THE CRANIAL NERVES. 597 



the coordinating and equilibratory power. Disordered movements, 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, received 

 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. 



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 

 petrosal and jugular ganglia. The single axon from each of these cells soon 

 divides into two branches, one of which passes centrally, the other peripher- 

 ally. The centrally directed branches collectively form the so-called roots, 

 four or five in number, which enter the medulla between the olivary and 

 restiform bodies. The peripherally directed branches collectively form the 

 two main divisions, from the distribution of which, to the tongue and pharynx, 

 the nerve takes its name. 



Distribution. The axons of the centrally directed branches after 

 entering the medulla pass toward its dorsal aspect, where they bifurcate, 

 give off collateral branches, and terminate in fine end-tufts in the immediate 

 neighborhood of two groups of nerve-cells, the sensor end-nuclei. The axons 

 of the peripherally directed branches, after emerging from the base of the 

 skull through the jugular foramen, pass forward and inward under cover of 

 the stylo-pharyngeal muscle; winding around this muscle they divide into 

 terminal branches which are distributed to the mucous membrane of the 



