224

ROOTS OF AUDITORY NERVE.

It goes without saying, that the

corpora restiformia, which are columns of fibres destined for the cerebellum, are excluded from this ring. The pyramids intertwining

in the formation of the after-brain.

Fig. 126.-Cross-section, fig. 117, i.-Ndt, Nucleus dentatus cerebelli; VIIIa, ascending root of auditory nerve and its large-celled nucleus; Nos, nucleus olivaris superior; VI, root-fibres of n. abducens; NVII, nucleus of facial nerve; VIIa, nuclear limb, VIIc, issuing limb of its root; NTr, nucleus corporis trapezoidis; Po, pons.

SECTION OF THE PONS.

225 with the fibres of the pons lose their distinctness, and take the opportunity, perhaps, of forming numerous connections with them. In examining the human brain it is better to cut off the cerebellum before hardening, leaving only the lingula in connection with the brachia pontis. In monkeys and small animals the cerebellum may be cut in the same sections as the pons (see fig. 17). Hence, we exclude the cerebellum at present from this description, and treat it subsequently by itself.

The most conspicuous difference between a section through this region (fig. 126) and sections through the after-brain is due to the appearance of the pons, Po.

The pons takes the form of great bundles of white fibres which start in the cerebellum, and running transversely across the middle line enclose amongst them irregular masses of grey substance, the nuclei pontis.

Every section through the pons is divided into two quite distinct portions one ventral, the other dorsal. The latter contains the continuation upwards of the structures of the hind-brain with the exception of the pyramids; the ventral half contains, in addition to the proper formation of the pons, the continuation upwards of the pyramids, Py. The dorsal portion may well be called the tegmental field, since most of its longitudinal fibres appear later in the tegmentum of the crus cerebri.

In figs. 126, 127, and 129, an artificial boundary between the cerebellum and the pons is traced. The chief nucleus of the auditory nerve, VIIIh, already diminished in size, still lies beneath the floor of the ventricle; to its outer side the reticular formation of the ascending auditory root, VIIIa, has become thicker, and is distinguished, especially in many animals, by conspicuous large multipolar cells; this region is, therefore, known as the large-celled nucleus of the auditory nerve (Deiters' nucleus). The mesial auditory root, VIIIm, is seen to proceed from the region of the large-celled nucleus and the lateral and ventral angle of the chief nucleus, between the corpus restiforme, Crst, and the ascending root of the trigeminus, Va, taking its exit at the lateral part of the pons. The accessory nucleus, VIIIac, lies on the convexity of the corpus restiforme, and is traversed a little to the ventral side of this by the lateral root, VIII. The transverse fibres which we have already described as proceeding for this group of cells form the largest part of the corpus trapezoides, Tr.

In the lateral part of the reticular substance the facial nucleus, NVII, becomes even more distinct. It takes the form of rounded groups of cells, from which separate bundles of fibres, VIIa, never

226

ROOTS OF ABDUCENS.

united into large tracts, wend their way obliquely in the direction of the dorsal surface and mid-line, towards the posterior longitudinal bundle as it seems. As they run at the same time somewhat forwards, it is only in later sections that we shall be sure that we have to do with the fibres of origin and the nucleus of the facial nerve. In the same section we see these same fibres traversing the pons obliquely near its margin, VIIc, close to the inner side of the ascending root of the fifth nerve, but this time in the form of a compact bundle. The two portions are united by a tract which undergoes various windings as subsequent sections (figs. 127 and 128) will reveal. The roots of the facial and trigeminal nerves towards their exit are distinguished by their course, the one on the mesial, the other on the lateral side of the ascending root of the fifth.

While in the most distal section through the region of the pons all the fibres of the pons surround the pyramids on their ventral side, we find in sections farther brainwards that scattered bundles of fibres and masses of grey substance, as well as the fillet, insinuate themselves between them. Farther forward still, scattered clumps of grey matter are found embedded in amongst the hitherto compact bundles of the pyramid; and, lastly, the farther forwards we make our sections, the more horizontal fibres do we find interlacing with the bundles of the pyramid, as well as lying to their dorsal side. The tracts which lie on the ventral side of the pyramid may be designated superficial bundles of the pons, those on its dorsal side, deep bundles, and those which traverse it, middle (or piercing) fibres.

In animals the pons is much less strongly developed than in Man, and consequently we find in the former, as a rule, that a considerable portion of the corpus trapezoides is left uncovered, and appears superficially on the ventral side of the medulla as a somewhat trapezoidal area, which occupies the whole space behind the pons and between the ventral margins of the cerebellum; [part of the corpus trapezoides runs over and part under the pyramids].

In the section shown in fig. 126 a number of fairly thick bundles of coarse fibres, VI, are to be remarked, which cross the tegment in a dorso-ventral direction, piercing also the fillet, the corpus trapezoides, and the pyramid. Neither their beginning nor their end is shown in this section. These are the fibres of the nervus abducens, the nucleus of origin of which, lying near the great brain, will be seen in fig. 127; while its exit from the medulla just behind the pons would be shown in a section taken between figs. 125 and 126 but not here delineated.

Between the facial nucleus and the roots of the sixth nerve is situate a somewhat ill-defined body of about the size of the facia

helps us to recognise the body. The nerve-cells scattered about in the corpus trapezoides make up the nucleus corporis trapezoidis, Ntr. The fibres of the corpus trapezoides reach the raphe, in which, collected into slender bundles, they pierce the fillet.

CORPUS TRAPEZOIDES.

227

nucleus, the superior olive, Nos. The superior olive descends almost into the corpus trapezoides, and presses its slender bundles close together. The appearance of this cup in which the superior olive lies

Fig. 127.-Transverse section, fig. 117, k.-VIIb, Ascending root of facial nerve; x, accession of crossed fibres to the nervus facialis; NVI, nucleus of nervus abducens; Nrtg, nucleus reticularis tegmenti.

228

THREE LIMBS OF THE FACIAL ROOT.

Dorsal to the fibres of the pons lie a number of structures which we have already studied, but may with advantage recapitulate. In addition to the transverse trapezoidal fibres, we find in order from the middle line outwards :-(1) The raphe, (2) the fillet, (3) the roots of the nervus abducens, (4) the nucleus trapezoides, (5) the superior olive, (6) the nucleus nervi facialis, (7) the issuing fibres of the trigeminal nerve, (8) the ascending root of the trigeminal, (9) the mesial root of the auditory, (10) the restiform body, (11) and (12) the lateral root of the auditory nerve with the accessory nucleus of the same.

On the mesial border of the superior olive is found a small tract of fibres cut transversely, the central tegmental tract, cH (Bechterew and Flechsig). It is not, as a rule, sharply defined. Its fibres are supposed to take origin in the inferior olive.

In the next section (fig. 127) the fillet, which lies just dorsal to the fibres of the pons, is broader, its dorso-ventral diameter being diminished to a corresponding extent. It is traversed in the manner already described by the fine bundles of the corpus trapezoides.

In this section, as in all those behind it, fibræ arcuata are seen curving through all parts of the tegmental region, from the pons fibres right up to the floor of the fourth ventricle. They traverse the posterior longitudinal bundle, Flp, in their course towards the raphe. One must be careful to avoid confounding with the posterior longitudinal bundle a medullated nerve, VIIb, which for a time finds its place between it and the surface of the ventricle.

This nerve, the ascending limb of the root of the facial nerve, is easily distinguished from the posterior longitudinal bundle by the fact that it is not traversed by fibræ arcuatæ. It is better defined too. Most of the fibres arising from the facial nucleus, which is already much diminished in size, incline at first towards the raphe, applying themselves gradually to the nerve-root as it lies beneath the floor of the ventricle close to the middle line, while at the same time they assume a longitudinal direction.

In this section, too, we see for a greater distance the descending limb, VIIc, of the root of the facial nerve, laterally to its nucleus. So it comes about that the root of the facial nerve, VIIa, b, c, is three times met with on its course from its nucleus to the surface, without the connection between the three pieces being visible in any one section.

Close to the nucleus of the facial nerve in the bay formed by the fibres of the corpus trapezoides lies the superior olive, which takes the form here of a narrow riband more or less folded.

Near the olive the central tract of the tegmental region is usually but slightly marked, and next to it the bundles of the abducent nerve are conspicuous, as they form arches convex towards the raphe in their