962 NUCLEI OF CRANIAL NERVES. [BOOK in. 



simply pass through the nucleus, and do not take origin from any 

 of its cells. 



620. The Sixth or Abducens Nerve. This nerve starts from a 

 compact oval nucleus (Fig. 115, VI.), lying at the level of the 

 hinder part of the pons, and therefore of the front part of the 

 fourth ventricle, in the central grey matter of the floor of the 

 ventricle, or rather just between it and the reticular formation, a 

 little on one side of the median line (Fig. Ill, VI.). A slight 

 swelling of the floor of the fourth ventricle, eminentia teres, 

 marks its position (Fig. 115, e. t). The nucleus contains fairly 

 large nerve cells, with distinct axis-cylinder processes. These are 

 gathered at the median side of the nucleus to form the thin 

 root, which passing ventrally and laterally, at some little distance 

 from the median raphe, through the reticular formation, runs 

 backward above the pyramidal bundles of the pons, and finally 

 comes to the surface at the hinder edge of the pons, opposite the 

 front end of the pyramid (Fig. 108, O). 



621. The Fifth or Trigeminal Nerve. This nerve, as it comes 

 to the surface on the ventral aspect of the pons (Fig. 108, C.), 

 near the front edge, at some distance from the median line, 

 consists of two parts, a smaller motor root and a larger sensory 

 root, the latter bearing the large ganglion of Gasser; and the 

 origin of the nerve is in many ways complex. Both roots may 

 be traced in an oblique direction (Fig. 112, V.) inwards and 

 towards the dorsal surface, through the pons to the reticular 

 formation beneath the floor of the front part of the fourth 

 ventricle, the smaller motor root taking up a position median to 

 the larger sensory root. 



Here the motor root comes into connection with a collection of 

 nerve cells (Figs. 115 and 112, V. m.), which may be regarded 

 as its nucleus; but this is not the whole nucleus of the motor 

 root. From the level of the nucleus there stretches forwards as 

 far as the level of the anterior corpora quadrigemina a bundle of 

 longitudinal fibres which, since it is usually traced from the front 

 backwards until it passes into the root of the nerve, is spoken of 

 as the descending root of the fifth nerve. 



This descending root begins as a few scattered bundles of fibres 

 at the level of the anterior corpora quadrigemina, in the peri- 

 pheral lateral part of the central grey matter surrounding the 

 aqueduct, dorsal and lateral (Fig. 1 14, V. d), to the nucleus of the 

 third nerve (Fig. 114, III. ??.). From thence the fibres pass 

 backward, augmenting in number, and soon form a compact 

 bundle, semilunar in transverse section, lying lateral to the fourth 

 nerve as this is rising dorsally (Fig. 113, V. d.) ; still increasing 

 in number in their course backward they gradually assume a 

 more ventral position as the aqueduct opens into the fourth 

 ventricle. All along its course this descending root has attached 

 to it large (70 /j, or more in diameter), sparse spheroidal nerve 



