THE CARDIAC NERVES. 593 



The Action of the Accelerator Fibers. In experimental 

 work the accelerators are usually stimulated in one or more of the 

 branches represented schematically as 5, 3, 6, in Fig. 243, or 3, in 

 Fig. 244, The effect is an increase in the rate of beat of the heart, 

 which may be very evident, amounting to as much as 70 per cent, 

 or more of the original rate, or may be very slight. When accelera- 

 tion is obtained the latent period is considerable and the heart 

 does not return at once to its" normal rate upon cessation of the 

 stimulus (see Figs. 245 and 246). In some cases the effect upon the 

 heart is an acceleration pure and simple, that is, the rate of beat is 



Fig. 245. To show the acceleration of the heart-rate in dog upon stimulation of the 

 accelerator fibers. The uppermost line gives the heart-rate as recorded by a Hurthle manometer 

 inserted into the carotid; the middle line indicates the beginning and duration of the stimulus 

 (tetanizing induction shocks) ; the bottom line marks seconds. The pulse-rate was increased 

 from 105 to 135 per minute. The heart did not recover its normal rate until thirty seconds 

 after the stimulation. 



increased without any evidence of an increase in the force of the 

 beats. The larger number of beats is offset by the smaller amplitude 

 of each beat ; so that the blood-pressure in the arteries is unchanged. 

 In other cases the effect upon the heart may be an increase not only 

 in rate but also in the force or amplitude of the beats, or the rate 

 may remain unaffected and only the amplitude of the heart 

 beats be increased. For these reasons most authors favor the 

 view that the accelerator nerves, so called, contain in reality 

 two sets of fibers, one, the accelerators proper, whose function 

 is simply to accelerate the rate, and one, the augmentors, that 

 cause a more forcible beat. The augmenting action is obtained 

 especially from the nerves of the left side. 



Tonicity of the Accelerators and Reflex Acceleration. 

 The results of the most careful work show, without doubt, that the 

 accelerators to the heart are normally in a state of tonic activity.* 



* For a discussion of this and other points in the physiology of the ac- 

 celerators see Hunt, "American Journal of Physiology," 2, 395, 1899, and 

 "Journal of Experimental Medicine," 2, 151, 1897. 

 38 



