THE HEART 107 



during vagus excitation the propagation of the cardiac excita- 

 tion is more difficult. A demarcation current derived from a 

 portion of the auricle is increased by vagus excitation, although 

 the auricle shows no visible change of form. The heart cannot 

 be continuously inhibited by prolonged stimulation. It escapes 

 from the influence of the vagus and resumes its former rhythm 

 with perhaps increased force. Immediate stimulation of the 

 second vagus after the heart has escaped from the influence 

 of the first is without effect, making it probable that both 

 nerves act upon the same mechanism in the heart. 



Stimulation of the sympathetic or augmentor fibers causes 

 an increase in the rate of the heart beat from 7 to 70 per cent., 

 the amount of increase depending upon the heart's rate before 

 stimulation. A long excitation produces no greater acceleration 

 than a short one. The force of the beat, the pulse volume, 

 and the speed of the excitation wave are all increased. The 

 latent period is usually a- long one, extending from two to ten 

 seconds. The acceleration may continue for several minutes 

 after the excitation has ceased. It has been found that pres- 

 sure brought to bear upon the human heart where a defect 

 in the chest wall makes it accessible can be felt by the subject, 

 and direct stimulation of the surface of the heart in animal? 

 may cause movements of the limbs. The latter event is absent 

 when the vagi are cut, so that it is thought the vagus carries 

 afferent fibers to the brain. Stimulation of the central end 

 of the cut vagus when the other is intact slows the heart rate. 

 This effect disappears when both vagi are cut. 



The depressor is a nerve whose fibers pass from the heart 

 to the central nervous system. Section and stimulation of 

 the peripheral end cause no appreciable change. Stimulation 

 of the central end causes a general fall of blood pressure to 

 one-half or one-third its former height, and lessens also the 

 pulse rate. Both are restored after stimulation ceases. When 

 both vagi are cut there is a fall of blood pressure upon stimula- 

 tion of the depressor, but no change in the pulse rate. This 

 shows that the impulses from the depressor may spread to 

 the cardio-inhibitory centre and through the vagi slow the 

 heart. It shows, moreover, that the fall in pressure is not 

 dependent upon the vagi. Section of the splanchnic nerve 

 causes dilatation of the abdominal vessels and a fall of the 



