Book - An Atlas of the Medulla and Midbrain 6
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Sabin FR. and Knower H. An atlas of the medulla and midbrain, a laboratory manual (1901) Baltimore: Friedenwald.
|Florence Rena Sabin (1871 - 1953) and her collaborator presents one of the very earliest atlases of the human central nervous system, describing the midbrain and brainstem. This atlas was extremely useful for later researchers attempting to both understand the development and mapping of the midbrain and medulla. Florence Sabin later work was as a key historic researcher in early 1900's establishing our early understanding of both vascular and lymphatic development in the embryo.
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Chapter VI.The Cerebral Nerves And Their Nuclei
The Lateral Group
The nerves of the lateral group are the 1ST. accessorius, JN". vagus, N". glossopharyngeus, ~N. cochleae, "N. vestibuli, N. intermedius, N". facialis and N". trigeminus. Instead of considering them serially, I propose to divide their nuclei into motor and sensory groups. The motor group contains four nuclei, the nucleus 1ST. accessorii, the nucleus ambiguus of the "N. vagus and !N". glossopharyngeus, and the nucleus ~N. facialis and the nuclei "N. trigemini, while the sensory group has many nuclei belonging to the N. vagus, "N. glossopharyngeus, N. acusticus, 1ST. intermedius and N. trigeminus. This grouping is of value in two ways: it brings out the contrast (1) between the motor nuclei of the median group and those of the lateral, and (2) between the motor and sensory nuclei of the lateral group. Both of these distinctions are, in reality, distinctions of development. In Plate v is shown the lateral group of motor nuclei and the course of the root-fibres of the nerves. The lateral funiculus of the cord has been removed in part in order to show the floor of the trough in which lie the ventral-horn cells.
A. Motor Nerves Of The Lateral Group
(a) N. accessorius (Fig. 25). The motor nuclei of the lateral Lateral motor group, as well as of the median group, show in Plate v. The part nerves " of the model representing the spinal cord has been added on from another series in this plate. Into the cord no details have been introduced. Thus it happens that only the extreme proximal part of the spinal portion of N. accessorius is illustrated. With regard to the study of the " vagus portion " of the nerve, it was not possible to find the fibres in the longitudinal sections and so it does not appear in the model.
Spinal portion. The nucleus of the spinal part of the N. Nucleus, N. . accessorius has not been represented. At the beginning of the medulla oblongata the large motor cells of the ventral horn scatter out into the formatio reticularis, so that it is quite impossible to group them into a nucleus. As has been said, some of these motor cells can be traced all the way from the definite ventral horn to the nucleus of the hypoglossal nerve. On the other hand, the cells lying farther to the side give rise apparently to the rootfibres of the "N. accessorius. In brief, here is the point at which the motor cells of the cord divide into the two groups, the median and the lateral groups of the medulla.
Root-fibres, N. H. It will be seen in the model (Plate v) that at least some of the root-fibres of the E". accessorius, instead of passing directly from their cells of origin to the lateral groove, pass dorsalward and then turn ventral ward and lateral ward toward the superficial origin. 1 The fibres, in their dorsal course, I shall refer to as the pars prima, and, in the ventrolateral course, as the pars secunda. Inasmuch as the bundle in the model represents but a few fibres of the nerve, it can be taken to represent the nerve as a whole only in a general way. The general direction of the loop of the fibres is of interest, namely, that the loop is placed obliquely with reference to the transverse plane and the pars prima lies slightly farther toward the cord than the pars secunda. From this it will be seen that the pars prima and pars secunda would not be cut in the same transverse section. This is true, however, only in part, for certain of the fibres do not pass so far dorsalward, and these fibres lie in approximately the same transverse plane throughout their course (Fig. 25). On the other hand, Fig. 26 shows the longer fibres of the !N". accessorius and includes only the pars secunda. The presence of these longer and shorter fibres shown in a diagrammatic form, gives an interesting suggestion of an easy transition from the condition of the cord.
N. M in sections. The two transverse sections given above represent the spinal portion of the nerve. The same portion is shown in the longitudinal series in Figs. 17 and 16. Fig. 17 shows a very definite pars secunda and a small pars prima, while the second section shows these two portions of the nerve nearer together, that is passing toward the loop.
The question of a "vagus portion" of the nerve has been a difficult one; first, on account of the indefiniteness of the nucleus ambiguus, and, secondly, on account of the difficulty of finding the fibres in longitudinal sections. A definite nucleus of the vagus part of the ~N. accessorius has not been made out in either series, but there are motor cells scattered throughout the formatio reticularis of the region between the N. accessorius and the nucleus ambiguus shown in Plate v.
1 Edinger, L. Bau der nervosen Centralorgane, Leipz. (1893), S. 168.
In tracing 1 the transverse series from the distal end of the medulla, where the spinal part of the nerve is clearly made out (Figs. 25 and 26) to a region, which is clearly that of the vagus nerve, it is certain that there are fibres corresponding to the description of the N. accessorius (Figs. 28 and 29). In the sections of this region certain points are of interest. First, no section contains a definite pars prima, though certain sections, as Fig. 27, have small bundles of fibres within the central gray matter suggesting a pars prima; a few sections show fibres of the nerve making a loop as well as a pars secunda. From a complete series of the N. accessorius it would be easy to construct a diagram showing how the N. accessorius is a transition in form between the nerves of the spinal cord and the N. vagus.
In the longitudinal series the nerve-fibres of the vagus portion must be cut across or nearly so and thus are exceedingly difficult to trace.
Near the level of Fig. 14 a few delicate fibres are seen cutting across the lateral funiculus of the cord. They were not definite enough to introduce into the model.
(b) N. glossopharyngeus and N. vagus (Fig. 30). The motor N . IX and x fibres of the N. hypoglossus and N. vagus make the second member 2JbSuu8? uc of the lateral motor group (Plate v). The nucleus ambiguus could not be determined in the horizontal series, for the longitudinal fibres of the formatio reticularis wholly obscure the cells. An oblong block representing the position of the nucleus has been made from a comparison with the transverse series and introduced into the model. This block does not represent the shape of the nucleus. In determining the length of the block, only those sections have been included which show a definite group of cells related to the fibres of the !N". glossopharyngeus and 1ST. vagus (Fig. 33). In this way the proximal limit of the nucleus could be made out definitely, but the distal limit was indefinite. I did not include in the nucleus any of the scattered motor cells lying in the formatio reticularis between the levels of the distal end of the dorsal accessory olive and the lower end of the medulla, though these cells are commonly called a part of the nucleus ambiguus. The block in the model corresponds to the region of the root-fibres of the glossopharyngeal and vagus nerves. It lies in the formatio reticularis, dorsolateral to the dorsal accessory olive and lateral from the tract from Belters' nucleus to the spinal cord (Fig. 33). It lies at the same horizontal level as the nucleus IN", facialis (Plate v).
Root-fibres, The root-fibres, starting from the nucleus, pass medialward and dorsalward to the floor of the fourth ventricle. In this part of their course I have called the fibres a pars prima. At the floor of the ventricle the fibres turn sharply to pass ventralward and lateralward, thereby making a pars secunda. The fibres of the pars secunda take the same course as the entering sensory fibres passing to the ala cinerea (Plate iv). In this plate it is well shown that the fibres of the two nerves pass through the tractus spinalis E". trigemini. The loop made by the root-fibres of the !N". glossopharyngeus and N. vagus lies transversely, so that the nucleiis, pars prima and pars secunda all show in one section. In longitudinal series the fibres are cut across, and the pars prima thus becomes too delicate to be made out, while the fibres of the pars secunda are plain (Fig. 12).
Nucleus, N. vii. (c) N. facialis (Fig. 38). The N. facialis is the third nerve of the group (Plate v). Its nucleus lies at the same horizontal level as the nucleus ambiguus. It is situated in the distal part of the pons just dorsal to the level of the corpus trapezoideum. Moreover, it lies in the slight angle which the tract from Deiters' nucleus to the spinal cord makes with the lemniscus later alis. The nucleus is a compact, oval mass of cells with its long axis parallel to the long axis of the pons. Its length is about 2 mm.
Root-fibres, The course of the fibres is well known. The pars prima leaves L the dorsal surface of the nucleus in the form of a round bundle 1 mm. in diameter. In this bundle the fibres are distinct and somewhat scattered. It passes medialward and dorsalward to the distal surface of the nucleus ~N. abducentis. Here the bundle curves around the latter nucleus, making the knee, or genu internum (Plates in, iv and v). In the knee the fibres form a small compact bundle which runs directly cerebralward along the medial margin of the nucleus, then turns sharply lateralward along the proximal border. This second part of the knee slopes very slightly ventralward (Plate v), and ends abruptly at the lateral margin of the nucleus, where the bundle turns ventralward. The pars secunda is a small dense bundle more than twice as long as the pars prima. It plunges into the corpus trapezoideum just lateral to the nucleus olivaris superior, and leaves, just ventral to the bundle which enters the trapezoid body from the ventral nucleus of the cochlear nerve. From here the bundle passes to its superficial origin at the distal end of the pons. In Plate v it can be seen that the pars prima and knee pass forward, that is, toward the cerebrum, but the pars secunda passes so far toward the cord that the superficial origin is in the same transverse plane as the nucleus.
The course of the fibres can be best followed by noting the lines of N. vn in the sections on the diagram in Fig. 52 and comparing them with Plate sections. v. For example, the section Fig. 36 passes through the nucleus and the pars prima, Fig. 37 includes the longitudinal part of the knee and part of the pars secunda, while Fig. 38 shows the transverse part of the knee and the pars secunda.
In tracing the nerve in the longitudinal series it will be best to start with a dorsal section and thus pass toward both the nucleus and the superficial origin (Figs. 6 to 19). Fig. 6 shows the longitudinal and Fig. 7 the transverse fibres of the knee. From this it can be seen that transverse fibres are farther ventral than the longitudinal. In Fig. 9 the fine fibres of the pars prima (N. vn, a.) are in contrast to the dense black bundle of the pars secunda. The crescent shape of the pars secunda is maintained almost the entire extent of the bundle. Fig. 12 shows the nucleus of the facial nerve with the pars secunda just lateral from it. The nucleus is seen similarly in the next section (Fig. 13) , but here the pars secunda is found plunging into the corpus trapezoideum. Within the trapezoid body the fibres scatter but can be made out in almost every section (Fig. 16). In emerging from the trapezoid body, the bundle again becomes clear and distinct (Fig. 19).
(d) N. trigeminus (Fig. 40). The last motor nerve of the lateral group is the N. trigeminus. The 4 entire nerve is shown in Plate iv, and the motor part can be seen in Plate v.
The nucleus motorius princeps !N". trigemini, lies in the middle Nucleus, N. v of the pons, just proximal to the transverse part of the knee of the (l facial nerve. It is at a level ventral to the nucleus of the N. abducens and dorsal to the nucleus of the N". facialis. The distal end lies opposite the proximal end of the nucleus olivaris superior. The nucleus is oblong in shape, and its long diameter (1.6 mm.) lies in a dorsoventral direction. From the distal dorsal angle a small spur projects. This part of the nucleus shows on Figs. 9 and 38.
In comparing the. motor and sensory roots of the N. trigeminus, R 00 t-nbres, as seen on Plate n, the two bundles seem of equal size; nevertheless, N ' v < motor )the motor root has been called the portio minor and the sensory the portio major. The reason of this apparent discrepancy is plain in Fig. 16, which shows that the motor fibres, which are few and scattered, cover as great an area as the closely packed sensory fibres. The fibres of the motor root border the entire lateral surface of the motor nucleus (Plate v). Against the nucleus the fibre bundle is curved so that in longitudinal section it forms a crescent (Fig. 8), but ventral to the nucleus it becomes triangular in section and passes directly ventralward, parallel to the entering sensory bundle (Fig. 16).
In longitudinal series the nucleus shows well on Figs. 8 and 9; the root-fibres can be traced through Fig. 21. The distinction between the motor and sensory roots shows especially clearly (Figs. 13 to 19). In the transverse sections the motor nucleus shows on Figs. 38 and 39. The fibres adjacent to the nucleus show in Fig. 39. Ventral to the nucleus they cannot toe definitely separated from the sensory root-fibres.
The question of the radix descendens !N~. trigemini is a difficult v one. The bundle is easy to model throughout most of its course, but at a most important point, namely, the distal end, where it comes into relation with the main part of the nerve, it is difficult to trace. The radix descendens (mesencephalica) ]^. trigemini, starts at the distal border of the stratum album profundum (Plate v). At first sight it looks as if the fibres of the deep white layer might be continuous with the nerve, but closer inspection shows that in reality the nerve bundle lies just medial to the deep white layer (Plate in). The descending root traverses the pons as a narrow bundle (.2 mm. in width) parallel to the raphe. The dorsoventral diameter of the bundle measures on the average l.>2 mm., and throughout this part of its course the bundle lies eniLOCUS cajruieus. bedded in the locus caeruleus (Plate in). The locus cseruleus is a long flat nucleus, triangular in shape (Fig. 6). (It is not labeled, but it lies just proximal (above) the line marked ff). It extends about half the length of the pons. 1 Within the locus caeruleus the nerve fibres are scattered, but opposite the main motor nucleus of the nerve the bundle becomes small and compact and then plunges directly ventralward. 2
1 The locus cseruleus of the model probably includes the cells related to radix descendens (m) N. trigemini.
2 This agrees with the description of Eamon y Cajal. Op. cit., S. 14.
These relations will be made clear in the longitudinal sections before N. v in sections, passing on to the more difficult question of the distal end of the bundle. The longitudinal sections are very valuable for the study of this part of the nerve, for a single section shows the entire course of the fibres from the proximal end of the bundle to the point at which the bundle turns ventralward. Fig. 5 shows the break in the continuity of the stratum album profundum just opposite the nucleus colliculi inferioris. The nerve bundle in this section corresponds to the bundle in Plate n of the model. Fig. 6 shows the locus caeruleus (not labeled) and the fibres of the descending root within it.
At the level of Fig. 7 there are many fibres apparently connecting the gTay matter of the pons and midbrain, and these fibres are hard to distinguish from those of the root of the N. trigeminus. After determining the proximal end of the nerve in the longitudinal series it is not difficult to fix the point on the transverse series by noting the shape of the stratum album profundum. This will be clear by tracing the series (Figs. 51 to 45). On the last section will be noted the distinct bend of the bundle toward the median line.
The distal or spinal end of the nerve is difficult to trace in sections. Starting with Fig. 5 the distal end of the nerve turns ventralward and is seen in Fig. 6 just lateral to the bundle marked 0. In Fig. 7 the fibres are near little groups of cells. In Fig. 8 the relation to the motor root is evident. The black coarse fibres of the motor root make a contrast with the delicate fibres of the mesencephalic root just lateral. Below this level the fibres are difficult to trace, but, together with the little groups of cells, they are seen as far as the level of the section (Fig. 9) which passes through the sensory nucleus of the 1ST. trigeminus (substantia gelatinosa Kolandi).
In the transverse series the fibres are cut longitudinally in their ventral course and so can be followed to much better advantage. In Fig. 39, it will be noted, that on the left side of the section the mesencephalic root runs directly past the motor nucleus, through the sensory nucleus to the root of the N. trigeminus. This section should be compared with a Golgi section of a foetal mouse given by Eamon y Cajal 1 in which the descending root definitely enters the main motor nucleus. In Fig. 39 the bundle on the left side just as definitely passes by the motor nucleus, but on the right side it runs directly to the motor root where it lies adjacent to the motor nucleus, thus corresponding with Eamon y Cajal's picture. Section Fig. 38 shows the mesencephalic root splitting up as it enters the sensory nucleus. Thus it is clear that neither the sections nor the model decide whether the mesencephalic root is related to the motor nucleus, to the sensory nucleus, or to the root-bundle. The locus cseruleus shows in Fig. 41 just external to the posterior longitudinal bundle.
In the view of the dorsal surface of the model (Plate in) can be seen a small bundle of decussating fibres which lies just proximal to the transverse part of the knee of the facial nerve. The bundle has been cut off to show the motor nucleus of the ~N. trigeminus.
1 Ramon y Cajal. Op. cit., Fig. 4, S. 15.
In reality, it spreads over this motor nucleus (see Fig. 7, N. v. dec.). This decussation has been called a motor decussation. Ramon y Cajal 1 says, however, that in no case has he seen any evidence of a motor decussation of the trigeminal nerve. The model adds weight to the view that this decussation is sensory, for the bundle passes directly over the motor nucleus and ends dorsal to the proximal end of the sensory nucleus.
Having now described the separate nerves of the lateral motor
group and their nuclei, it will be well to note the points which
they have in common. In this consideration the radix descendens
!N". trigemini, will be left out, since I am unable to place it.
summary of the The group characteristics . are as follows: (1) The four motor
lateral motor - cerebral nerves, nuclei in the series all lie in the lateral region at a level distinctly ventral from the central canal. (2) They all lie in the formatio reticularis. (3) As regards the course of the fibres, it is easy to group the root-bundles of the first three together, namely, those of the "N. accessorius, "N. glossopharyngeus et vagus and !N". facialis, inasmuch as each root-bundle has a pars prima and a pars secunda. The first two form a sharp angle at the junction of these two parts, while the N". facialis, in curving around the nucleus E". abducentis, has its angle developed into a " knee." The pars prima of all these nerves consists of fine scattered fibres, the pars secunda of dense compact bundles. Such an agreement, of course, cannot but suggest development under similar circumstances. The motor root of the E". trigeminus, however, forms no such loop. Its fibres pass directly ventralward from the main nucleus. In considering this point, it is to be noted that its nucleus lies farther dorsal than any of the other nuclei of the group, and that the horizontal plane occupied by the nuclei of the group is filled in this region, not by formatio reticularis fibres, but by the definite and compact bundles of the lateral and medial lemnisci. The motor part of the IS. trigeminus then seems to lack a pars prima.
B. Sensory Nerves of the Lateral Group
sensory nerves. The model shows the nuclei of four sensory cerebral nerves: (1) the sensory part of the 1ST. vagus and 1ST. glossopharyngeus, (2) N. intermedius, (3) the N. vestibuli and (4) the T. cochleae. The optic nerve, though it has an end station in the superior colliculus, will not be included in the group, for it is probably not comparable with the other cerebral nerves, but corresponds rather to a tract inside the central nervous system. 1 All of the nuclei of this group lie in the lateral region dorsal to the level of the lateral motor nuclei; this is in accord with their correspondence morphologically to the dorsal horn of the cord.
1 v. Ramon y Cajal. Op. cit., S. 16.
(a) N. glossopharyngeus and N. vagus (sensory) (Figs. 30 and N. 33). In describing the sensory nerves, I shall start in each case with the fibres as they enter the neural tube. The entering fibres of the glossopharyngeus and vagus nerves can be divided readily into two distinct groups : first, scattered fibres passing directly to the ala cinerea, and, second, a compact bundle passing into the tractus solitarius. Almost immediately after entrance, the fibres of both sets plunge either through or over the tractus spinalis !N". trigemini, and hence are shown in the plate that represents that nerve (Plate iv). The ala cinerea and the tractus solitarius, however, are seen best from the dorsal surface (Plate in).
Fibres to the ala cinerea. The entry zone of the root-bundles Root-fibres to
j* 11 i ala cinerea.
01 the glossopharyngeal and vagus nerves corresponds to about the proximal third of the medulla oblongata (Plate iv). The fibres passing into the ala cinerea cover fully two-thirds of this area. Their entry zone begins just proximal to the nucleus funiculi cuneati (Burdach), at about the point where the most proximal of the direct cerebellar fibres pass dorsalward to enter, the corpus restiforme (Plate n). It extends to the place of entry of the compact bundle making the tractus solitarius. The fibres to the ala cinerea enter as small scattered bundles, not to be distinguished from the motor fibres passing out. Since these bundles are too small to be modelled accurately, they are represented diagrammatically in Plate iv. The sensory fibres pass dorsalward and medialward through the fibre-bundle and nucleus of the tractus spinalis N. trigemini. Beyond this nerve they continue in a direct course to the floor of the fourth ventricle. Here the fibres end in the nucleus alse cinerese. In entering the nucleus, the fibrebundle as a whole does not split into an ascending and descending bundle, as do the trigeminal and vestibular root-bundles. 2
1 Barker, op. cit., pp. 541-543 and pp. 782-783.
2 Eamon y Cajal, op. cit., S. 44. Barker, op. cit., S. 479.
Nucleus ai The nucleus alse cinerese, as seen in Plate in, is a long narrow nucleus placed obliquely across the floor of the fourth ventricle. It lies parallel to the tractus solitarius, and is, at the same time, both dorsal and medial to it. The extent of the nucleus is about 6.2 mm., and its proximal end is 2.9 mm. from the median line. The distal end is placed directly dorsal to the nucleus "N. hypoglossi near the raphe. The nucleus is nearly uniform in width, averaging about 1 mm. The model shows an interesting relation between the nucleus and the root-bundle, namely, that the root-bundle runs simply to the proximal end of the nucleus, and does not cover even a half of its length. Moreover, the fibres from the entering bundle cannot be traced running to the distal end of the nucleus, as in the tractus solitarius. However, there are fibres running between the nucleus alaa cineress and the tractus solitarius throughout the region not covered by the root-bundle fibres to the nucleus alee cinerese 1 (Plate in).
Ala cinerea The full extent of the nucleus alas cinerese can be seen in Figs. 5 and 6.
in sections, rp^e secon( j section shows the nucleus more clearly, for the cells have taken the stain deeply. It is not possible to outline the nucleus accurately with low powers, for there are almost no fibres within the nucleus. Both sections show the fibres crossing 1 between the nucleus alas cinerese and the tractus solitarius. With a Leitz Obj. 3 these fibres are much more abundant than they appear in the low-power drawings. In transverse series the nucleus shows in Figs. 31 and 29. In the second section it is made out only with high powers.
Fibres to tractus Tractus solitarius (Fig. 34). The fibres that make up the 1US ' tractus solitarius enter as a definite, compact bundle on the lateral surface of the proximal part of the medulla (Plate iv). This bundle enters at a level proximal to the entry zone of the fibres which go to the nucleus alse cinerese. The bundle passes dorsalward along the lateral surface of the tractus spinalis N. trigemini, rather than plunging obliquely through it, as do the fibres to the nucleus alee cinerese (Plate iv). Thus it happens that the fibres which go to the tractus solitarius do not pass through the substantia gelatinosa (Fig. 34). At the 'dorsal surface of the tractus spinalis !N". trigemini, the bundle arches medialward and dorsalward, curving over the substantia gelatinosa Rolandi, and then passes spinalward between the nucleus alee cinerese and the nucleus funiculi cuneati (Plate in). The bundle in the model includes not only the fibres of the tract, but also the cells which surround it, namely, its f^ e jf s J itaritls nucleus. Cells can be traced all along the length of the tract, but in two regions the nucleus is especially well developed, namely, at the proximal and distal ends. The nucleus of the proximal end is a small mass of cells which lies medial to the root-bundle at the point, where it turns spinalward, just ventral to the level of the medial vestibular nucleus (Plate in). It is not shown in the plate, but it lies between the proximal ends of the tractus solitarius and the nucleus alse cinerese. This nucleus extends farther cerebralward than the root-bundle fibres, and in this connection it should be noted that the entire root-bundle turns spinalward. The nucleus is readily made out in longitudinal sections by the basket-work-like arrangement of its cells and fibres (Fig. 7, Nu. tr. Sol.). It has been suggested that this part of the nucleus is related to the fibres of the 1ST. intermedius.
1 According to Held certain of the root-fibres send one branch to the tractus solitarius and the other to the ala cinerea. Held, H., Die Encligungsweise der sensiblen Nerven im Gehirn., Arch. f. Anat. und Physiol., Anat. Abth., Leipz. (1892), S. 33-39.
The nucleus of the distal end of the tractus solitarius, possibly the commissural nucleus of Ramon y Cajal 1 is seen in Plate in. The fibres of the tractus solitarius lie parallel to the nucleus alse cinerese; but beyond the spinal end of the nucleus alse cinereae a bundle of the fibres from the tract turns ventralward and plunges into a small nucleus which lies close to the raphe just medial to Burdach's nucleus. The complete course of the tractus solitarius, measuring from the proximal edge of its root-bundle to the distal end of this commissural nucleus is 9.1 mm.
The points illustrated by the model are readily confirmed in sections. Tractus solitarius The compact entry bundle, and the fibres running to the distal nucleus, in sections, or commissural nucleus of Barnon y Cajal, are shown in Fig. 9. The entry bundle is more definite on the left side of the section.
The series from Fig. 7 to Fig. 5 will show the relations of the tract itself; its position, its oblique course, its relation to the nucleus alse cinerese and to the nucleus funiculi cuneati. The sections in which the fibres are fewest, show best the cells that accompany them, as well as the fine fibres crossing to the nucleus alae cinereae. In Fig. 7 the nucleus of the proximal end is seen, but the nucleus of the distal end is farther ventral.
In transverse section, the distal nucleus is well shown in Fig. 28. The tract itself hardly needs illustration, Fig. 31. Fig. 34 shows how the definite compact bundle skirts along the border of the fibres of the tractus spinalis N. trigemini, and passes medialward to the proximal nucleus of the tract. This proximal nucleus is still to be seen in Fig. 35 (left side).
1 Ramon y Cajal, op. cit., S. 46. Barker, L. F., op. cit., S. 486.
(6) N. trigeminus (Fig. 39). The motor part of the N. trigeminus has already been described, but it will be of value to refer to it again on account of its sensory part. Plate iv is designed to show the entire nerve which, it will be noted, covers almost the extent of the model, namely, the entire medulla oblongata, the pons, and at least half of the midbrain.
Sensory root of The sensory fibres of the X. trigeminus enter the lateral surface ' spi r naii" s of the pons just distal to the motor bundle. The fibres form a compact bundle, which passes nearly directly dorsalward past the lemniscus lateralis into the region of the pars dorsalis pontis (Plates ii and iv). After passing dorsalward a distance of 6.2 mm. from the surface of the pons, the bundle bifurcates into a Y. Into the notch of this Y fits the upper end of the nucleus. The lateral and larger arm of the Y passes dorsalward a distance of 2.7 mm. and then turns at an angle of 90 toward the spinal cord, making the long tractus spinalis N". trigemini. The medial and smaller arm of the Y passes dorsalward just medial to the nucleus. It does not show in the plates, being hidden by the nucleus; but it grows thinner in its dorsal course as if many of its fibres were ending. The model suggests that the inner arm of the Y contains the ascending branches of the bifurcating root-fibres, and that they end in the proximal part of the nucleus which makes a wedge in the fork of the Y.
The tractus spinalis N. trigemini is unfortunately shown in the plate only from the lateral aspect in which it appears as a long band of fibres (Plate iv, labeled Radix N. trigemini). It was not introduced into the view from the medial aspect (Plate vn), inasmuch as it would entirely hide the cochlear nuclei and their relation to the corpus trapezoideum. Its position can be imagined on the left side of Plate vn. It extends from the middle of the pons to the substantia gelatinosa Rolandi of the spinal cord, which is to be seen just lateral to the nucleus funiculi gracilis. It lies medial to the cochlear nuclei and radix N. facialis, pars secunda, and lateral to the nuclei of the dorsal funiculi.
The tractus spinalis N. trigemini is a long tract in the shape of a trough, the lateral wall of which is nearly complete, while the floor and medial wall are complete only in the proximal or cerebral half. However, the division line between the fibre-bundles and nucleus is not a clear-cut one, for fine bundles of fibres pass spinalward between the cells, especially in the proximal part of the nucleus.
The lateral wall of the trough is derived from the expansion of the lateral limb of the Y (Plate iv). As seen from the surface, it is a broad band which gradually narrows in passing toward the spinal cord to a point about 3 mm. from the distal end of the medulla oblongata, where it expands into a sort of cap which fits over the nucleus. This wall completely conceals the nucleus from a lateral view except at the two ends. 1 At the proximal end the large wedge of the nucleus, which fits into the fork of the Y, is only in part covered by fibres, and at the distal or spinal end the ventral part of the nucleus projects beneath the level of the tract. This distal end is concealed in Plate iv by the lateral funiculus of the spinal cord (cf. Fig. 25). The lateral wall of the trough is cut by the root-fibres of the N. glossopharyngeus and -N". vagus in the manner already described. In the proximal third of the tract the lateral wall underlies the descending root of the vestibular nerve, and it w r as impossible to determine the absolute line of separation between the two (Plate iv). As seen from the medial aspect, the lateral wall curves into the ventral wall or floor so that the two make a crescent, in which the nucleus lies. From the medial surface of the lateral wall fibres enter the nucleus all along its course, but at the distal end one large bundle cuts across the dorsal part of the substantia gelatinosa. In consequence of the ending of the fibres, the walls grow progressively thinner. In Plate iv is to be seen the relation of this lateral wall of the tract to the spinal cord, namely, that the tractus spinalis !N". trigemini passes directly into Lissauer's zone. The ventral and medial walls of the trough are not shown in the plates. The ventral wall appears to be simply a curving of the lateral wall underneath the nucleus. It is clearly derived from the lateral arm of the Y and extends a little more
1 For a resume of the different views which have been advanced concerning- the distal end of the tractus spinalis N. trigemini, see Barker, L. F., op. cit., pp. 517-528.
than half the length of the tract. The medial wall is shorter than the ventral. Its fibres are derived from the medial arm of the Y. It fades out at its distal end opposite the area of the motor root-fibres of the glossopharyngeus and vagus nerves (Plate iv), and the internal arcuate bundle from the nucleus funiculi cuneati (Plate vn). These medial descending fibres of the 1ST. trigeminus are most clearly shown in a section by Ramon y Cajal. 1
sensory nucleus, The sensory nucleus of the trigeminal nerve has already been continuation referred to in describing the fibres (Plate iv). The nucleus cong'eiatinosa forms in shape to the tract except at the two ends. The proximal ' end is a wedge which fits into the fork of the Y, while the distal end projects below the ventral border of the tract. The proximal end of the nucleus shows best in the small view accompanying Plate iv. The fact that the entering sensory bundle bifurcates into a Y has already been mentioned and is indicated by the lines on the sensory root-bundle. The medial arm of the Y runs just behind the nucleus, as seen from the side, and is parallel to the motor root, the edge of which shows just dorsal to the sensory nucleus. The proximal end of the nucleus is a forked wedge that fits into this fork of the fibre-bundle. Internal to this wedge of the nucleus and covering much of its mesial surface, are the ascending sensory fibres side by side with the motor root, while external to the nucleus, at least in its ventral part, are the descending sensory fibres. Inasmuch as the nucleus is forked, a little spur of cells must be imagined as projecting on the medial surface of the fibre-bundle to correspond with the ventral tip of the nucleus seen from the side.
The dorsoventral diameter of this wedge of the nucleus is 2.7 mm., fully twice the corresponding diameter of the central part of the nucleus. It consists of cells so closely packed that it is evident that here the nucleus is well developed. Reference has already been made to the fact that the ascending branches of bifurcation of the sensory nerve fibres appear to end in it.
The nucleus of two-thirds of the spinal tract corresponds so closely to the shape of the fibres as not to be seen from the surface view of the model. In the distal third, however, the nucleus again shows an enlargement. This makes the nucleus project ventralward beneath the fibres. In the model the direct cerebellar tract lies just lateral from this part of the nucleus and, therefore, hides it (Plate iv). The point, however, is shown clearly in section (Fig. 25). The lateral view shows that the nucleus of the tractus spinalis N. trigemini is a direct continuation of the substantia gelatinosa of the spinal cord. The dorsal horn of the spinal cord will be seen to swell into a rounded mass in approaching the medulla oblongata (Plate vn), which is continuous with the nucleus of the spinal tract of the N. trigeminus (Plates n and iv).
1 Ramon y Cajal, op. cit., S. 4. The fibres of this tract are shown in Bruce's Atlas, as the descending- fibres of the vestibular nerve.
The proximal wedge, or main nucleus, is continuous with the substantia gelatinosa. The dorsal edge of the nucleus is somewhat irregular. In the proximal third it shows an indentation where the fibres of the lateral wall of the trigerninal trough almost meet the medial wall. At the junction of the upper and middle thirds, the edge slants ventralward as it passes toward the spinal cord. This dorsal edge gradually broadens out into a surface toward the spinal end; this surface is cut obliquely by a bundle of fibres from the lateral wall. Over the end of the nucleus the lateral wall rounds up and fits like a cap. The ventral edge of the nucleus has been described; it is smooth except for the projecting forked proximal end and the bulging, rounded distal end.
The course of the nerve is seen far better in longitudinal sections N v ( Sensor y) than in the transverse (Figs. 19 to 12). Starting with the root-bundle in sections. in Figs. 19 and 17, it will be noticed how easy it is to separate the sensory from the motor root. Fig. 14 shows the ventral tip of the main nucleus or wedge, where it projects even farther ventralward than the tract itself. The next section (Fig. 13) passes through the ventral wall of the tract and shows the main nucleus. Fig. 12 passes through the entire nerve and shows many of its relations. Beginning at the proximal end, the main nucleus is well developed and lies just lateral from the motor root and medial arm of the Y. The medial and lateral arms of the Y are both plainly shown. The lateral arm of the Y becomes the lateral wall of the tract. The medial wall shows best on the right side and it ends opposite the bundle of the fibres arcuatse internse. The nucleus likewise is seen in its full extent; its cells can be traced all the way from the main nucleus to the large substantia gelatinosa of the spinal cord. Between the lateral and medial bundles it is narrow, but toward the lower end of the section, it gradually broadens out, until it projects markedly from the surface of the cord.
The nerve can be traced through a long series of transverse sections (Figs. 39 to 25). Fig. 39 makes an interesting study in connection with Plate iv. It shows (1) the sensory root-bundle at the point of bifurcation into a Y, (2) the forked sensory nucleus which fits into the Y, the medial tip of the fork being very clear, and (3) the motor nucleus with its root-bundle just external. In the next five sections can be seen the nucleus, the thick lateral and ventral walls, and the thin medial tract. In Fig. 31 only the lateral fibre Avail remains, but the nucleus is growing larger. Fig. 29 shows the lateral wall becoming more massed against the dorsal part of the nucleus, while in the last section, the dorsal fibre cap is clear. In this section the nucleus is evidently the rounded substantia gelatinos'a.
In describing the motor nucleus, I referred to a decussation lying in the floor of the fourth ventricle just proximal to the transverse part of the knee of the facial nerve. The model can not settle the question as to whether this decussation is motor or sensory, but it certainly adds weight to the idea that it is sensory, for the fibres pass beyond the motor nucleus.
N. vestibuii, vm. (c) N. vestibuli. Thus far, in considering the sensory nerves, it has been found, first, that their nuclei are related to the dorsal part of the medulla oblongata and pons, and, secondly, that the intracerebral parts of the nerves consist in the main of long descending tracts with cells immediately adjacent. One part of the acoustic nerve, namely, the vestibular, conforms in the main to this type, while the other part, the cochlear nerve, is wholly different. The cochlea* and vestibular nerve-roots enter together at the lower end of the pons (Plate n). The vestibular bundle lies proximal to the cochlear and is easily distinguished from it. The two nerve-roots pass dorsalward together, showing a slight spiral curve around each other so that the cochlear root partially surrounds the vestibular (Plate u). The cochlear nerve soon breaks up into the nucleus ~N. cochleae ventralis, which, together with the fibres passing from it, completely surrounds the vestibular root-bundle (Plates n and vn). Dorsal to the level of the ventral cochlear nucleus, the vestibular root continues its course between the corpus restiforme and the tractus spinalis N. trigemini until it reaches a level dorsal to the latter; here the bundle breaks up into ascending and descending bundles which lie exactly parallel to the tractus spinalis N". trigemini (Plates n and iv). The vestibular nerve does not cover as extensive an area in the model as does the "N. trigeminus. In reality, the areas of these two nerves cannot be compared, however, until the full course of the vestibular fibres to the cerebellum is known.
The view of the dorsal surface of the model shows the general relation of the ascending and descending vestibular fibres (Plate in). The point at which the root divides into its two parts is in the same transverse plane as the distal surface of the nucleus IN", abducentis. Ascending and The descending fibres make a long, narrow tract which passes toward the cord parallel to the corpus restiforme. The dorsoventral diameter of the tract is greater than the transverse (cf. Plates in and iv). In Plate in can be seen that the descendingtract is tilted so that the dorsal surface is nearer the median line than the ventral.
The ascending bundle is broader than the descending. It passes obliquely lateralward and cerebralward adjacent to the corpus restiforme (Plate in).
In order to understand the ascending bundle in the model, it is necessary to make certain explanations: (1) its relation to the cut made in removing the cerebellum and (2) its relation to the superior vestibular nucleus. In regard to the cerebellum, its relation to the model is best seen in Plate 11, where the cut surfaces of two of the cerebellar peduncles, namely, the corpus restiforme and brachium conjunctivum are plain. In Plate in the cut surface extends across (1) the edge of the corpus restiforme, (2) the surface of the ascending vestibular bundle, (3) the edge of the adjacent superior vestibular nucleus, and (4) the edge of the brachium conjunctivum. Thus it is seen that in cutting off the cerebellum a portion of the ascending vestibular bundle has been removed. The relation of the ascending fibre-bundle of the model to the nucleus N. vestibuli superior will be plain in Fig. 5. On the right side of Fig. 5 the line labeled ISTu. n. vest. S. ends in the lateral part of the nucleus. This part of the nucleus contains the ascending vestibular fibres and has been modelled as a fibre-bundle (Plate in, R. n. vestibuli), dotted, however, with blue to show that it contains cells also. The superior vestibular nucleus in the model lies just internal to this dotted fibre-bundle and shows in Fig. 5 as the gray mass external to the lateral recess of the fourth ventricle. The bundle of the ascending vestibular fibres is a complex one; it contains at least three sets of fibres and many cells of the nucleus N. vestibuli superior. These fibre-bundles are (1) a lateral set, which lie adjacent to the corpus restiforme (Plate in) (Fig. 5. F. v. c. o.), and run toward the nucleus N. vestibuli lateralis (Plate iv), (2) short scattered fibres that enter the nucleus N. vestibuli superior, (3) scattered fibres that enter the cerebellum from the cut surface shown in Plate in. They constitute a medial bundle. These two bundles will be made plain later in a study of the sections.
vestibuiar Plate in gives the best idea of the form and relations of the ' nuclei of the vestibuiar nerve. In general, the vestibuiar nuclei might be divided into two parts. First, a great mass of cells lying for the most part medial to the vestibuiar tract, including its ascending and its descending roots. It includes the medial and superior vestibuiar nuclei. Secondly, two small masses of cells situated opposite the root-bundle, one lateral and the other medial to the tract and main nucleus. These are the parts of the nucleus N. vestibuli lateralis (cf. Plate in). The vestibuiar nuclei are difficult to outline from the central gray matter both on the medial and the lateral borders.
Taking first the main mass of cells, its outlines are irregular, but they can be related to surrounding structures (Plate in). The nucleus is divided by an hour-glass-like constriction into two unequal parts. This constriction is opposite the root-bundle and its meaning is evident if one notes that into the angle thus formed the lateral recess of the fourth ventricle dips down. This is to be seen by comparing the two sides of Plate in. The nucleus N. abducentis lies opposite this angle of the vestibuiar nuclei. The part of this nuclear mass lying proximal to the constriction corresponds to the ascending fibres and has been called the nucleus N. vestibuli superior, while the distal and larger portion corresponds to the descending root and has been called the nucleus nervi vestibuli medialis. Medial vestibuiar Nucleus N. vestibuli medialis. The nucleus 1ST. vestibuli me' dialis lies in the floor of the fourth ventricle (Plate in) between the nuclei of the N. hypoglossus and "N. abducens. It is 6.8 mm. long. In the proximal part it consists of a mass of cells reaching nearly to the median line. Toward the distal end the nucleus narrows rapidly as it runs along the border of the radix descendens N". vestibuli. The curve made by this rapid narrowing is occupied by the nucleus of the tractus solitarius and the ala cinerea. The dorsal surface of the nucleus is practically level in the lateral part, but the medial part slopes like a terrace toward the middle line.
By this curve of the surface the nucleus conforms in shape to that of the floor of the fourth ventricle. In the depth of the proximal part of the nucleus the cells are closely crowded. This area was referred to as nucleus Y in a previous paper, but the fact that it is an integral part of the nucleus IsT. vestibuli medialis is clearly shown in the model. In among the fibres of the descending vestibular tract can be traced cells which are not shown in the model. These cells have been called the nucleus nervi vestibuli spinalis (radix descendens).
Nucleus N. vestibuli superior. As has been said, a portion of superior the nucleus N. vestibuli superior has been removed in cutting oft' nucleus, the cerebellum, but nevertheless the main features can be made out. The relation of the superior nucleus to the ascending rootbundle has already been pointed out. The superior nucleus and fibre-bundle together extend well into the region of the motor nucleus of the N. trigemiiius. The medial border of the nucleus makes an angle with the medial nucleus and extends forward parallel to the raphe. The lateral border is the line of the cut by which the cerebellum was removed.
Nucleus N. vestibuli lateralis (Deiters 7 nucleus). Thus far the Lateral vestivestibular nuclei, both superior and medial, have conformed to the (Delfe^s'Nu. 8 type of sensory nuclei already described; that is to say, they are masses of cells scattered along, ascending and descending fibretracts. On the other hand, the two small masses previously mentioned as occurring opposite the root-bundle, are quite different in character. Turning to the lateral view (Plate iv) there is a small nucleus, trapezoid in shape, lying embedded in the root-bundle just dorsal to the point at which it divides into ascending and descending branches. On the other hand, from the dorsal view (Plate in), two small nuclei can be seen: (1) the mass just mentioned, lying lateral to the vestibular tract and (2) a mass in the angle between the superior and medial vestibular nuclei. These two masses lie opposite one another at the same dorsoventral level. They are in reality parts of a single nucleus, the nucleus 1ST. vestibuli lateralis, for the motor cells of which they are made are scattered among the fibres that separate the two parts in the model.
The longitudinal sections give the best idea of the vestibular nuclei N. vestibuli and roots. The main points, except the entering" root-bundle can be in sectionseen in two sections (Figs. 5 and 6). The ascending and descendingtracts can be distinguished, for the division line is marked, first, by the nucleus IN", vestibuli lateralis; and second, by the distal border of the nucleus N. abducentis (see Fig. 5). An interesting point in connection with the shape of the descending tract is shown, namely, that its proximal third curves lateralward and thereby gives room for the widest portion of the medial nucleus. The root-bundle can be traced through Figs. 6 to 17. On Fig. 13 it is enclosed by the nucleus N. cochleae dorsalis.
In regard to the nuclei, Figs. 4, 5 and 6 show that there is a continuity between the cells of the medial and superior nuclei as well as all the cells scattered between the ascending and descending root-fibres. The lateral portion of the nucleus N. vestibuli lateralis is plain in Figs. 5 and 6. The medial part is only made out with higher powers. In transverse section the radix descendens N. vestibuli can be traced through Figs. 31 to 36. The last section shows the entering root-bundle. All of these sections show the cells between the fibres and the medial nucleus. The medial nucleus is well developed from Figs. 33 to 36. Fig. 36 shows the root-bundle, the nucleus N. vestibuli lateralis, and the distal part of the nucleus N. vestibuli superior. This nucleus can be traced in Figs. 37 and 38.
Thus far, in describing sensory nuclei, no tracts of the second order from these nuclei, nor any special tracts, save the nerve-roots, have been described. This is because such tracts have not been definite enough to reconstruct. In connection with the vestibular nuclei, however, several such tracts are to be described. (1) Intracts ternal arcuate fibres from the medial nucleus, (2) a commissure co the e ves e tib^iar between Bechterew's nuclei, (3) a tract from Deiters' nucleus to nerve> the spinal cord, and (4) two sets of fibres relating to the vestibular area with the cerebellum.
(1) Thus far only two sets of arcuate fibres have been mentioned, namely, the bundles from the nucleus f uniculi gracilis and nucleus funiculi cuneati. A glance at one longitudinal section (Fig. 9) will show that the entire region proximal to the nucleus funiculi cuneati is very rich in fine arcuates. These are to be related to the sensory cerebral nuclei. Fig. 33 shows such fibres from the trigeminal and vestibular nuclei. In the longitudinal series it can be seen that a great bed of arcuate fibres underlies the medial vestibular nucleus, as can be made out by tracing the nucleus through the sections in Figs. 5, 6 and 7. These fibres were not modelled, since they cannot be separated into a definite group.
(2) In describing the superior cerebellar peduncle, reference was made to the fact that the distal decussation was really related to the superior vestibular, or Bechterew's nuclei (Plates n and viu). This bundle underlies the proximal end of Bechterew's nucleus. The decussating fibres are readily distinguished from the brachium conjunctivum in the model, first, by the fact that they turn lateralward toward the superior vestibular nucleus, and, second, by the fact that they form a ridge on the medial surface of the brachium conjunctivum, which can be traced to the decussation.
The entrance of this bundle into Bechterew's nucleus is best seen in Figs. 6 and 7. Fig. 6 is slightly torn. This decussation has been traced in connection with the brachium conjunctivum through Figs. 7 to 16 and Figs. 43 to 37. In Fig. 37 the brachium conjunctivum meets Bechterew's nucleus.
(3) The bundle from Deiters' nucleus to the spinal cord has already been described and notice taken of the fact that it is not possible in these sections to trace this bundle all the way from the nucleus.
(4) In a previous paper 1 mention was made of two bundles of fibres connecting the vestibular nuclei with the cerebellum, one lying in the medial border of the inferior peduncle or corpus restif orme, the other passing through the superior peduncle or brachium conjunctivum. Fig. 6 (F. v. c. [o.]) shows the lateral bundle with its coarse black fibres cut obliquely. The bundle appears to run toward Deiters 7 nucleus. The medial bundle passing through the brachium conjunctivum cannot be distinguished in the longitudinal series from the fibres ending in the superior vestibular nucleus. In the transverse series it can be traced in Figs. 34 and 35. The contrast between these two bundles is brought out by following their direction. The lateral bundle follows the course of the corpus restiforme, and is seen entering the cerebellum on section No. 182, Fig. 36 (cf. with the oblique lines on Fig. 52). The mesial bundle is last seen in the roof of the cerebellum on section ISTo. 146, Fig. 33. It passes spinalward, while the other bundle passes cerebralward. It is important to relate these two bundles as nearly as possible to the vestibular cerebellar bundles as described by Ramon y Cajal. 2
1 On the Anatomical Relations of the Nuclei of Eeception of the Cochlear and Vestibular Nerves. Johns Hopkins Hosp. Bull., Bait., vol. viii (1897), pp. 253-259.
2 Ramon y Cajal speaks of the vestibular cerebellar bundle as lying in the border of the corpus restiforme and corresponding to the fibres described by Forel, Sala and v. Koelliker, namely, the lateral vestibular fibres of v. Koelliker. He says that the bundle has end stations in Deiters' nucleus, Bechterew's nucleus, Ramon y Cajal's cerebello-acustic nucleus and the nucleus of the roof. It is the medial bundle of my series that runs to the nucleus of the roof (Fig. 35) but this bundle passes through the brachium conjunctivum, not the corpus restiforme.
N. cochiese vm. (c) N. Cochleae. The nerve enters the pons just distal to the N. vestibuli (Plate n). The bundle passes dorsalward a distance of
1.6 mm., growing gradually wider until it spreads out into a nucleus. This nucleus is rectangular in shape, placed vertically against the corpus restiforme. From the surface it appears to be one continuous nucleus, but it has received two names, nucleus N. cochleae ventralis and the nucleus N". cochleae dorsalis. There is a reason for this division, however, even from form alone, for the ventral portion of the nucleus is thick and projects beneath, that is, ventral to the corpus restiforme, while the dorsal part, on the other hand, is a thin layer lying against the surface of the corpus restiforme (Plate n). The division is also justified on histological grounds. I wish to lay especial emphasis on the relative position of these two nuclei on account of an error in a previous paper. The
ventral cochiear previous flat reconstruction was made from cross-sections taken nu <2ntrai pith! slightly obliquely which threw the dorsal nucleus farther spinalward than is shown in the model. In point of fact, the dorsal nucleus lies dorsal to the ventral. The relative thickness of the two nuclei is best seen in the view from a dorsal aspect, which is given to show the relations to the corpus trapezoideum (Plate vn). From what has already been said, it is evident that the cochiear nerve has no long descending tract similar to the other sensory cerebral nerves. The central paths leading from the nuclei of termination of the cochiear nerve pass cerebralward.
Trapezoid body. In Plate vii the fibres from the ventral nucleus can be traced. Two bundles leave the nucleus and pass medialward, forward and ventralward. They enclose the vestibular root, then fuse and pass medialward to become part of the trapezoid body (Plate iv). The trapezoid body has already been mentioned in connection with its relation to the medial lemniscus. It is a bar of fibres stretching across the floor of the pars dorsalis pontis. The bar is thickest in its dorso ventral diameter (2.7 mm. approximately). On either side,
2.7 mm. from the raphe the nucleus olivaris superior lies embedded in the trapezoid body. Between the two nuclei of either side the bar is straight; but lateral to the superior olive the bar curves dorsalward and spinalward owing to the position of the nucleus nervi cochleae ventralis.
superior olive. The superior olive is a mass of cells ovoid in shape. It does not show the scroll shape of the corresponding nucleus of the cat. The other nuclei relating to the trapezoid body could not be outlined with sufficient definiteness to be introduced into the model.
The farther course of the auditory path beyond the trapezoid body is shown in the views from the side (Plates n, v and vi). These views show well the relation of the lateral lemniscus to the corpus trapezoideum. The lateral lemniscus starts as a large Lateral bundle of fibres directly dorsal to the superior olive, passes dorsalward through the pons and into the midbrain. It forms a narrow band which lies at first adjacent to the medial lemniscus, but soon becomes separated from the superior lemniscus by a narrow area.
Though called lateral lemniscus, it does not lie as far lateralward as the medial lemniscus in this region. In the bundle of the model is a long groove which runs the full length of the bundle and opens into the space occupied by the superior olive. This groove contains the nucleus of the lateral lemniscus. The nucleus is, in reality, surrounded by fibres, but the lateral fibre-wall has been removed to show the area occupied by the cells. 1 At the beginning of the midbrain, the band of the lateral lemniscus fuses with the superior inferior lemniscus and becomes molded into the capsule of the nucleus c colliculi inferioris. The shape of this capsule is best seen in Plates vn and vm, which show that the fibres make a lateral, distal and dorsal wall for the nucleus. This capsule is not clearly defined in sections as is the capsule of the nucleus funiculi cuneati. However, in the lateral border of the nucleus the fibres are more densely crowded than elsewhere. Only the distal wall of the capsule is thick, for it receives the main bulk of the lateral lemniscus fibres (Plate vn). The nucleus colliculi inferioris itself is oval in shape, with the long diameter in the dorsoventral direction. It fits accurately into the capsule made by the fibres.
There are four small bundles of fibres to be considered in relation to the lateral lemniscus: (1) a bundle which projects from the proximal edge of the capsule and appears to pass toward the region of the medial geniculate body (Plate in), not labeled. (2) a small bundle which runs between the lemniscus lateralis and the brachium
1 That the nucleus is directly connected with the nucleus olivaris superior is quite in accord with the views of Koller and Held. Kamon y Cajal on the other hand does not ag*ree, inasmuch as the character of the cells and the course of the fibres differ. Eamon y Cajal, op. cit., S. 100.
conjunctivum (Plates in, vn and vm a ). It is placed just distal to the capsule of the nucleus colliculi inferioris. (3) Many scattered fibres appear to leave the medial surface of the capsule to enter the superior colliculus. (4) In Plate in there are two small bundles that decussate dorsal to the central canal and connect the nuclei of the inferior colliculi of the two sides.
Dorsal cochiear Nucleus N. cochleae dorsdlis. Turning now to the dorsal cochstrKSSiaS lear nucleus (Plate n), the model does not trace the fibres from it as completely as those from the ventral nucleus. In Plate in there is a small bundle of fibres the striae acusticse running from the dorsal edge of the nucleus obliquely forward and medialward across the descending vestibular tract. 1 It becomes lost in the medial vestibular nucleus, inasmuch as its fibres probably plunge ventralward to join the corpus trapezoideum, and in so doing pass through a complex formatio reticularis region.
As has been said, the model covers a considerable portion of the cochiear path the dorsal and ventral nuclei, the fibres of the trapezoid body, the superior olive, the lateral lemniscus, the nucleus of the inferior colliculus and the arm of the medial geniculate body.
N. cochleae in The form relations seen in the model are readily verified in the secsections, tions (Figs. 20 to 4). In the longitudinal series, the entering root-bundle of the nerve is seen on Fig. 20. The same section shows a few fibres of the extreme ventral portion of the trapezoid body. In passing dorsalward, the region of the trapezoid body is reached before the ventral cochiear nucleus. In Fig. 16 the trapezoid body is well seen; its relations to the superior olive and the straightness of its fibres between the two olivary nuclei are evident. On the right side of the section can be seen the fibres passing from the ventral cochiear nucleus to the trapezoid body. Fig. 14 shows the micleiis N. cochleae ventralis, as well as the origin of the lateral lemniscus. In passing still farther dorsalward two structures can be traced: (1) the lateral lemniscus passing into the nucleus inferioris colliculi, and (2) the dorsal cochiear nuclei and its striae acusticae. In Fig. 12 the nucleus of the lateral lemniscus is well marked, and on the right side, the beginning of the dorsal cochiear nucleus. The longitudinal sections bring out the contrast between the dorsal and ventral cochiear nuclei well. Compare Figs. 14 and 9. In Figs. 7 to 5 the relations of the lateral lemniscus to the nucleus inferioris colliculi are well brought out as well as the bundle passing to the medial geniculate body. In the last two sections, the bundle passing between the lateral lemniscus and the superior peduncle is clearly shown. Still farther dorsalward, the nucleus colliculi inferioris be comes more and more developed and the fibres of the striae acusticae are seen (Fig. 4).
1 According to Held the striae acusticae are only sparingly medullated in the cat at birth, v. Held, H., Die centralen Bahnen des N. acusticus bei der Katze. Arch, f . Anat. u. Phys., Anat. Abth., Leipz. (1891) , S. 274.
In the transverse series the nucleus N. cochleae dorsalis is seen in Fig. 33, the nucleus N. cochleae ventralis, in Fig. 38. Fig. 36 shows the beginning of the corpus trapezoideum, which is seen best on Fig. 39. From the superior olive the lateral lemniscus can be traced in its course to the midbrain through Figs. 40, 41 and 42. Fig. 43 shows well the relative positions of the lateral lemniscus and the superior lemniscus. The gap between them, it will be noticed, is filled by very fine fibres, apparently belonging to the lemniscus lateralis. The lemniscus lateralis lies slightly farther medialward than does the lemniscus superior. 1
x The preceding description corresponds, in the main points, with the work of Held. It does not, of course, illustrate the details of the direction and course of the fibres. Cf. Held, Arch. f. Anat. u. Phys., Anat. Abth., Leipz. (1893).
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An Atlas of the Medulla and Midbrain (1901): Chapter I. Introductory | Chapter II. The Long Tracts | Chapter III. The Columns Of The Spinal Cord | Chapter IV. Cerebellar Peduncles | Chapter V. The Cerebral Nerves And Their Nuclei | Chapter VI. The Cerebral Nerves And Their Nuclei (Continued). Lateral Group | Chapter VII. The Inferior And Accessory Olives | Chapter VIII. The Midbrain | Chapter IX. The Formatio Reticularis Alba And Grisea | General Summary of what Is shown In Reconstruction | References To Literature | Abbreviations | Description of Figures and Plates
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