THE HEART 261 



and the heart is most likely to result in a quickening 

 of the beat. This indicates that the prevailing influence 

 of the centers is inhibitory. The paths of the nerve- 

 impulses by which the heart is thus restrained from 

 "running away" are in the two vagus nerves. These 

 nerves have been described as being the tenth pair in 

 the cranial series. They have many other functions 

 besides that of cardiac inhibition, though this is the 

 one the student is most apt to associate with them. 



The discovery, in 1845, that the heart is inhibited when 

 the vagus nerve is stimulated was one of the most sig- 

 nificant in the history of physiology. It showed that the 

 activity of living tissues can be restrained as well as 

 excited through nerves leading to them. This is a 

 conception which it was most important for the scientist 

 to grasp and it remains to this day unfamiliar to the 

 layman. Cardiac inhibition is quite different from the 

 inhibition of the tone of skeletal muscles which was 

 described in connection with reciprocal innervation 

 (Chapter VI). With the heart it is the contractile sub- 

 stance which is prevented from full activity. In the 

 other case the restraint is applied to certain nerve cells. 



The facts of cardiac inhibition were first observed in 

 the frog. The heart of this animal stops beating when 

 the vagus is stimulated and can be kept at rest for an 

 indefinite time. Anyone noting the behavior of the 

 heart for the first time would be likely to infer that the 

 muscle had been thrown into a sustained, cramp-like 

 contraction resembling the tetanus of the skeletal type. 

 That is to say, he would think that the beating of the 

 heart ceased because no diastole occurred. However, 

 close inspection makes it plain that the inhibited heart 

 is in diastole and not systole; it is not prevented from, 

 relaxing but from contracting. 



Complete inhibition of the heart involves complete 

 arrest of the circulation. This would result in death 

 within a few minutes in any of the warm-blooded animals. 

 But it is scarcely possible to kill a frog or a turtle by this 



