THE CARDIAC NERVES. 525 



therefore, as true antagonists, acting in opposite ways upon the 

 same part of the heart. The existence of the accelerator 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, particu- 

 larly by emotional states. The natural explanation of such ac- 

 celerations is that they are 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 fibers or by a reflex inhibition of the cardio-inhibitory 

 center. Hunt especially has presented many experimental facts 

 which seem to indicate that increase in heart rate from reflex action 

 is usually due to an inhibition of the tonic activity of the cardio- 

 inhibitory center. He finds, for instance, that when the two 

 vagi are cut stimulation of various sensory nerves fails to give any 

 increase in the already rapid heart rate, while, on the contrary, 

 when the two accelerator paths are cut a reflex increase in heart 

 rate may be obtained readily. It is perhaps dangerous to draw 

 positive conclusions from such experiments in regard to the work- 

 ings of these delicate and independent mechanisms under normal 

 conditions, but since our only positive knowledge must rest upon 

 experiments we may accept this result provisionally at least. We 

 may assume, therefore, that the accelerator and the inhibitory 

 fibers are working constantly on the heart, and its rate is the re- 

 sultant or algebraic sum of their effects, and that sudden changes 

 in this rate, such as result from sensory or psychical disturbances 

 of any kind, may be referred mainly to a reflex effect upon the car- 

 dio-inhibitory center. When this center is stimulated to greater 

 activity, a slower rate results; when it is inhibited, a faster rate. 

 The tonic activity of the accelerator from this standpoint acts as a 

 more or less constant opposing force to the inhibitory influence, 

 so that this latter works against a constant resistance which may be 

 likened figuratively to that exerted by a spring. 



The Accelerator Center. The accelerator fibers arise primarily in 

 the central nervous system. Since stimulation of the upper cervical region 

 of the cord causes acceleration, it seems evident that the path must begin 

 somewhere in the brain. It has been assumed that, like the inhibitory fibers, 

 the path starts in the medulla, and that, therefore, the cells in that organ 

 which give rise to the accelerator fibers constitute the accelerator center 

 through which reflex effects, if any, take place. As a matter of fact, the 

 location of these cells of origin has not been made out satisfactorily. The 

 matter offers unusual difficulty on the experimental side, owing to the existence 

 of the cardio-inhibitory center in the medulla and the absence of any entirely 

 satisfactory method of distinguishing certainly between reflex acceleration 

 through this center and through the accelerator center. 



