CARDIAC INHIBITION AND ACCELERATION 313 



purposes of experimentation, either the right or the left nerve may be 

 the more effective. As the inhibitor impulses pass from the vagus 

 center to the periphery, i.e., in an efferent direction, it may readily 

 be gathered that the inhibition may also be obtained by stimulating 

 solely the distal end of the divided vagus. In the frog, turtle and 

 allied animals, it is also possible to arrest the heart by applying the 

 electrodes directly to its sino-auricular region, because this particular 

 area gives lodgment to a plexus which possesses an inhibitor function 

 and may therefore represent the principal peripheral relay station 

 of the vagus. 



The inhibition of the heart is characterized by a gradual pre- 

 ponderance of its diastolic period. Its systolic movement is hindered 

 moxe and more until its musculature temporarily enters a state of com- 

 plete relaxation. The organ as a whole becomes greatly distended 



Fig. 164. — Record op the Contractions of the Frog's Heart During Stimulation of 



THE Vagus Nerve. 

 The time is given in seconds, the stimulation is indicated by the signal. 



with blood and exhibits a pronounced venous appearance. The in- 

 hibition appears as a rule after a brief latent period and continues for 

 a few moments after the cessation of the excitation. Furthermore, 

 while the principal effect of the stimulation consists in a diminution of 

 the frequency of the heart, this inhibition is frequently associated with 

 a reduction in the amplitude of the individual contractions. Weak 

 stimuli, for example, are prone to affect solely the rate and to give 

 merely a partial cessation of the contractions, while stronger stimuli 

 diminish the height as well as the frequency of the contractions until 

 a complete stoppage has been obtained. The strength of the stimulus, 

 however, is not the only factor determining these effects; in fact, 

 we shall see later on that they find their origin in certain functional 

 peculiarities of the inhibitor mechanism. 



The inhibited heart resumes its activity by giving a contraction 

 which is either smaller or much larger than normal. In either case, 

 the beats regain their former amplitude gradually within a few 

 moments. It is to be noted, however, that the heart cannot be 

 kept in the inhibited state for any length of time, because it resumes 



