3I4 TEXT-BOOK OF PHYSIOLOGY 



level they emerge from the cord in company with the nerve-fibers composing 

 the anterior roots of the second third, and fourth thoracic nerves. After 

 a short course, they enter the white rami communicantes, then the sympa- 

 thetic chain and pass upward to the ganglion Stella turn (the first thoracic), 

 and by way of the annulus of Vieussens (in the dog) to the inferior cervical 

 ganglion as well, around the nerve-cells of both of which their terminal 

 branches arborize. 1 From the nerve-cells of both the stellate and inferior 

 cervical ganglia, the sympathetic nerves proper arise, which after emerging 

 from the ganglia pass toward the heart and become associated with the fibers 

 of the vagus and assist in the formation of the cardiac plexuses. On reach- 

 ing the heart they may terminate directly in the muscle-cell or indirectly 

 through the intermediation of intra-cardiac nerve-cells. The former mode 

 of termination is the more probable. Experiment has shown that both 

 the pre- and post-ganglionic fibers are efferent in function. 



The Origin and Distribution of the Vagus Nerve in Mammals. 

 The vagus nerve-fibers which influence the heart are connected on the one 

 hand with the heart, through the intermediation of the intra-cardiac cells, and 

 on the other hand with the central nerve system. Histologic investigation has 

 shown that the vagus nerve-trunk of man and mammals generally, contains 

 medullated fibers of large and small size. Experiment has shown that the 

 large fibers are afferent, the small fibers efferent in function. 



The large afferent fibers arise in the ganglia situated on the trunk of the 

 nerve. From their contained nerve-cells a short axon process proceeds 

 which soon divides into a central and a peripheral branch. The 

 central branch passes toward and into the gray matter beneath the floor of 

 the fourth ventricle, where its end-tufts arborize around nerve-cells; the 

 peripheral branch passes toward tlie general periphery to be distributed to 

 the mucous membrane of the lungs, stomach, intestine, etc. 



The small efferent fibers are the peripherally coursing axons of nerve- 

 cells situated in the gray matter beneath the floor of the fourth ventricle at 

 the tip of the calamus scrip torius. The exact course of these fibers from 

 their origin into the trunk of the vagus is not positively known. According 

 to some investigators, they leave the medulla by way of the spinal accessory 

 nerve and enter the trunk of the vagus through its internal or anastomotic 

 branch; according to recent investigations made by Schaternikoff and 

 Friedenthal, they leave the medulla along the path by which the afferent 

 fibers enter and never become associated with the spinal accessory nerve 

 at its origin. 



In the neighborhood of the inferior or recurrent laryngeal nerves, branches 

 containing efferent fibers are given off, which pass to the heart by way of the 

 cardiac plexus. The terminal branches of these fibers are not distributed 

 directly to the heart-muscle, but to the intra-cardiac nerve-cells, around 

 the bodies of which they end in basket-like formations. The fibers in the 

 vagus are pre-ganglionic; those of the nerve-cells post-ganglionic. (See 

 Fig. 140.) 



The Origin and Distribution of the Sympathetic and Vagus Nerves 

 in the Frog. In the frog and allied animals the relation of these two sets of 

 nerve-fibers, viz., the efferent sympathetic fibers and the efferent vagus 



B 1 In man the annulus of Vieussens connects the inferior cervical with the middle cervical 

 ganglion. 



