228 



THE CIRCULATION OF THE BLOOD 



as postganglionic fibers, proceeding to the heart through branches com- 

 ing off from the stellate ganglion itself, or from the ansa subclavii or. 

 inferior cervical ganglion. (Fig. 70). In cold-blooded animals, such as 

 the frog, the sympathetic fibers run up to the upper end of the cervical 

 sympathetic and join the vagus immediately after it leaves the cranial 

 cavity. They then proceed along with this nerve forming the vago- 

 sympathetic to the heart. The effect of stimulation is shown in Fig. 71. 

 The sympathetic nerve differs from the vagus in that a much longer la- 

 tent period elapses before its influence becomes effective, and this persists 

 for a much longer period after the stimulus is withdrawn. If the vagus 



B. 



Fig. 71. Tracings showing the effects on the heartbeat of the frog resulting from stimulation of 

 the sympathetic nerves prior to their union with the vagus nerve. (From Brodie.) 



and sympathetic are stimulated at the same time, as by exciting the vago- 

 sympathetic in the frog, the first effect observed is that of the vagus 

 usually followed, after removal of the stimulus, by the sympathetic ef- 

 fect. If the stimulus is maintained for a long time, so that the vagus 

 becomes fatigued, escapement will occur earlier than with pure vagus 

 stimulation, and augmentation may become apparent. The sympathetic 

 influence is, however, never so strong as that of the vagus. The two 

 nerves are therefore not antagonistic in the sense that the one neutralizes 

 the effect of the other; but when both are stimulated, the heart responds 

 first to the vagus and later to the sympathetic. 



