THE CRANIAL NERVES. 599 



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 in all probability their origin in the nucleus salivatorius. 



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- 

 erals, and terminate in fine end-tufts in the immediate neighborhood of two 

 groups of nerve-cells, the vagal sensor end-nuclei. 



The axons of the peripherally directed branches unite to form a portion 

 of the common trunk, which, as it descends the neck and enters the thorax and 

 abdomen, gives off a number of branches which are ultimately distributed to 

 the mucous membrane of the esophagus, larynx, lungs, stomach, and in- 

 testine, and also to the heart. The afferent fibers thus serve to bring into 

 anatomic and physiologic relation the mucous membrane of these organs 

 and certain sensor end-nuclei in the medulla oblongata. 



Origin of the Efferent Fibers. The efferent fibers take their origin 

 from nerve-cells located in the lateral portion of theformatio reticularis at 

 some distance below the floor of the fourth ventricle. These cells constitute 

 the lower portion of the nucleus ambiguus. 



Distribution. From their origin the efferent axons pass dorsally to 

 near the sensor end-nuclei, then turn outward and forward, and finally 

 emerge from the medulla in close association with the afferent branches. 

 They are ultimately distributed to the muscles of the lower two-thirds of the 

 esophagus; to the muscle-fibers of the stomach and perhaps the intestines; 

 to the walls of the gall-bladder and to the sphincter of the common bile duct; 

 and to the non-striated muscle-fibers of the bronchial tubes, and to the heart. 

 Among the efferent fibers are some which are distributed to the gastric 

 glands and to the pancreas (?). From this distribution it is apparent 

 that the efferent fibers in the vagus are largely if not entirely members of 

 the autonomic system of nerves. 



The efferent fibers serve to bring the nerve-cells from which they arise 

 into anatomic and physiologic connection with a portion of the musculature 



