594 CIRCULATION OF BLOOD AND LYMPH. 



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 control 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 

 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, therefore, as true antagonists, acting 

 in opposite ways upon the heart. The existence of the accel- 

 erator nerves makes possible, of course, their reflex stimulation. 

 Experimentally it is found that stimulation of various sensory 

 nerves those of the limbs or trunk, for instance may cause 

 reflexly either an increase or decrease in the heart rate, and as 

 a matter of experience we know that our heart rate may be 

 increased by various changes, particularly by emotional states. 

 The natural explanation of such accelerations is that they are 



Fig. 246. To show the acceleration and augmentation produced by a strong stimulus. 

 Isolated cat's heart, stimulation on left side. The upper curve gives the ventricular 

 contractions, the lower one the auricular contractions. The lowermost line gives the 

 time in seconds and the line above indicates the duration of the stimulation of the accel- 

 erator nerve. 



due to reflex stimulation of the nerve cells in the central nervous 

 system which give rise to the accelerator fibers. But another 

 point of view is possible. An increase in heart rate may be 

 brought about either by a reflex stimulation of the accelerator 



