THE CENTRAL NERVOUS SYSTEM. 91 



lesions. A partial decussation of the auditory tracts occurs, similar to 

 that of the optic nerves, so that each cerebral hemisphere receives 

 impulses from both ears. 



The audito-sensory area is located in the temporal lobe, and has 

 been supposed to be limited to the middle region of the superior 

 temporal convolution, the audito-psychic area being adjacent to it. 

 But whereas, so far as can be judged, extirpation of one temporal lobe 

 is followed by partial deafness, and removal of both lobes by complete 

 deafness, Schafer has shown that removal of the superior temporal 

 convolutions alone from both sides in monkeys does not result in 

 complete deafness. The superior temporal convolution is probably the 

 chief centre for hearing, but there may be subsidiary areas outside it. 



FIG. 26. Diagram showing the paths for auditory impulses in the pons. 

 a, Accessory nucleus ; 6, tuberculum acusticum ; c, trapezoid nucleus ; d, lateral fillet. 



Stimulation of the superior temporal convolution in monkeys is followed 

 by pricking of the opposite ear and turning of the head towards the 

 opposite side. 



Impulses are conveyed from the cochlea to the auditory centre by 

 the cochlear division of the eighth nerve, the auditory tract, and the 

 auditory radiation. The fibres of the cochlear nerve are derived from 

 the spiral ganglion, and terminate in the tuberculum acusticum and 

 the accessory nucleus. The axons from the tuberculum acusticum turn 

 over the restiform body to become the strice acusticce (strice medullares) 

 in the floor of the fourth ventricle. These dip into the substance of 

 the pons at the middle line, some passing to the trapezoid nucleus and 

 superior olivary nucleus of the same side, others to the corresponding 

 nuclei of the opposite side. Some fibres terminate in these nuclei on 

 each side, others are continued directly into the lateral fillet. The fibres 

 from the accessory nucleus constitute the trapezium, and also enter 

 into the formation of the lateral fillet (fig. 26). Many of the trapezoid 



