THE CARDIAC NERVES. 



583 



Analysis of the Action of the Inhibitory Fibers. The prom- 

 inent effect of the action of the inhibitory fibers is the slowing 



Fig. 240. To show the inhibition of the heart from stimulation of the vagus in the 

 dog. Record B is, the blood-pressure tracing. The vagus was stimulated twice. The 

 marks x, x, indicate the beginning and end of the stimulus. The first stimulation was 

 weak ; it will be noted that the heart escaped and began beating before the stimulus was 

 withdrawn. The second stimulus was stronger ; the inhibition lasted some time after re- 

 moval of the stimulus. The upper curve (K) is a plethysmographic (oncometer) tracing 

 of the volume of the kidney. It will be noted that when the heart stops and blood-pressure 

 falls the kidney, like the other organs, diminishes in volume. (Dawson.) 



of the rate of the heart beat. Numerous observers have called 

 attention to the fact that the vagus fibers may also cause a weaken- 

 ing in the force of the beat as well as a slowing in the rate, or, 

 indeed, the two effects may be obtained separately. This fact has 

 been shown especially for the auricles.* In the heart of the terrapin 

 one may, by using weak stimuli, obtain only a weakening of the 

 auricular beats without any interference with the rate (Fig. 241), 

 while by increasing the stimulus the slowing in rate becomes evident 

 combined with a diminution in force or extent. Although the 



* Bayliss and Starling, "Journal of Physiology," 13, 410, 1892. 



