THE ENCEPHALIC OR CRANIAL NERVES 647 



lemniscus of the same side. The lemniscus then passes upward to the in- 

 ternal geniculate body around the nerve-cells of which its fibers terminate. 

 From the geniculate body the acoustic radiation arises which then passes 

 upward and outward through the posterior limits of the internal capsule 

 to terminate finally around the cells of the super-temporal convolution. 



From the nuclei around which the vestibular branch terminates, axons 

 arise which in all probability pursue a somewhat similar course and 

 terminate in the temporal lobe. 



Properties. Stimulation of the cochlear nerve is unattended by either 

 motor or sensor phenomena. Division of the nerve is followed by a loss 

 of the sense of hearing. Irritative pathologic lesions give rise to sensations 

 of sound of varying character and intensity. Degeneration of the nerve or 

 destruction by tumors, etc., will also be followed by a loss of the sense 

 of hearing. 



|h<| Experimental lesions of the semicircular canals involving a destruc- 

 tion of the physiologic relations of the vestibular nerve are followed by a 

 loss of the coordinating and equilibratory power. Disordered move- 

 ments, such as rotation to the right or left, somersaults backward and 

 forward, follow destruction of these canals. Pathologic lesions in the 

 peripheral distribution of the nerve are attended in man by disturbances 

 of equilibrium, e.g., vertigo, or a sense of swaying, pitching, and staggering. 



Functions. The function of the cochlear nerve is to convey nerve 

 impulses from its origin to the pons, from which they are transmitted by 

 the auditory tract to the acoustic area in the cerebral cortex where they evoke 

 sensations of sound and its different qualities, intensity, pitch, and timbre. 

 The specific physiologic stimulus to the development of these impulses is 

 the impact of atmospheric undulations on the tympanic membrane, re- 

 ceived and transmitted by the chain of bones to the structures of the internal 

 ear the organ of Corti with which the peripheral terminations of the 

 nerve are connected. 



The function of the vestibular nerve is the transmission of nerve impulses 

 to the pons, whence they are transmitted to the cortex of both the cerebrum 

 and cerebellum and to other centers. The specific physiologic stimulus is 

 supposed to be a variation in pressure in the ampullae of the semicircular 

 canals caused by inertia of the endolymph during changes in the position of 

 the head and body. The impulses carried by the vestibular nerve give rise 

 reflexly to certain adaptive and protective movements by which the equi- 

 librium of the body in both dynamic and static conditions is maintained. 



THE NINTH NERVE. THE GLOSSO-PHARYNGEAL 



The ninth cranial nerve, the glosso-pharyngeal, consists, as shown by 

 both histologic and experimental methods of research, of both afferent and 

 efferent nerve-fibers, of which the former, however, are by far the more 

 abundant. Near its exit from the cavity of the skull the nerve presents two 

 ganglionic enlargements known as the petrosal and jugular ganglia. 



Origin of the Afferent Fibers. The afferent fibers serve to bring cer- 

 tain end-nuclei in the medulla oblongata into anatomic and physiologic 

 relation with portions of the mucous membrane of the tongue, pharynx, and 

 middle ear. The afferent fibers are axons of the monaxonic cells of the 



