THE AUDITORY AREA 217 



other. It is believed to belong to the auditory path. There 

 are fibers passing from the chiasma into the floor of the third 

 ventricle which are thought to connect with the nuclei of the 

 third nerve and to be concerned in the light reflex of the iris. 

 Finally, there is supposed to be a superior commissure along 

 the anterior margin of the chiasma connecting the two retinae. 

 The termination of the optic radiations is mainly in the cuneus, 

 particularly along the calcarine fissure, and probably, in addi- 

 tion, in portions of the lingual lobe and on the external surface 

 of the occipital lobe. Stimulation in these regions causes 

 movements of the head and eyes. 



The Auditory Area. It is very probable that the auditory 

 area lies in the superior temporal gyrus and in the transverse 

 gyri extending into the fissure of Sylvius. Entire ablation of 

 both temporal lobes is followed by complete deafness. Abla- 

 tion on one side, however, is followed by impairment of hearing, 

 so that it is probable that the fibers end partly on the same side 

 and mainly on the opposite side of the cerebrum. 



The fibers constituting the cochlear branch of the eighth 

 cranial nerve arise from the cells in the spiral ganglion. These 

 cells are bipolar. One axon passes to the cochlea, where the 

 impulses aroused by sound waves are generated. The other 

 axon passes to the pons. Here it may end in one of two nuclei, 

 one lying ventral to the restiform body and known as the acces- 

 sory nucleus; and one, dorsally, known as the tuberculum acus- 

 ticum. The axons that arise from the accessory nucleus pass 

 mainly to the opposite side by slightly different routes. Some 

 go directly across toward the ventral side of the pons, forming 

 the corpus trapezoideum ; others pass dorsally around the 

 restiform body, and then down through the tegmental region 

 to join the corpus trapezoideum. The fibers of this cross 

 band end in certain nuclei of gray matter on the opposite side 

 of the pons, especially in the superior olivary body and in the 

 trapezoidal nucleus, and then the path is continued forward by 

 a third neuron. At the level of the superior olivary body the 

 auditory fibers enter into the lateral fillet or lateral lemniscus. 

 The secondary sensory fibers from the tuberculum acusticum 

 pass dorsally and transversely, forming the band of fibers 

 known as the auditory striae, on the floor of the fourth ventricle. 

 The fibers then dip inward at the raphe, and some of them cross 



