552 TEXT-BOOK OF PHYSIOLOGY. 



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, independent of those derived in its course frohi adjoin- 

 ing 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 plexifprm). 



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 peripherally directed branches 

 collectively form a portion of the common trunk of the nerve. 



Distribution. The axons of the centrally directed branches 

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

 bifurcate, give off collaterals, 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 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 intestine, and also to the heart. The 

 afferent fibers thus serve to bring into anatomic and physiologic 

 relation the mucous membrane of these organs with 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 the formatio 

 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 dor- 

 sally to near the sensory 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 levator 

 palati, azygqs uvulae, and tensor palati muscles; to the superior and 

 inferior constrictor muscles of the pharynx, and to the muscles of 

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

 intestines; and to the non-striated muscle-fibers of the bronchial 

 tubes. Among the efferent fibers are some which are distributed to 

 the gastric glands and to the pancreas. 



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



