THE CIRCULATION. 263 



same result follows: there is a diminution in the number of the 

 heart-beats. 



PECULIARITIES. Some of the points of peculiarity of the vagus 

 and its action are: 1. The heart is arrested in diastole; so that the 

 slowing depends upon the period of diastole. 2. The irritation of 

 one nerve alone acts upon the two sets of inhibitory ganglia in the 

 heart by reason of association fibers. 3. After the arrest of the heart 



Fig. 80. Arrest of the Heart of a Rabbit by Irritation of the Peripheral 

 End of the Pneumogastric in the Neck. (GLEY.) 



PC, Carotid pressure equals 12 centimeters of mercury. E, Excitation of 

 the nerve by an induction current. T, Time every 2 seconds. 



by excitation of the vagus the heart begins its contractions first in the 

 auricles. 



Cardio-inhibitory Reflexes. A tap upon the abdominal wall is 

 able to throw the pneumogastric into greatly increased action; so 

 that the heart is often stopped and death ensues. In this case the 

 sympathetic nerves of the solar plexus convey the impression up the 

 spinal cord to the center of the pneumogastric in the medulla. From 

 the medulla the impulse is sent down the inhibitory fibers of the 

 pneumogastric, which causes arrest of the heart. The arrest occurs 

 always in diastole, never in systole. 



