928 THE CENTRAL NERVOUS SYSTEM 



nerve. And the cochlear and vestibular roots are physiologically 

 as well as anatomically distinct. For the cochlea subserves the 

 function of hearing, the semicircular canals and vestibule the function 

 of equilibration. As they enter the medulla oblongata, the fibres of 

 the dorsal root bifurcate. Of the two branches, one is considerably 

 thicker than the other. Many of the thicker branches terminate by 

 arborizing around the cells of the accessory auditory nucleus, whose 

 position is indicated by a swelling on the ventral surface of the resti- 

 form body at the junction of the dorsal and ventral roots; but some 

 pass over the restiform body to end in another nucleus (lateral nucleus), 

 also indicated by a swelling (tuberculum acusticum) lying over the 

 restiform body. The nerve-cells of the accessory nucleus and the 

 acoustic tubercle, therefore, constitute nuclei of reception for the 

 dorsal root-fibres. The more slender branches of the cochlear root- 

 fibres run downwards for some distance before breaking up into fibrils. 



The path to the higher parts of the brain is continued by the axons 

 of nerve-cells in the accessory nucleus and the acoustic tubercle. The 

 fibres from the accessory nucleus pass into the trapezium, a mass of 

 transverse fibres lying in the pons behind the pyramidal fibres. In 

 their course through the trapezium some of the fibres terminate around 

 the cells of the nucleus of the trapezium, others run into the superior 

 olive of the same side, and end there ; but most of them cross the middle 

 line, and enter the trapezoid nucleus and superior olive of the opposite 

 side, where many of them terminate. Others, however, run through 

 those nuclei and pass into the lateral fillet, to end in its nucleus or in 

 the posterior corpora quadrigemina. The path of the fibres which 

 terminate in the nuclei of the trapezium, superior olive, and lateral 

 fillet, is continued by another relay of fibres, which link them also to 

 the posterior corpora quadrigemina. The axons of the cells of the 

 acoustic tubercle enter for the most part the stria acusticce, a series of 

 prominent strands that run transversely across the floor of the fourth 

 ventricle. Passing across the raphe, they join the fibres from the 

 accessory nucleus on their way to the superior olive, and accompany 

 them into the lateral fillet, which terminates in the grey matter of the 

 posterior corpus quadrigeminum. We must assume, from clinical and 

 experimental data, that the dorsal root is ultimately connected with 

 the first or first and second temporo-sphenoidal convolutions on the 

 opposite side. From the posterior corpora quadrigemina the auditory 

 path to the convolutions seems to run in the brachium to the internal 

 or mesial geniculate body, whence it is continued in the posterior 

 extremity of the internal capsule. 



The fibres of the ventral root of the eighth nerve, better termed the 

 vestibular nerve, after entering the medulla oblongata, pass to a 

 nucleus called the principal nucleus of the vestibular division. Here 

 each bifurcates into a descending and an ascending branch. The 

 descending branches running down in the medulla terminate at dif- 

 ferent levels around cells in the principal nucleus, and the grey matter 

 continued down from it (descending vestibular nucleus). The ascending 

 branches run up on the inner side of the restiform body towards the 

 nucleus of the roof (nucleus tecti) in the cerebellar worm. On their 

 course they enter into relation through their collaterals with the nuclei 

 of Deiters and Bechterew. The nucleus of Deiters, as already stated, 

 sends fibres into the posterior longitudinal bundle. Through ascend- 

 ing branches of these fibres a communication is established with the 

 nuclei of the third and sixth nerves, and through descending branches 

 that pass into the antero-lateral descending tract of the cord with the 

 anterior horn cells. It is obvious that through these connections 



