264 HUMAN PHYSIOLOGY 



means. The oxygen requirement of the tissues is so 

 very moderate that fatal asphyxia does not take place 

 either in the heart itself or elsewhere. The heart of a 

 turtle has been held motionless by vagus stimulation 

 for four hours and has resumed beating when the arti- 

 ficial restraint has been removed. When we make ex- 

 periments, upon cats and dogs we find that it is not 

 possible to kill these animals either by inhibition of the 

 heart. Life is preserved through the trial because the 

 warm-blooded heart soon resumes beating in spite 

 of the application of stimuli to the vagus. The heart, 

 in such a case, is said to " escape" from inhibition. 



The extent to which the heart can be inhibited varies 

 greatly in different species. In the cat it can scarcely 

 be stopped at all, but it can be slowed and thus its output 

 can be diminished. Perhaps the student is somewhat 

 misled by the striking experiment of stopping the heart. 

 He is to consider that the laboratory trial does not illus- 

 trate a normal function but an exaggeration of one. 

 It could never be advantageous to arrest the heart of 

 any animal. So we must think that the true service 

 of the mechanism reached by the vagus fibers is to 

 secure economy and to provide a reserve for emergencies. 



The point has been made that the average condition 

 of the living heart is one in which some degree of vagus 

 inhibition is operative. We say that the vagus influence 

 is a "tonic" or sustained one. If this is true, it follows 

 that a heart which is speeding up may be displaying the 

 results of a withdrawal of the usual restraint. Its quick- 

 ening may be like the acceleration of the train on the 

 down grade when the brakes are taken off. The nervous 

 system is not hurrying the heart but merely giving it 

 liberty to make its own pace. 



Nevertheless, the heart is provided with nervous con- 

 nections through which it can be definitely excited to 

 work more rapidly and more powerfully. These con- 

 nections are the accelerator nerves of the heart. They 

 reach it otherwise than by the vagus, and the general 



