I40 



NERVOUS SYSTEM OF VERTEBRATES. 



It is usually stated that the cochlear nerve includes a branch 

 to the macula acustica of the sacculus in addition to the fibers 

 to the organ of Corti. Recent studies on the human embryo show, 

 however, that the branch in question belongs to the vestibular 

 nerve and is quite independent of the cochlear nerve (Streeter). 

 The fibers of the cochlear nerve bifurcate on entering the medulla 

 oblongata and end for the most part in two nuclei, the ventral 

 and dorsal cochlear nuclei, which lie lateral and dorsal to the 

 vestibular nuclei. Two important things are to be noticed in 



Cerebellum 



C.genic. med, 

 PuKinar 



C.genic 

 lat 



N. cochlearis 

 f'estibularis 



Comm 



Tr. opticus 



Fig. 70. — A diagram to show the central endings of the vestibular and cochlear 

 nerves and of the optic tract in man and the chief secondary tracts related to them. 

 Compare Fig. 60. 



regard to the centers for the cochlea. The first is that these 

 nuclei are superficial with respect to the vestibular nuclei. In 

 this they offer a clear illustration of the general law that the more 

 highly speciahzed structures in the brain, and hence those which 

 have appeared later in the phylogeny, are placed toward the outer 

 surface with respect to older structures to which they are related. 

 It is probable that these nuclei have been developed from the 

 acusticum of lower vertebrates and have taken up the superficial 

 position as they developed. The second point is that no cochlear 

 fibers go to the cerebellum. Apparently the development of the 

 cochlea has come so late as compared with the evolution of the 

 brain that the cerebellum had already assumed functions of 



