cells, but from the cells of the Gasserian ganglion (His), and secondly, if the sensory root of the fifth is cut at its exit from the pons, the fibres of the so-called ascending root degenerate, but the cells of the adjacent gelatinous substance remain unaffected (Bechterew). There may, however, be a physiological connection established with these cells by means of collateral fibres, as in the case of the fibres of the posterior columns of the cord and the substance of Rolando of the posterior horn.

The grey matter of the base of the posterior horn undergoes a considerable increase as we trace it upwards in sections. Portions of grey matter are soon found to extend from it into the funiculi graciles and cuneati, forming the so-called nuclei of those columns (fig. 43, n.g., n.c.). These nuclei are at first narrow in transverse section; but as the central canal approaches the posterior surface of the medulla they appear as comparatively thick masses, which produce externally the

Fig. 45.-SECTION ACROSS THE MEDULLA OBLONGATA A

LITTLE BELOW THE POINT OF THE CALAMUS SCRIP

TORIUS (Lockhart Clarke).

c, central canal; f, anterior median fissure; f.g., funiculus gracilis ; f.c., funiculus cuneatus; t. R., tubercle of Rolando; o, olivary body; a.p., pyramid; XI, XII, spinal accessory and hypoglossal nerves; XI'. XII', their nuclei.

t.R

XI

eminences of the clava and the cuneate 'tubercle. Outside the nucleus of the funiculus cuneatus an accessory or external nucleus becomes formed (fig. 43, n.c.). From this nucleus fibres pass directly into the restiform body. The nerve-cells of the gracile nucleus are for the most part larger than those of the cuneate nucleus, but those of the accessory cuneate are larger than either (50μ to 80μ). The accessory cuneate nucleus is supposed to represent a continuation of Clarke's column of the cord (Blumenau), while in the grey matter of the nucleus gracilis and principal nucleus cuneatus most of the ascending fibres of the posterior column of the cervical cord become lost.

From the lower parts of these nuclei fibres are seen to emerge and to sweep forwards and inwards in a curved manner (internal arched or arcuate fibres) towards the raphe or septum which unites the two halves of the medulla oblongata. Having here intercrossed with those from the opposite side in a decussation which lies above that formed by the fibres of the pyramids (decussation of the fillet), they form a considerable bundle of longitudinally coursing fibres which lies just dorsal to the pyramid and is known as the fillet (lemniscus). Its fibres receive their myelin much earlier than those of the pyramid. On section of the fillet higher up some of its fibres degenerate downwards and the degeneration also affects internal arched fibres connecting them with the opposite nuclei. These therefore have their origin from cells higher up in the brain.

Upper or ventricular part of the medulla oblongata.-When the slit-like upper end of the central canal ope out into the fourth ventricle, the small remaining portion of the base of the anter rn, which in the closed part was ventro-lateral to the central canal, comes to the surface at the floor of the ventricle, and as the sections are traced upward increases gradually in size, producing the prominence of the funiculus teres. In it, both in the lower part of the bulb where the canal is still closed and above where it has opened out, a group of large nerve-cells (n. XII.) is seen in all transverse sections. From this group (column) of cells the successive bundles of the roots of the hypoglossal or twelfth cranial nerve arise and pass obliquely through the substance of the bulb to leave it on its anterior aspect. The tract of nerve-cells is accordingly known as the hypoglossal nucleus.

At the fourth ventricle the hypoglossal nucleus lies a short distance from the surface covered by a flattened bundle of longitudinally running white fibres, which gives this mesial triangle of the calamus scriptorius (trigonum hypoglossi) a white appearance. Fig. 46.-SECTION OF THE MEDULLA OBLONGATA

P nar

C.r:

-g -a.V.

f.

AT ABOUT THE MIDDLE OF THE OLIVARY

BODY. (Schwalbe.)

f.l.a., anterior median fissure; n.ar., nucleus arciformis; p., pyramid; XII., bundle of hypoglossal nerve emerging from the surface; at b, it is seen coursing between the pyramid and the olivary nucleus, o. ; f.a.e., external arciform fibres; n.l., nucleus lateralis; a., arciform fibres passing towards restiform body partly through the substantia gelatinosa, g., partly superficial to the ascending root of the 5th nerve, a. V.; X., bundle of vagus root, emerging; f.r., formatio reticularis; c.r., corpus restiforme, beginning to be formed, chiefly by arciform fibres, superfi cial and deep; n.c., nucleus cuneatus; n.J., nucleus gracilis; t, attachment of the ligula ; f.s., funiculus solitarius; n.X., n.X'., two parts of the vagus nucleus; n. XII., hypoglossal nucleus; n.t., nucleus of the funiculus teres ; n.am., nucleus ambiguus; r., raphe; A., continuation of anterior column of cord; o', o", accessory olivary nuclei; p.o.l., pedunculus oliva.

Nearer to the surface of the floor and nearer also to the median groove is a small group of cells, known sometimes as the nucleus of the funiculus teres (fig. 46, n.t.). The cells are small and appear to give origin to fibres which belong to the vago-glosso-pharyngeal roots.

Fig. 47.-TRANSVERSE SECTION OF THE UPPER PART OF THE MEDULLA OBLONGATA. (Schwalbe.) py., pyramid; o., olivary nucleus; V.a., ascending root of the fifth nerve; VIII., root of the auditory nerve, formed of two parts, a. and b., which enclose the restiform body, c.r. ; n, VIII.p.. principal (dorsal) nucleus of the auditory nerve; n. VIII.ac.. accessory nucleus; g, ganglion cells in the root; n.f.t., nucleus of the funiculus teres; n.XII., nucleus of the hypoglossal; r., raphe.

At the base of the posterior horn in the lower part of the bulb, and near the centra canal, a group of cells (fig. 43, n. XI.) is seen in section, which if traced upwards is found to be pushed to the side as the central canal opens, so that in the

floor of the ventricle it lies lateral or dorso-lateral to the hypoglossal nucleus. These cells form the upper or bulbar part of the nucleus of the spinal accessory.1 Above the level where the roots of this nerve cease to come off a mass of grey matter with numerous cells is seen lying lateral to the hypoglossal nucleus in a situation near the floor of the ventricle corresponding to the prominence of the ala cinerea (trigonum vagi) which appears on the surface, and it extends upwards as far as the fovea inferior. In connection with it there arise successively bundles of fibres of the roots of the vagus and glosso-pharyngeal nerves (10th and 9th cranial nerves): those of the vagus beginning at the commencement of the ventricle, and arising along the length of the ala cinerea; and those of the glosso-pharyngeal coming for the most part from the upper part of the ala cinerea, and from beneath the inferior fovea. The grey matter in question forms then the principal nucleus of the pneumogastric and glosso-pharyngeal nerves.

It has been inferred from the clinical and pathological evidence met with in cases of bulbar paralysis that the motor fibres to the palate and vocal cords, which leave the medulla oblongata by the spinal accessory roots, have their real origin in the hypoglossal nucleus. On the other hand the pars intermedia of the seventh nerve is said to arise from the upper end of the glosso-pharyngeal nucleus (Duval).

Close to this nucleus, but placed somewhat more deeply in the grey matter, is a round longitudinal bundle of white fibres termed the funiculus solitarius (figs. 44, 8., 46, f.s.). This bundle, which is surrounded by gelatinous grey matter with many small nerve-cells, occupies the same relation to the ninth and tenth nerves that the so-called ascending root of the fifth occupies to the trigeminal. Its fibres appear to lose themselves amongst the cells of the enclosing grey matter, and this and the bundle gradually disappear when traced towards the spinal cord: traced upwards they pass out with the bundles of nerve-roots which go to form the vagus and glossopharyngeal, especially the latter. Both this bundle and those forming the ascending root of the fifth have their myelin sheath developed at an early period. As His has shown, they grow into the medulla oblongata from the ganglia of the vagus and glosso-pharyngeal, in the same way as the posterior roots grow into the medulla spinalis from the spinal ganglia.

Lying in the reticular formation and ventral to the principal mass of grey matter which here occupies the floor of the fourth ventricle, is a small detached pear-shaped mass of grey matter containing nerve-cells, which is connected by a kind of stalk with the rest of the grey matter. This nucleus, which was formerly termed the nucleus ambiguus, gives origin to fibres which pass along the stalk obliquely towards the floor of the fourth ventricle and then turn outwards and forwards to issue with the root-bundles of the tenth nerve from the side of the bulb. It is therefore an accessory vagal nucleus and, in its relation to the grey matter and in the size and character of its cells it is a counterpart of the nucleus of the seventh nerve, which appears in sections somewhat higher up (in the pons). A prolongation of this nucleus gives origin higher up to fibres of the glosso-pharyngeal.

The issuing bundles of the auditory nerve pass partly dorso-lateral and partly ventro-mesial to the restiform body. The dorsal division contains a large number of nerve-cells (ganglion radicis cochlearis), which probably give origin to many of its fibres. Ventral to the restiform body and between the two roots is another mass of ganglion-cells, which has been termed the accessory auditory nucleus (Schwalbe) (fig. 47, n.VIII.ac). From these cells fibres are seen both in the upper part of the bulb and in the pons passing transversely towards the opposite side; they belong to the

:

1 The bulbar or accessory part of the nerve the spinal part of the nerve takes origin in the ventrolateral group of cells of the anterior horn of the spinal cord (cervical region).

.

system of the trapezium (p. 47). Higher up this nucleus blends with the ganglion of the lateral root, the two together forming a ventral nucleus for the auditory nerve (fig. 49, VIII.v.).

Towards the upper part of the bulb an extensive tract of grey matter containing small scattered nerve-cells becomes developed outside the vago-glossopharyngeal nucleus. This tract corresponds to the lateral triangular area (trigonum acustici) which is seen on the surface outside the ala cinerea. Into it most of the fibres of the ventral or vestibular division of the auditory nerve apparently pass; it is termed the inner or dorsal auditory nucleus (fig. 47, n.VIII.p). Ventral to this nucleus is a collection of grey matter with large nerve-cells, the nucleus of Deiters. Its cells appear to be connected with the cerebellum, for it becomes atrophied after removal of the cerebellar hemisphere of the same side in the newborn animal.' Associated with this group of cells is a longitudinal bundle of nervefibres which has been termed by Roller the ascending root of the auditory nerve (fig. 49, R.), but the precise connection of these fibres with the roots of the eighth nerve has yet to be established. Most of these collections of cells will be again noticed in dealing with the structure of the pons.

The nerve-cells in the hypoglossal nucleus are largest; those in the principal nucleus of the spinal accessory of moderate size, and those in the vago-glosso-pharyngeal nucleus are small and fusiform; those of the principal (dorsal) auditory nucleus are the smallest. There are a number of small cells in the ventral part of the hypoglossal nucleus (small-celled hypoglossal nucleus of Roller), but they do not give origin to any of the fibres of the nerve (Forel).

Nucleus of the olivary body.-Besides those collections of grey matter which are traceable from the grey matter of the spinal cord, portions occur in certain parts of the medulla oblongata, which are not represented in the cord. Of these the most striking is the nucleus of the olivary body, which has been termed, from its appearance in section, the corpus dentatum of the olive (fig. 44). It is enclosed in the olivary prominence, and is therefore situated in the lateral area of the bulb, but the grey matter is not visible from the surface, being covered by both longitudinal and transverse white fibres. It takes the form of a thin wavy lamina, which is curved round at its edge so as to form an ovoid scalloped capsule. The open part or hilum of this looks towards the middle line and receives a considerable tract of white fibres, which emanate from the raphe, being derived to all appearance from the opposite olive, and pass into the hilum along its whole extent, forming the socalled olivary peduncle (p.o.). Under the microscope the nucleus appears as a wavy band of neuroglia, with small multipolar nerve-cells embedded in it. The fibres of the olivary peduncle diverge as they pass to the grey lamina. They are partly lost in the grey matter of the olivary nucleus but mostly pass in small bundles through the lamina, those which are more posterior turning backwards and coursing obliquely through the posterior part of the lateral area to join the restiform body and thus to pass to the cerebellum as internal arched fibres. These internal arched fibres are easily distinguishable by their small diameter from the large internal arched fibres which belong to the tract of the fillet moreover they develope their myelin sheath later. Others after coursing through the grey lamina and running between the longitudinal fibres which cover the olive reach the surface, where they bend round and are continued as part of the layer of external arched fibres into the restiform body. Through the restiform body, the arched fibres and the fibres of the olivary peduncles, the cerebellar hemisphere of one side is connected therefore with the olivary nuclei

1 The nucleus of Deiters was formerly regarded as giving origin to part of the auditory nerve, and is also known as the outer or superior auditory nucleus (see p. 62).

2 Some fibres emerge from the hilum and turn sharply round the ventral and lateral borders of the dentate nucleus, to which they form a kind of capsule (fig. 44, s.o.).

of both sides. But the connection with the opposite side is the more intimate, for it is found that in cases of atrophy of the cerebellar hemisphere of one side, the olive of the opposite side is atrophied while that of the same side is intact. And it was found by Gudden that after removal in the new-born animal of the one cerebellar hemisphere, the opposite olive was atrophied. On the other hand the olivary nucleus appears to be connected with the cerebral hemisphere of the same side by a tract of longitudinal fibres which lies lateral and dorsal to the nucleus in the medulla oblongata, and passes up towards the brain in the reticular formation. Thus

Fig. 48.-PART OF THE RETICULAR FORMATION OF THE MEDULLA OBLONGATA (Henle).

r.a., reticularis alba; r.g., reticularis grisea; between them a root-bundle of the hypoglossus (XII), The longitudinal fibres of the reticular formation are cut across; the transversely coursing fibres are internal arcuate fibres, passing on the right of the figure towards the raphe.

the olives are intermediary stations between the cerebrum and cerebellum: they have no direct connection with the cord (Flechsig).

Besides the main olivary nucleus two smaller isolated bands are generally seen (figs. 43, 44, 46), looking like separated portions of the chief nucleus. They are situated one on the dorsal, and the other on the mesial aspect of the chief nucleus, and are known as the outer and inner accessory olivary nuclei. They are traversed like the main nucleus by bundles of internal arched fibres going to the restiform body, and are frequently connected at one or two places to the main nucleus. The inner accessory nuclei are sometimes termed the pyramidal nuclei, for they lie immediately behind the pyramids. The root-bundles of the hypoglossal nerves generally pass between them and the chief olivary nucleus after traversing the olivary peduncle, but sometimes the nerve pierces the chief nucleus near its mesial edge.

Other small collections of grey matter and nerve-cells are scattered in certain parts of the formatio reticularis, as well as one or two distinct tracts in connection