524 CIRCULATION OF BLOOD AND LTMPH. 



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 seem to 

 incline to the view that the accelerator nerves, so called, contain in 

 reality two sets of fibers, one, the accelerators proper, whose func- 

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

 cause a more forcible beat. Under normal conditions we may sup- 

 pose that these fibers act either separately or in combination. 



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. 



Fig. 217. To show the acceleration of the rate of heart beat upon stimulation of the 

 accelerator nerve, and the long latent period (Beaunis) : PF, The pulse beat registered by 

 a mercury manometer connected with the femoral artery. The duration of the stimulus 

 is indicated by the broad marking on line E. The line A shows the beginning of accelera- 

 tion, and indicates the long latent period. 



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 

 studied experimentally the effect upon the heart of stimulating 

 simultaneously the inhibitory and the accelerator nerves. The work 

 done upon this subject by Hunt seems to make it very certain that 

 in all such cases the result, so far as the rate is concerned, is the 

 algebraic sum of the effects of the separate stimulations of the nerve. 

 The inhibitory and the accelerator fibers must be considered,. 



* 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. 



