386 TRACING OF IMPULSES. 



1. The Cochlear Path (Figs. 89, 70 and 54). Impulses of hear- 

 ing originate in the organ of Corti. They are transmitted by 

 the rods and hair cells of Corti to the dendrites of the spiral gan- 

 glion. Traversing the dendrites and cell-bodies of that ganglion, 

 they enter the axones, which form the cochlear nerve, and run 

 backward to the terminal nucleus of that nerve hi the medulla. 

 Both the ventral and the lateral portions of the cochlear nucleus 

 receive the impulses of hearing. From the cochlear nucleus they 

 run either lateral and dorsal to the restiform body and cross to 

 the opposite side through the medullary striae and trapezoid body, 

 or they run medial to the restiform body and enter at once into 

 the trapezoid body. By either course they reach the lateral fil- 

 let, and chiefly the opposite one. The lateral fillets conduct the 

 impulses to the inferior quadrigeminal colh'culi; the brachia in- 

 feriora to the medial geniculate bodies, and the temporo-thalamic 

 radiations to the third and fourth fifths of the superior temporal 

 and to the transverse temporal gyri of the cerebrum. Through 

 the lateral fillet, impulses producing reflex reach the quadrigemi- 

 nal colliculi and, thence, by the anterior longitudinal bundle, 

 pass to motor nuclei; and, also, through the olivary pedicle and 

 medial longitudinal bundle, they reach the nuclei of the sixth, 

 fourth and third cerebral nerves. 



2. Vestibular Path. The extent of the vestibular conduction 

 path is from the acustic areas of the utricle, saccule and semi- 

 circular canals to the vestibular nuclei in the floor of the fourth 

 ventricle; and, thence, to the cerebellum, and to the cortical area 

 of equilibrium, according to Mills, hi the temporal cortex. It is 

 the path of space sense. Through the vestibular nerve the im- 

 pulses reach the dorso-medial, the dorso-lateral and superior nu- 

 cleus, and the nucleus of the descending root in the floor of the 

 fourth ventricle. 



The impulses may pursue, from the terminal nuclei in the ven- 

 tricular floor, either a direct or an indirect course to the cerebral 

 cortex. 



(i) By the direct course they run through the opposite medial 

 fillet and certain fibers in the cortical fillet, perhaps the ventral 

 stalk of the thalamus, to the middle and inferior temporal gyri. 



