THE NERVOUS SYSTEM 499 



The auditory nerve is divided into two parts: the cochlear nerve (ganglion 

 spirale) and the vestibular nerve (ganglion of Scarpa). The fibres of the coch- 

 lear root enter at a lower level than those of the vestibular. Some of them enter 

 the ventral cochlear nucleus; the remainder pass dorsalward to the dorsal 

 cochlear nucleus, or nucleus of the acoustic tubercle. According to some author- 

 ities, some root fibres pass to the superior olivary and trapezoid nuclei. The 

 axones of the cells of the ventral and dorsal nuclei form the secondary cochlear 

 tract (lateral fillet). These fibres decussate and send collaterals to, or are 

 partially interrupted in, the nucleus olivaris superior, trapezoideus, nucleus 

 of lateral fillet and inferior coUiculus (posterior corpus quadrigeminum). 

 According to some authorities all the fibres of the lateral lemniscus terminate 

 in the inferior coUiculus. From the inferior colliculus the path is formed by the 

 arm or brachium of the latter to the medial geniculate body and thence to the 

 temporal cortex cerebri. It is thus not possible to state definitely how many 

 neurone systems are involved, but the principal ones are: (i) ganglion spirale, 

 (2) dorsal and ventral nuclei and (decussation) lateral lemniscus, (3) posterior 

 corpus quadrigeminum and its brachium, (4) medial geniculate body of the thal- 

 amus and geniculo-cortical fibres. If the lateral lemniscus fibres be regarded 

 as simply passing by the posterior corpus quadrigeminum, giving collaterals to 

 it (Cajal), the path would in part consist of three neurone systems analogous 

 to those of the paths from the cord, trigeminus and eye. (Figs. 331, 338.) 



The fibres of the vestibular root enter higher and mesial to those of the coch- 

 lear root, passing dorsally along the inner side of the restiform body to four 

 terminal nuclei, which cannot all be clearly seen in any one section; (o) Deiters' 

 nucleus (lateral vestibular nucleus) situated at the end of the main bundle of 

 root fibres, just internal to the restiform body; (5) von Bechterew's nucleus (supe- 

 rior vestibular nucleus) situated somewhat dorsal to Deiters' nucleus in the lateral 

 wall of the fourth ventricle; (c) the median or principal nucleus of the vestibular 

 division — a large triangular nucleus, occupying a considerable part of the floor 

 of the fourth ventricle; (d) the descending vestibular nucleus which accompanies 

 the descending fibres of the vestibular root (spinal eighth). Fibres also pass to 

 the cerebellum. The axones of the cells of the terminal vestibular nuclei form 

 the secondary vestibular tracts, some axones going to (a) the cerebellum (?),(b) 

 the midbrain, especially to the nuclei of nerves III and IV (via Deiters and von 

 Bechterew, the former by the medial longitudinal fasciculus), (c) the medulla 

 and cord, probably to various motor nuclei, via the medial longitudinal fasciculus, 

 lateral tract from Deiters' nucleus and other tracts in the reticular formation. 

 (Figs. 331, 339 and 345.) 



The descending vestibular root is large, as is also its terminal nucleus and the 

 medial terminal vestibular nucleus, in the present section. 



The spinal V is unchanged, its terminal nucleus being rather indistinct. 

 Secondary trigeminal tracts cannot be distinguished — such fibres probably either 

 join the medial lemniscus or form an independent ascending tract in the reticular 

 formation. The fillet is about the same. The ventral spino-cerebellar and spino- 

 thalamic tracts are in the same positions. 



Other Afferent Cerebellar Neurones. — The olives are still larger and send 

 many bundles of olivo-cerebellar fibres to the opposite restiform body which has 



