THE MEDULLA OBLONGATA AND PONS VAROLII. 



outer side of the motor root. The cells are small and arranged in clusters sepa- 

 rated by the fasciculi of origin of the sensory nerve-root. This collection of cells 

 is more extensive than the motor nucleus, being seen in sections higher up the 

 pons, and passing below into the so-called " lower sensory nucleus," which is a 

 continuation of the grey matter of the tubercle of Rolando, and ultimately, 

 therefore, of the substantia gelatinosa of the posterior horn of the spinal cord. 

 Passing towards this nucleus into the issuing roots of the fifth nerve is seen, as 

 has just been mentioned, in all sections through the middle part of the pons 

 a well marked tract of fine fibres which are traceable over the posterior longi- 

 tudinal bundle to the raphe, and ultimately pass, it is believed, upwards 

 towards the mid -brain. This tract is known variously as the crossed root, 

 the raphe-root, or the central tract of the trigeminal. Its fibres pass by and 



Fig. 51. OBLIQUE TRANSVERSE SUCTION OF THE PONS 



ALONG THE LINE OP EXIT TRAVERSED BY THE FIFTH 

 NERVE (E.A.S.). 3 



The section passes through the lower part of the 

 motor nucleus (nv 1 ) from which a bundle of fibres of 

 the motor root, V, is seen passing ; a part of the upper 

 sensory nucleus (nv) is also shown in the section in 

 the form of a number of small isolated portions of 

 grey matter. Amongst these are a few bundles of 

 the ascending root cut across, but most of these 

 have already become diverted outwards to join and 

 assist in forming the issuing part of the main or 

 sensory root, V ; ar, arcuate fibres near the fourth 

 ventricle, which come partly from the raphe, partly 

 from a small longitudinal bundle of fibres (I) near 

 the median sulcus (m.s. ), and pass outwards to join 

 the root of the fifth nerve ; f.r., formatio reticularis ; 

 r., raphe ; ./., substantia ferruginea. 



may in part be continuous with a small oval 

 bundle of longitudinal fibres (fig. 51, Z), 

 which lies in the grey matter not far from 

 the median sulcus, and which resembles in 

 appearance the ascending part of the facial 

 root. 



The rest of the grey matter of the pons lies near the dorsal surface and appears 

 in the floor of the upper part of the fourth ventricle. Besides scattered nerve-cells, 

 others are collected at certain parts into definite groups or nuclei from which some 

 of the remaining cranial nerves take origin. Like the similarly placed nuclei in 

 the medulla oblongata, these also do not lie close to the epithelium which covers the 

 surface, but are separated from it by a layer of gelatinous substance (neuroglia) free 

 from nerve-cells, termed the ependyma of the ventricle. 



The dorsal nucleus of the auditory nerve (fig. 47, n. VIII p.\ is prolonged 

 upwards underneath the striae acusticae into the pons (fig. 49, Vlll.d.}. It is widest 

 at about the junction of the medulla and pons, where it extends almost to the middle 

 line ; further up it rapidly narrows and becomes shifted towards the lateral boundary 

 of the ventricle as the nucleus of the sixth nerve makes its appearance between it 

 and the, median sulcus. Its cells are small, and it is much broken up by the pas- 

 sage through it of fine transverse nerve-fibres. 



The so-called outer or superior nucleus of the auditory nerve, or the nucleus of 

 Deiters (Laura) (fig. 49, D ; fig. 50, n.VIH\ is characterized by the large size of its 

 cells, and lies immediately ventro-lateral to the dorsal nucleus, which has just been 

 described. It does not begin to be visible so far down as the latter, but is 

 continued as far upwards, rather increasing in size superiorly, whereas the dorsal 



