54 GRAY SUBSTANCE OF THE TRAPEZIUM, 



directions ; some join the numerous bundles ( C) which either pass into or originate 

 from the cerebellum, or pass into the restiform body; while others join the bundles 

 (B) which pass out into the antero-lateral portion of the meduUa, forming with 

 other transverse fibres a very beautiful network around the longitudinal bundles. 



Fig. 43, Plate XII, represents a group of the very large cells found in the pos- 

 terior portion of the auditory nucleus of the sheep, showing their connection with 

 the numerous and distinct bundles of fibres which radiate from this part of the 

 nucleiis towards the cerebellum. 



The Boots. — The auditory nerve, as is well known, consists of two portions, 

 anterior and posterior, which, as Stilling has shown,^ take a slightly ascending- 

 course as traced from without inwards, this course being more oblique in man than 

 in most of the mammalia, the great thickness of the pons pressing the roots down- 

 wards, while in some of the lower mammaUa in which the pons is very thin, the 

 roots of the auditory nerve may be traced in a direct course to their nucleus. 



Clarke has given in a few words an accurate description of the course pursued 

 by the posterior division of the auditory root ; as stated by him, this portion of the 

 nerve takes its origin from both parts of the nucleus, " and winds outward as a broad 

 convex band over the restiform body. In this course it contains, at first, a few small 

 cells, elongated in the direction of its fibres ; but as it proceeds, the cells gradually 

 become larger and more numerous, until at the anterior border of the restiform body 

 it enlarges into a pyriform ganglion, which is crowded with nerve-cells, similar in 

 appearance to those of the inner nucleus. The nerve is also reinforced by fibres 

 radiating from the centre of the restiform body as it winds round the latter."^ 



In Plate XIV, Fig. S'', we have the first appearance of the posterior auditory root, 

 as a thickened band of fibres proceeding from the group of rather small, oval and 

 fusiform cells in the posterior portion of the nucleus. In Fig. 9" the fibres are seen 

 arising from a similar group ; some of the cells are of considerable size and are 

 arranged mostly with their longer axes turned in the direction of the entering 

 fibres; these cells are also connected with a little group near the flocculus, with which 

 a few of the fibres of the posterior root seem to be connected. As the roots proceed 

 onward through this cell group, it increases in size, and finally spreads out, as noticed 

 by Clarke, into a pyriform mass containing numerous cells (z), among which some of 

 the fibres interlace before passing outwards. The fibres from the restiform column, 

 though mostly pursuing a longitudinal course, will not unfrequently be seen turning 

 off at an angle more or less acute to reinforce the posterior root as it winds round it 

 (Figs. 8, 9, 10). In the cat the posterior root is especially large and conspicuous 

 (Plate IX, Figs. 34, 35), the pyriform enlargement being very evident ; the fibres 

 Avhich constitute it appear to take a somewhat wavy course, many of them are 

 obliquely ascending or descending fibres, interlacing among the ceUs at a great' 

 variety of angles resulting from the somewhat oblicjuely curved course taken by the 

 root while winding round the restiform body. 



The fibres of the posterior root are constantly reinforced by little cell-groups, 

 until they arrive within the nucleus ; some of them pass towards the large multi- 



' Pous Varolii, 39. ^ Proceedings of tlie Royal Society, June 20, 18G1. 



