282 MANUAL OF PHYSIOLOGY. 



INHIBITORY NERVES OF THE HEART. 



It was observed by Weber that electric stimulation of the vagus 

 nerve caused a slowing of the heart's rhythm, and if increased 

 gave rise to a standstill of the heart in diastole ; the heart beat 

 gradually recommencing some time after the stimulus had been 

 removed. On the other hand, the section of both vagi produced 

 an increase in the rapidity of the heart beat varying according 

 to the kind of animal experimented upon. 



Section of only one vagus, however, has not this effect. From 

 these experiments it would appear 1. That some fibres of the 

 vagus bear impulses of a checking or inhibitory nature to the 

 intrinsic nerves of the heart. 2. That these influences are con- 



FIG. 323. 



Tracing showing the effect of weak Stimulation of Vagus Nerve. Stimulus applied 

 between vertical lines. (Recording surface moved from left to right.) 



stantly in operation, or, in other words, the vagi exert a tonic 

 inhibitory influence on the rapidity of the heart beat. 3. The 

 tonic action of one vagus bears inhibitory influence sufficient to 

 regulate the heart's action. This toriicity of the vagus inhibition 

 is moreover more marked in man than in dogs and rabbits, and 

 is reduced to a minimum in frogs, where section of the vagi pro- 

 duces very little effect on the rate of the beat. 



Vagus inhibition is increased by the following circumstances : 

 (a) certain psychical phenomena, such as terror, which is said to 

 produce a temporary standstill ; (6) deficiency of arterial blood 

 in the medulla oblongata ; (c) increase of the blood pressure 



