial interest is shown in figure 17, which is from the 34.5 cm. plane in figure 1C and through the transverse arm of a tract designated as tractus (Z). In this region the nucleus rnotorius dorsalis nervi vagi (Xd.N.) is lateral to the canalis centralis, more or less separated from the nucleus nervi hypoglossi (XII N.), below, by the nucleus intercalatus Staderini (Int.S.N.). Dorsal and lateral will be seen the tractus solitarius (T.S.), TRACTUS SOLITARIUS IN THE GUINEA PIG 195 here completely isolated, circular in outline and composed of a number of small scattered bundles of degenerated fibers. Numer- ous degenerated tractus solitarius fibers are present in the nucleus tractus solitarii (T.S.N.), situated mainly median and dorso- median of the tractus solitarius. This nucleus has maintained its original size, but occupies a more median position, overlying the nucleus motorius dorsalis nervi vagi to some extent, and it is perfectly clear that degenerated tractus solitarius fibers extend into the dorso-lateral corner of this motor nucleus (fig. 17, Xd.N. and indicated by S in fig. 1C). Also in the rabbit series degener- ated fibers appear in the dorso-lateral corner of the nucleus motor- izes dorsalis nervi vagi at this level. The tractus solitarius and its sensory nucleus is bounded dorsally and laterally in figure 17 by the nucleus fasciculi gracilis (Grac.N.), the nucleus fasciculi cuneati (Cun.N.) and by the radix descendens nervi vestibularis (VIII Ves.D.), here reduced to a small fasciculus composed mainly of tractus arcuatus Russell fibers. A tract, which might readily be taken for a part of the tractus solitarius in Weigert sections of this region, is a bundle designated previously as tractus (Z), for the reason that its relationships have not been fully established. It apparently takes origin from the nucleus cuneatus or nucleus proprius corporis restiformis, the level varying somewhat in different individuals. As shown in figures 1C and 17, it bends abruptly caudad after crossing the tractus solitarius dorsally. Then for some distance it pursues a caudal course, dorso-mesad, of the tractus solitarius, to ulti- mately bend ventrad, parallel with a bundle of cortico-spinalis dorsalis fibers, with which it decussates, and apparently enters the pyramis rather than the lemniscus medialis. If so, it is probably a descending tract. Sometimes this tract is present on both sides , consist of one or more bundles. To determine accurately the course and relationships of this tract it should be studied from a series showing this tract in degeneration. It is possible that future study may show tractus (Z) to be the same as Kosaka and Yagita fasciculus solitario-spinalis of the dog or Hirose bulbospinale tract of the rabbit. 196 WILLIAM F. ALLEN The remainder of the tractus solitarius is largely concerned with its termination in the nucleus commissuralis or ganglion com- rnissurale. Figure 18, which is from a level immediately in front of the 37th cm. plane in figure 1C, demonstrates the tractus solitarius (T.S.) directly ventrad of a bundle of fibrae arcuate internae (I.Arc.) and a little lateral and ventral to tractus (Z). It is reduced to a few small bundles of degenerated fibers. The terminal sensory nucleus of the tractus solitarius (T.S.N.) not only surrounds the tract at this level, but extends inward to the mid-line to meet the corresponding nucleus of the opposite side. This common nucleus is usually termed the nucleus commissuralis or ganglion commissurale (T.S.N. or Com.N.). From figure 18 it is apparent that this nucleus receives a very rich supply of degenerated tractus solitarius fibers. It is also obvious that very few of these degenerated fibers cross to the opposite side as corn- rnissural fibers (C.F.). Most of these fibers are shown in figure 1C. In this series the commissural fibers were mainly normal fibers, taking origin apparently from the nuclei of the general reticular substance; for in another Marchi series, where the retic- ular substance was destroyed at this level, there were many more degenerated commissural fibers. In the rabbit series not a single degenerated tractus solitarius fiber was seen crossing to the opposite side in the region of the nucleus commissuralis, and no commissural fibers are shown in Van Gehuchten figure 18, which is a section from this region. In fact, the entire number of commissural fibers is relatively small in mammals, as was demon- strated from a Marchi series where the entire nucleus corn- missuralis was severed longitudinally on one side. At this level the nucleus commissuralis is situated considerably dorsad of the nucleus nervi accessorii or caudal continuation of the nucleus motorius dorsalis nervi vagi (XI N.) and no degenerated tractus solitarius fibers were observed near this,nucleus. Fig. 18 Dorso-central portion of a transverse section through the medulla spinalis from the same series as figure 11. This section passes through the nucleus commissuralis and in figure 1C would lie in the plane (C. F.). X 18. Fig. 19 Left half of a transverse section from the same series as above at a level of the decussatio pyramidum, situated in the 39th , X 6. --- C.C.xr N. ............. 2aL N. Cl Gra.c.N. z corn N C.S. 18 I.Arc. z, Cor.Sp.D. Cun.N. Pyr. 19 197 I Arc-. T .S. T.S.N.'---- - C .C. I.Arc. - ERM.-- I.Ar c. 198 WILLIAM F. ALLEN From the region of the 34th cm. plane in figure 1 C, caudad, the- tractus solitarius (T.S.) becomes reduced from very small bundles to individual fibers. In figure 19, which is from the 39th cm. plane in figure IC, through the decussatio pyramidum, there are a few de- generated tractus solitarius fibers in the caudal end of the nucleus commissuralis (Com.N.) and a few in the region occupied by the tractus solitarius in more cephalic sections (T.S.N.). The de- generated fibers in the area of the nucleus commissuralis terminate in the third or fourth section caudal to figure 19, while the more lateral area of degenerated tractus solitarius fibers continues a few sections further caudad in the general reticular area. In figure 19, tractus (Z), a bundle of fibrae arcuatae internae (I. Are.) and a bundle of tractus cortico-spinalis dorsalis fibers (Cor.Sp.D.) will be seen between these two areas of degenerated tractus soli- tarius fibers and the nucleus fasciculi gracilis and the fasciculi cuneati. No degenerated fibers are present in the nucleus nervi accessorii, which is situated lateral to the canalis centralis and dorsal to the nucleus nervi hypoglossi. Both the fibrae arcuatae internae and the tractus cortico-spinalis dorsalis are decussating, and since both are normal fibers it is difficult to discriminate be- tween them. Tractus (Z) does not make its ventral bend until a lower level is reached. SUMMARY AND GENERAL CONSIDERATIONS As in other mammals, the tractus solitarius in the guinea pig is made up of descending sensory fibers from the VII, IX and X nerves. It begins at the level of the entrance of the radix sensibilis nervi facialis and extends caudally to the level of the decussatio pyratnidum. This tract permits of a division into two almost equal parts at about the entrance of the third or fourth radix sensibilis nervi vagi. The cephalic portion is embedded in the ventro-lateral corner of the radix descendens nervi vestibularis, is an area for the reception of sensory root fibers and distributes both gustatory and general visceral sensa- tions to a primary sensory nucleus. On the other hand, the caudal portion is isolated from the radix descendens nervi vesti- bularis, receives no sensory roots and distributes no gustatory TRACTUS SOLITARITIS IN THE GUINEA PIG 199 fibers to its sensory nucleus. Caudally the tractus solitarius breaks up into small bundles, which terminate in the nucleus tractus solitarii. Two of the three components of the tractus solitarius, namely, the radix sensibilis nervi facialis and the radix sensibilis nervi glossopharyngei, are very similar; both separate into at least four bundles or rootlets in traversing the medulla, to reunite in a common fasciculus that gives rise to, or enters the tractus soli- tarius. No facialis fibers appear in the tractus solitarius caudal to the entrance of the first radix sensibilis nervi vagi, and no glossopharyngeal fibers are present in the tractus solitarius caudal to the entrance of the third radix sensibilis nervi vagi. In the guinea pig four distinct sensory fasciculi or roots of the X nerve were recognized. The first two are composed of from two to four bundles or rootlets and are readily comparable to the sensory roots of the VII and IX nerves; while the last two consist of one bundle each. The various roots entering into the formation of the tractus solitarius, cross ventrally the previously formed portion of the tractus solitarius and for the most part continue caudally median to it. This signifies that the VII fibers could be separated laterally from the tractus solitarius with little injury to the IX fibers, and likewise the IX with little injury to the first root of the X. It was recorded, however, that there was some inter- mingling of these root fibers. None of the sensory fibers of the VII, IX or X (having cell bodies in the ganglion nodosum) nerves are distributed anywhere but to the tractus solitarius. If the relative size of degenerated myelin is any indication of the corresponding thickness of medullary sheaths, the VII and IX sensory fibers would be finely medullated and the X would be coarse. In the guinea pig the nucleus tractus solitarii extends through- out the entire length of the tractus solitarius. At first it is a light area situated ventro-medially to the tractus solitarius, which soon expands, joining other nuclear masses in the. formation of the so-called dorsal vagus nucleus. This oval nuclear mass 200 WILLIAM F. ALLEN can be divided into three parts: A median mass of cells, the nucleus motorius dorsalis nervi 61ossopharyngei et vagi; a lateral mass of cells next to the tracti solitarius, the nucleus tractus solitarii; and an intermediate mass of small cells may represent a visceral reticular nucleus for short relays in the medulla. Cau- dally to the entrance of the last sensory root of the vagus the nucleus tractus solitarii assumes enormous proportions, dorsally and medially, to ultimately unite with its fellow in the spinal portion of the medulla in forming the nucleus comraissuralis. It was noted that the maximum number of facialis tractus solitarius fibers are dispensed to the nucleus tractus solitarii opposite the entrance of the IX sensory root, and that none are found opposite the entrance of the first vagus sensory root. Between the entrance of the IX and first X sensory roots, numerous glossopharyngeal tractus solitarius fibers are distributed to the nucleus tractus solitarii, but none appear caudal to the entrance of the third vagus sensory root. Very few vagus tractus solitarius fibers are present in the nucleus tractus solitarii opposite the entrance of the vagus sensory roots. Caudad, these fibers become very abundant, especially in the nucleus commissuralis. In at least two places vagus fibers or collaterals are distributed from the tractus solitarius to the nucleus motorius dorsalis nervi vagi (fig. 1, S.). One of these areas is in the caudal part of the medulla and the other is in the medulla spinalis. In many other places the neurons of this motor nucleus would not have to possess very long dendrites to come in contact with fibers from the tractus solitarius. No fibers or collaterals from the tractus solitarius were seen in the nucleus nervi hypoglossi, in the nucleus parasolitarius, or in the nucleus nervi vestibularis spinalis. A so-called tractus (Z), which may be the same as Kosaka and Yagita fasciculus solitario-spinalis or Hirose bulbospinale tract, was briefly mentioned on account of its intimate association with the tractus solitarius in the medulla spinalis region. Since practically all, if not all, of the taste fibers are carried by the VII and IX nerves, and these fibers have ended in the tractus solitarius at the level of the entrance of the third radix TRAcrus SOLITARITTS IN THE GUINEA PIG 201 sensibilis nervi vagi, it is evident that no part of the tractus solitarius of the guinea pig, caudal of this level, is concerned with conducting taste sensory impulses. It must be associated with general visceral and possibly muscle sense. Visceral or smooth muscle sense will be included under general visceral sensations. Such endings have been described by Ploshko between the smooth muscle bundles of the trachea; muscle of the stomach and intestine, and by Larsell in the smooth muscle of the bronchial tree. Parts or all of the nucleus tractus solitarii, cephalad of the entrance of the third radix sensibilis nervi vagi into the tractus solitarius, must also relay nervous impulses of general visceral and possibly muscle sense in addition to those of taste. The description of the entrance, disposition and distribution of the VII, IX and X nerves in the tractus solitarius suggests an original segmental grouping of these roots, corresponding to the existing segmental arrangement of the motor roots throughout the medulla. Apparently certain segmentally arranged sensory roots have been massed in three places and joined with little intermingling, in forming the tractus solitarius, which extends caudad to the level of the first cervical nerve. To reproduce the original primitive segmental condition, all that would be necessary would be to scatter the sensory roots of the VII, IX and X nerves into the intervals between and caudal of these roots, where no sensory root fibers are penetrating the medulla. If any reduction has occurred in the sensory system of the me- dulla in mammals, it is in connection with the somatic and not the visceral system. As a result of this study the writer has been impressed with the possibilities of the caudal part of the medulla and the medulla spinalis as a region for correlating the vital activities of the body. 202 WILLIAM F. ALLEN LITERATURE CITED Anrf;xs EAPPERS, C. U. 1920 Die Vergleichende Anatomic des Nervensy,stems der Wirbeltiere und des Menschen. 2 Abschnitt. 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