THE CIRCULATION OF THE BLOOD. 305 



gastric. Experimental investigation has established the fact that the sympa- 

 thetic is the motor nerve to the heart, the nerve which accelerates the rate 

 and augments the force of the normal beat; while the vagus is the inhibitor 

 nerve, the nerve which inhibits or controls the rate and the force of the beat 

 in accordance with the necessities of blood distribution. For this reason 

 these two nerves will be considered in the order stated. The course of 

 the fibers composing these nerves, from their origin to their termination, and 

 the relation they bear to one another and to neighboring structures, vary 

 somewhat in different animals. 



The Origin and Distribution of the Sympathetic Nerves in Mammals. 

 The sympathetic nerve-fibers which influence the action of the heart, are 

 connected on the one hand with the heart-muscle itself and on the other 

 hand with nerve-fibers coming from the central nerve system. The former 

 are non-medullated and post-ganglionic, the latter medullated and pre- 

 ganglionic. 



The pre-ganglionic fibers have their origin in the medulla oblongata and 

 very probably from nerve-cells in the gray matter beneath the floor of the 

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

 level of the second and third thoracic nerves. At this level they emerge from 

 the cord in company with the nerve-fibers composing the anterior roots of 

 the second and third thoracic nerves. After a short course, they enter the 

 white rami communicantes, enter the sympathetic chain and pass upward 

 to the ganglion stellatum, and by way of the annulus of Vieussens to the 

 inferior cervical ganglion as well, around the nerve-cells of which their 

 terminal branches arborize. 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 reaching 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 the 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 scriptorius. The exact course of these fibers from their origin into 



