THE CIRCULATION OF THE BLOOD 325 



increase in the rate of the beat. It develops steadily and reaches a maximum 

 at from the twenty-fifth to the thirtieth year. In advanced years the tone 

 again declines. For these and other reasons it is believed that this center 

 is in a state of tonic activity In many if not all mammals, discharging nerve 

 impulses which exert a regulative influence on the cardiac mechanism in ac- 

 cordance with its needs and especially in reference to the variable resistances 

 offered to the flow of blood which the heart must overcome. 



The Factors which Determine the Activity of the Cardio-inhibitor 

 Center. The question has also been raised as to whether the tonic activity 

 of this center is maintained by central or peripheral stimuli, i.e., whether it is 

 maintained by causes within itself the result of an interaction between the 

 constituents of the cell substance and those of the surrounding lymph, or 

 whether it is maintained by nerve impulses transmitted to it through various 

 afferent or sensor nerves. Though both factors play an important part in 

 the maintenance of its activity, it is difficult to state which is the more 

 important of the two. That nerve impulses transmitted to the center 

 through afferent nerves lead now to an inhibition, now to an acceleration 

 of the heart has been abundantly established by the stimulation of afferent 

 nerves in almost any region of the body. Thus stimulation of the dorsal 

 roots of the spinal nerves, the trunks of the cranial sensor nerves, the 

 splanchnic nerves, the pulmonic branches of the vagus, etc., gives rise to a 

 more or less pronounced inhibition of the heart. On the other hand 

 stimulation of afferent nerves sometimes leads to the opposite result, namely, 

 acceleration of the heart. As a rule, stimulation of the peripheral termina- 

 tions of these nerves is more effective than stimulation of their trunks. 

 From facts such as these, it has come to be believed that nerve impulses 

 developed in many regions of the body are being constantly transmitted 

 to this center, which not only stimulate it to activity but modify its activity 

 in one direction or another from moment to moment. 



The first explanation, that acceleration of the heart, the result of a 

 peripherally acting stimulus, is due to a stimulation of the cardio-accelerator 

 center by the arrival of nerve impulses coming through afferent nerves, 

 having been made questionable and improbable by the results of Hunt's 

 experiments, the alternative explanation must be that while inhibition of the 

 heart is due to an excitation of the normal activity of the cardio-inhibitor 

 center, the acceleration of the heart is due to an inhibition of the normal 

 activity of the cardio-inhibitor center, and hence there follows the corollary 

 that afferent nerves contain two sets of nerve-fibers which are in physiologic 

 relation with the cardio-inhibitor center, one of which when stimulated 

 peripherally excites or augments its activity, the other of which when stimu- 

 lated inhibits or depresses its activity, thus giving rise to a freer play of the 

 cardio-accelerator center. 



The extent to which both sets of fibers are present in any one afferent 

 nerve is unknown. In the trigeminus it is believed the excitator fibers 

 preponderate for the reason that peripheral stimulation of this nerve is 

 followed by inhibition of the heart; in the sciatic, it is believed the inhibitor 

 nerves preponderate, for the reason that stimulation of the central end of the 

 divided nerve is followed generally by acceleration of the heart. 



It is probable from the effects which follow gastro-intestinal disorders, 

 that the vagus nerve contains both classes of fibers as represented in Fig. 137, 



