306 TEXT-BOOK OF PHYSIOLOGY. 



the trunk of the vagus is not positively known. According to some investi- 

 gators, 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. 147.) 



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. 



The sympathetic nerve-fibers in this animal are also in connection with 

 the heart on the one hand and with nerve-fibers coming from the central 

 nerve system on the other hand. The pre-ganglionic fibers take their origin 

 very probably in nerve-cells in the medulla oblongata. From this origin 

 they descend and emerge from the spinal cord in the anterior roots of the 

 third spinal nerve, then pass through the white rami communicantes to the 

 third sympathetic ganglion around the nerve-cells of which their terminal 

 fibers arborize. 



From the nerve-cells of this ganglion, the sympathetic nerves proper, 

 the post-ganglionic, non-medullated fibers arise. From this origin they 

 ascend, passing successively through the second sympathetic ganglion, the 

 annulus of Vieussens, the first sympathetic ganglion, to the ganglion on the 

 trunk of the vagus, at which point they enter the sheath of the vagus fibers 

 and in company with them pass to the heart. For this reason the common 

 trunk is generally spoken of as the vago-sympathetic nerve. 



The vagus nerve is connected with the medulla oblongata by a series 

 of from six to eight roots. A short distance from the medulla, the nerve 

 trunk passes through a large opening in the cranium beyond which it presents 

 an enlargement, termed the vagus ganglion. The peripheral end of this 

 ganglion gives off two trunks, one the glossopharyngeal, the other the vagus 

 proper. 



The vagus nerve proper in the frog also consists of both afferent and 

 efferent fibers which have practically the same origin, distribution and 

 termination as the corresponding fibers in the mammal. 



After the union of the sympathetic fibers with the vagus fibers, the com- 

 mon trunk passes forward to the angle of the jaw, winds around the pharynx 

 just beneath the border of the petro-hyoid muscle and in close relation with 

 the carotid artery. As the nerve approaches the heart it divides into two 

 branches, the pulmonary and the cardiac. At the sinus venosus some of 

 the fibers become related, histologically and physiologically, with the ganglion 



