604 THE NEKVOUS SYSTEM. 



The oculomotor nucleus is connected (1) with the occipital part of th 

 cerebral cortex by fibres which reach it through the optic radiation ; (2) with th 

 vestibular, trochlear and abducent nuclei (and probably with other nuclei) b 

 fibres which come to it through the medial longitudinal bundle ; (3) possibly wit! 

 the facial nerve by fibres which pass out from it into the medial longitudina 

 bundle (p. 589) ; (4) with the visual system by fibres which enter it from the cell 

 of the superior colliculus. 



It is important to recognise that although the main part of the oculomoto 

 nucleus belongs to the medial somatic group, which also includes the trochleai 

 abducent and hypoglossal nuclei, it also includes a representative (the Edingei 

 Westphal group of small cells) of the column of splanchnic efferent nuclei in serie 

 with those of the facial, glossopharyngeal, and vagus nerves. Its axons pass on 

 along with the other fibres of the oculomotor nerve and enter the ciliar 

 ganglion, where they end in relationship with the cells that innervate the ciliar 

 muscle and the circular muscle of the iris. 



Nervus Acusticus. This large nerve enters the brain at the inferior borde 

 of the pons. Its fibres spring from bipolar ganglionic cells in the immediat 

 neighbourhood of the labyrinth or internal ear (see section dealing with th 

 Organs of Sense). One group of these forms the spiral ganglion, the periphery 

 branches of which are distributed to the organ of Corti in the cochlea : anothe 

 group constitutes the vestibular ganglion (often called Scarpa's), which distribute 

 fibres to the ampullae of the semicircular ducts, the utricle, and the saccule. Althoug 

 the central processes of the cells in these two ganglia accompany one another and ai 

 known collectively as the acoustic nerve they really remain distinct throughout, i 

 their mode of termination in the brain as well as in their peripheral distributioi 

 Beaching the brain the acoustic nerve divides into two parts, viz., the nervu 

 cochlearis and the nervus vestibularis, which present totally different connexion 

 corresponding to their distinct functions. In their further course these frw 

 divisions deviate from each other so as to embrace the restiform body th 

 vestibular part entering the pons on the medial aspect of the restiform bod; 

 whilst the cochlear part sweeps round its lateral surface. Special nuclei < 

 termination require to be studied in connexion with each part of the nerve. 



The cochlear nerve is composed of finer fibres than the vestibular nerv 

 and its fibres acquire their medullary sheaths at a later period. It is the true nen 

 of hearing, and its fibres end in a nucleus which lies in intimate relation to tt 

 restiform body. It may be described as consisting of two parts. Of the* 

 one, called the dorsal cochlear nucleus, is a piriform mass which is placed on tl: 

 dorsal aspect of the restiform body between it and the flocculus of the cer< 

 bellum. The second part, termed the ventral cochlear nucleus, is placed o 

 the ventral aspect of the restiform body in the interval between the cochlear au 

 vestibular divisions of the acoustic nerve, after they have separated from eac 

 other. The fibres of the cochlear nerve enter these two ganglia and end arouc 

 the cells in arborisations, which are finer, closer, and more intricate than thos 

 met with in any other nerve-ending in the brain. 



The vestibular nerve enters the brain at a slightly higher level than the cochlea 

 nerve and on the medial aspect of the ventral cochlear nucleus. It proceec 

 backwards through the pons between the restiform body, which lies on its later; 

 side, and the spinal tract of the trigeminal nerve, which is placed on its medial sid 

 Its fibres end in a series of terminal nuclei (Fig. 530, p. 597), viz. : (1) the nuclei 

 vestibularis dorsalis, often known as the principal nucleus, (2) its inferior pr< 

 longation, nucleus tractus descendentis, (3) the nucleus vestibularis lateralis (Deiters 

 (4) the nucleus vestibularis superior (Becliterews), and (5) the cerebellar cortex. 



The principal nucleus (Figs. 498, p. 565, and 535, p. 605) is a large diffui 

 nuclear mass, which lies in the floor of the fourth ventricle subjacent to the surfa< 

 district known as the area acustica (Fig. 482, p. 550). It is situated, therefor 

 in both the pons and the medulla oblongata to the lateral side of the fovea superi< 

 and the fovea inferior. In transverse section it is prismatic in outline, and cros 

 ing the surface of its upper or pontine part immediately under the ependyma i 

 the ventricle are the striae medullares. 



