3 i6 TEXT-BOOK OF PHYSIOLOGY 



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 v ago-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 

 cells, while others plunge into the heart, course along the auricular septum 

 on the left side and finally terminate at or near the ganglion cells of the base 

 of the ventricle. The mode of termination of both the vagus and sympa- 

 thetic fibers is similar to that observed in the mammals. 



The Physiologic Actions of the Sympathetic Nerves in the Frog. 

 The information now possessed regarding the influence which the central 

 nerve system exerts on the heart through these nerves, has been derived 

 largely from experiments made on the nerves of the frog, toad, and turtle. 

 Inasmuch as the sympathetic and vagus nerves in the frog and related 

 animals are bound up in a common sheath, it is necessary in order to demon- 

 strate their respective functions first to divide the nerves, above their union 

 at the vagus ganglion, and then stimulate their peripheral ends. The heart 

 should be exposed and attached to a recording lever so that its movements 

 may be taken up and recorded on a moving recording surface. 



Stimulation of the sympathetic fibers with induced electric currents, 

 prior to their union with the vagus, is followed by an increase in the rate, 

 or an augmentation in the force of the heart-beat or both, at the same time. 

 The effects of such a stimulation with induced currents of moderate intensity 

 are graphically shown in Fig. 138. The upper tracing shows that the heart was 

 first accelerated, the beats increasing from 15 per minute before stimulation, 



