THE CARDIAC NERVES. 543 



paralysis of the inhibitory fibers in the heart by atropin, stimulation of the 

 vagus causes an acceleration of the heart. Little attention has been paid to 

 the physiology of these fibers, since it seems evident that the great outflow of 

 accelerators is made via the sympathetic system. 



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. 223, or 3, in 

 Fig. 224. 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 Fig. 225). In some cases the effect upon the heart 

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

 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 energy of the beats, or the rate may 

 remain unaffected and only the force 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.* 

 When these nerves are cut upon both sides the heart rate is decreased. 

 We must believe, therefore, that under normal conditions the heart 

 muscle is under the constant influence of two antagonistic influ- 

 ences, one through the inhibitory fibers tending to slow the rate, 

 one through the accelerator fibers tending to quicken the rate. The 

 actual rate at any moment is the resultant of these two influences. 

 While such an arrangement seems at first sight to be unnecessary 

 from a mechanical standpoint, it is doubtless true that it possesses 

 some distinct advantage. Possibly it makes the heart more 

 promptly responsive to reflex regulation. Balanced mechanisms 

 of this kind are found in other parts of the body where smooth and 

 prompt reactions to stimulation seem to be especially necessary, 

 for example, the constrictor and dilator fibers of the iris, the ex- 

 tensor and flexor muscles of the joints, etc. Physiologists have 



* 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. 



