THE CRANIAL NERVES 



653 



number of them cross the midline to attain the lateral lemniscus of 

 the opposite side, whence they reach the gray matter of either the 

 inferior colliculus or median geniculate body. These structures are 

 connected with the psychic area for hearing by way of the auditory 

 radiation which passes through the inferior extremity of the internal 

 capsule. 



We observe, therefore, that the auditory nerve finally gives rise to 

 a decussation which bears a close resemblance to that effected by the 

 optic nerve, but the degree of crossing has not been determined as 

 yet with certainty. As we shall see later, this fact is very important, 

 because it helps to explain some of the symptoms resulting from uni- 

 lateral destruction of the center of hearing. In the second place, it 



tub.ac. 



FIBRES TO NUCL.LEMNISCI 

 &CORPORA QUADRIGEMINA 



NERVE-ENDINGS 



IN ORGAN OF CORTi 



FIG. 325. THE COURSE AND CONNECTIONS OF THE FIBRES FORMING THE COCHLEAR ROOT 



OP THE AUDITORY NERVE. 



r., Restiform body; V, descending root of the fifth nerve; tub.ac., tuberculum 

 acusticum; n.acc., accessory nucleus; s.o., superior olive; n.tr., nucleus of trapezium; 

 n.VI, nucleus of sixth nerve; VI, issuing root-fiber of sixth nerve. (Schafer.) 



will be seen that the median geniculate body may serve the purpose of 

 a secondary auditory center and hence, assume a position similar to 

 that of the lateral geniculate body which is really a subordinate center 

 for vision. Thirdly, sufficient experimental evidence is at hand to 

 show that the auditory centers form the starting points of certain motor 

 paths which are used in the reflex actions resulting in consequence 

 of auditory stimuli. 



9. The glossopharyngeus nerve is motor and sensory in its function. 

 It emerges from the side of the medulla, its motor fibers originating 

 from two nuclei, known as che nucleus ambiguus which forms the 

 ventral area of the vagus nucleus, and the nucleus dorsalis which is 

 situated below the floor of the fourth ventricle. Its sensory fibers are 

 derived from the ganglion superiore and ganglion petrosum. The 

 peripheral branches of these pass to the receptors and their central 



