55 o TEXT-BOOK OF PHYSIOLOGY. 



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

 and cerebellum and to other centers. The specific physiologic stim- 

 ulus is supposed to be a variation in pressure in the ampullae of the 

 semicircular canals caused by movements of the endolymph induced by 

 changes in the position of the head and body. The impulses carried 

 by the vestibular nerve give rise reflexly to certain adaptive and pro- 

 tective movements by which the equilibrium 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 certain 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 peripherally. The centrally 

 directed branches collectively form the so-called roots, four or five 

 in number, which enter the medulla between the olivary and resti- 

 form 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 r 

 the sensor end-nuclei. The axons of the peripherally directed 

 branches7"aTter 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 posterior one-third of the tongue, pharynx, soft palate, uvula, 

 and tonsils (Fig. 257). 



Origin of the Efferent Fibers. The efferent fibers serve to 

 bring the nerve-cells from which they arise into connection with a 

 portion of the musculature of the fauces and pharynx. These nerve- 

 cells are located in the lateral portion of the jormatio reticularis at 

 some distance below the floor of the fourth ventricle. They consti- 



