THE CIRCULATION OF THE BLOOD 315 



fourth ventricle. From this origin they descend the spinal cord as far as the 

 level of the second, third, and at times the fourth thoracic nerves. At this 

 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 verte- 

 bral ganglionic chain and pass upward to the ganglion stellatum (the first 

 thoracic), and by way of the annulus of Vieussens (in the dog) to the in- 

 ferior cervical ganglion as well, around the nerve-cells of both of which 

 their terminal branches arborize. 1 The post-ganglionic fibers, i.e., the 

 sympathetic nerves proper, arise from the nerve-cells of both the stellate 

 and inferior cervical ganglia; after emerging from the ganglia they pass 

 toward the heart and assist in the formation of the cardiac plexuses. On 

 reaching the heart they may terminate, directly in the muscle-cell or 

 indirectly through the termination 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. 

 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-celts 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 the general periphery to be distributed to 

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



The small efferent fibers in the trunk of the vagus nerve also consist 

 of two consecutively arranged and histologically related neurons, pre- 

 ganglionic and post-ganglionic. Experiment has shown that some of 

 the small efferent fibers are in function, inhibitor nerves of the heart. 



The pre-ganglionic fibers originate in cells in the gray matter beneath 

 the floor of the fourth ventricle at the tip of the calamus scrip torius. 

 From their origin they pass outward and downward in the trunk of the 

 vagus. In the neighborhood of the heart they leave the vagus and 

 pass by way of the cardiac plexus to the heart, where they form synapses 

 with the dendrites of the intra-cardiac cells. 



The post-ganglionic fibers originate in the cells of some of the intra- 

 cardiac ganglia from which they pass to terminate directly in the cardiac 

 muscle cells. There is some evidence for the view that nerve-cells here 

 referred to constitute a part at least of the Keith- Flack node. 



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 

 fibers, is somewhat different; and because of the fact that these nerves in this 

 animal are largely employed for determining experimentally their respective 

 actions on the heart, this relation should be clearly understood. 



x ln man the annulus of Vieussens connects the inferior cervical with the middle cervical 

 ganglion. 



