THE ENCEPHALIC OR CRANIAL NERVES 649 



The Taste Fibers. The taste fibers distributed to the posterior one- 

 third of the tongue have become dissociated from the taste fibers found 

 in the chorda tympani nerve. They have their origin in nerve-cells in 

 the one or the other of the ganglia on the course of the glosso-pharyngeal. 

 Each nerve-cell gives origin to a peripherally coursing branch which is 

 distributed to the mucous membrane of the tongue and to a centrally 

 coursing branch which enters the medulla and becomes associated with a 

 sensor end nucleus in common with the taste fibers in the chorda tympani 

 nerve. 



The glosso-pharyngeal nerve also contains vaso-dilatator and secreto- 

 motor nerve-fibers which doubtless have their origin in the vaso-dilatator 

 center and the salivary center in common with corresponding fibers con- 

 tained in the trunk of the nerve of Wrisberg. After a short course they 

 leave the glosso-pharyngeal by way of Jacobson's nerve. 



Stimulation of Jacobson's nerve is followed by dilatation of the blood- 

 vessels of, and secretion from, the mucous membrane of the lower lip, cheek, 

 and gums, and from the parotid gland. Division of the nerve is followed 

 by the opposite results. The course of the fibers which give rise to these 

 results is by way of the lesser petrosal to the otic ganglion, around the cells 

 of which the fibers arborize. From the cells of this ganglion non-medul- 

 lated fibers pass to the blood-vessels and secretor cells of the parotid 

 gland by way of the auriculo-temporal branch of the trigeminal nerve. 

 These nerve-fibers are thus members of the autonomic system of nerves. 



Functions. The afferent fibers of the glosso-pharyngeal transmit 

 nerve impulses from the parts to which they are distributed to the cerebral 

 cortex, where they evoke sensations of pain and sensations of taste; they 

 also assist in all probability in the performance of certain reflexes connected 

 with deglutition. The afferent fibers are therefore divisable into nerves of 

 general sensibility and nerves of special sense. The efferent fibers transmit 

 impulses to muscles, exciting them to activity, and to the otic ganglion, 

 which in turn dilates blood-vessels and excites secretion. The fibers excit- 

 ing secretion have their origin in the nucleus salivalorius, from which the 

 efferent autonomic fibers in the chorda tympani nerve arise. 



THE TENTH NERVE. THE PNEUMOGASTRIC OR VAGUS 



The tenth cranial nerve, the pneumogastric or vagus, consists, as shown 

 by histologic methods of research, of both afferent and efferent fibers, in- 

 dependent of those derived in its course from adjoining motor or efferent 

 nerves. Near the exit of the nerve from the cavity of the cranium it presents 

 two ganglionic enlargements known respectively as the ganglion of the root 

 (the jugular) and the ganglion of the trunk (the plexiform). 



Origin of the Afferent Fibers. The afferent fibers take their origin 

 in the monaxonic cells of the ganglia on the root and trunk. 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, ten to fifteen in number, which enter the 

 medulla between the restiform body and the lateral column. The periph- 

 erally directed branches collectively form a portion of the common trunk 

 of the nerve. 



Distribution. The axon of the centrally directed branches after entering 

 the medulla pass toward its dorsal aspect, where they bifurcate, give collat- 



