THE ENCEPHALIC OR CRANIAL NERVES 635 



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

 groups of nerve-cells, the vagal sensor end^nudei. 



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 enoVnuclei 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 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 auto- 

 nomic 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 

 of the alimentary canal and its diverticulum, the lung as well as the heart and 

 gastric glands. 



Communicating Branches. At or near the ganglia the vagus receives 

 communicating branches from the eleventh nerve, the spinal accessory, the 

 facial, the hypoglossal, and the anterior branches of the two upper cervical 

 nerves. Owing to this manifold origin of the efferent fibers in the trunk and 

 peripheral branches of the vagus, it is, in some instances, difficult, if not 

 impossible, to determine to which of these nerves a given muscle contraction 

 is to be referred 



Vagal Branches. As the vagus passes down the neck it gives off the 

 following main branches: 



1. The pharyngeal nerves, which, after entering into the formation of the 



pharyngeal plexus, are distributed to the mucous membrane and to the 

 muscles of the pharynx ; e.g., superior and inferior constrictors, the levator 

 palati, and the azygos uvulae; according to B eevor and Horsley the nerves 

 for these muscles are branches of the spinal accessory. 



2. The esophageal nerves, which after entering into the formation of the 



esophageal plexus, are distributed to the mucous membrane, and to the 

 muscles of the lower two-thirds of the esophagus. 



3. The superior laryngeal nerve which, entering the larynx through the 



thyro-hyoid membrane, is distributed to the mucous membrane lining 

 the interior of the larynx and to the crico-thyroid muscle. From the 

 superior laryngeal and the main trunk small branches are given off 

 whirh in thp rabbit unite to form a single nerve, the so-called depressor 



