600 THE NEKYOUS SYSTEM. 



and perhaps moves slightly upwards, i.e. towards the mesencephalon. The facial 

 nucleus, however, receives most of its stimuli from the nucleus tractus spinalis 

 nervi trigemini, and therefore, as the walls of the metencephalon thicken during 

 their growth, this nucleus retains its proximity to the trigeminal nucleus 

 (Fig. 531), and so migrates along a course which remains mapped out by its 

 emerging fibres. Streeter, working with human embryos, and Ariens-Kappers, 

 on comparative and therefore broader lines, have elucidated the meaning of this 

 peculiar intracentral course of the facial nerve. 



Nervus Abducens (Figs. 498 and 531). The abducens nerve is a small 

 motor nerve which emerges from the brain at the inferior border of the pons 

 above the lateral side of the pyramid of the medulla oblongata. It is the nerve 

 of supply to the lateral rectus muscle of the eyeball. Its nucleus of origin is 

 a small spherical mass of gray matter, containing large multipolar cells, which 

 lies in the dorsal part of the tegmental portion of the pons, close to the median 

 plane and immediately subjacent to the gray matter of the floor of the fourth 

 ventricle. Its position can be easily indicated on the ventricular floor, seeing 

 that it is placed subjacent to the colliculus facialis and immediately above the 

 level of the striae medullares. Its peculiar and intimate relation to the intra- 

 pontine portion of the facial nerve has already been indicated. It lies on the ventral 

 aspect of, and within the concavity formed by, the two limbs of the loop of that nerve. 



The axons of the multipolar cells of this nucleus emerge from the medial aspect 

 of the nucleus in the form of several bundles, which proceed through the whole 

 dorso- ventral thickness of the pons towards the place of exit. As they pass 

 forwards they incline downwards and slightly laterally. In the dorsal part 

 of the pons they proceed forwards on the medial side of the nucleus olivaris superior, 

 whilst in the basilar part of the pons they keep for the most part to the lateral 

 side of the pyramidal bundles, although several of the nerve fila pierce these 

 on their way to the surface. 



.It would appear probable that certain of the axons of the cells of the abducens nucleus enter 

 the medial longitudinal fasciculus and proceed upwards in it to end in the oculomotor 

 nucleus of the opposite side. Fibres and collaterals from the basis pedunculi of the opposite 

 side enter the nucleus, and, ending around the cells, bring the nucleus into connexion with the 

 motor area of the cerebral cortex. The pedicle of the nucleus olivaris superior ends partly 

 within the nucleus of the abducent nerve (Fig. 530). 



Nervus Trigeminus. The trigeminal nerve strikes its roots deeply into the 

 brain and establishes a connexion with it which extends from the upper part of 

 the mesencephalon above to the level of the second cervical nerve below. No 

 other cerebral nerve presents so extensive a connexion (Fig. 530, p. 597). It 

 is composed of two roots a large afferent or sensory root and a small efferent 

 or motor root. Both roots appear close together on the surface of the pons, 

 rather nearer its superior than its inferior border, and in the same line as the 

 facial, and glossopharyngeal and vagus nerves (Fig. 527, p. 594). 



The sensory root of the trigeminal nerve is composed of fibres which arise outside 

 the brain from the cells of the semilunar ganglion. They end within the brain in a 

 somewhat tadpole-shaped terminal nucleus, the swollen body of which is situated 

 in the pons and is termed the main sensory nucleus of the trigeminal nerve: 

 the tail is a long column of gray matter which is directly continuous below 

 with the substantia gelatinosa of the spinal medulla. 



The main sensory nucleus (Fig. 532) is an oval mass of gray matter placed 

 half-way up the pons in the lateral part of its dorsal or tegmental portion. It lies 

 close to the lateral surface of the pons and immediately subjacent to the ventral 

 submerged margin of the brachium conjunctivum. It is directly continuous with 

 the substantia gelatinosa, and may be regarded as being merely the enlarged 

 superior end of that column of gray matter. 



The fibres of the sensory root of the trigeminal nerve, on reaching the sensory 

 nucleus, divide, in the same way as the fibres of the entering posterior roots of the 

 spinal nerves, into a system of ascending and descending branches (Fig. 530, p. 597). 

 The ascending fibres are short, and almost immediately enter the sensory nucleus 

 and end within it ; the descending fibres turn sharply downwards and form the 



