THE CIRCULATION OF THE BLOOD. 



307 



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. 



FIG. 141. TRACINGS SHOWING THE EFFECTS ON THE HEART-BEAT OF THE FROG FROM STIMU- 

 LATION OF THE SYMPATHETIC NERVES PRIOR TO THEIR UNION WITH THE VAGUS NERVE. The 

 upper tracing shows an increase in the rate, which before stimulation was 15 per minute and 

 during stimulation 30 per minute. Before stimulation the height of the ventricular beat was 

 9 mm. and during the stimulation it was 12 mm. The lowest tracing shows a similar series of 

 effects, the differences being only of degree. (Brodie.) 



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. 141 . The upper tracing shows that the heart was 

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

 to 30 per minute during stimulation. On the cessation of the stimulation, 

 the heart slowly returned to its former rate. Coincidently with this acceler- 

 ation of the rate there was an augmentation of the force of the ventricular 

 contraction as shown by an increase in the height of the ventricular con- 

 traction which before stimulation was 9 mm., but during stimulation 12 mm. 



