586 CIRCULATION OF BLOOD AND LYMPH. 



fibers or by a reflex inhibition of the cardio-inhibitory center. 

 Hunt especially has presented many experimental facts which 

 indicate that an increase in heart rate from reflex action may be 

 produced by 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 con- 

 trary, when the two accelerator paths are cut a reflex increase in 

 heart rate may be obtained readily. The negative result after 

 previous section of the vagi may well be due, however, to the 

 fact that the heart is then beating at a very rapid rate, too 

 rapid for the production of an additional acceleration through 

 the ordinary physiological mechanism. Acting on this view. 

 Hooker* has shown that if the heart is kept slowed by artificial 

 stimulation of the peripheral end of the vagi, then various 

 sensory stimuli will provoke a reflex acceleration which can 

 only occur through the accelerator center. We may conclude, 

 therefore, that the accelerator and the inhibitory fibers are work- 

 ing constantly on the heart, and that its rate is the resultant or 

 algebraic sum of their effects, and that sudden changes in this 

 rate, such as follow from sensory or psychical disturbances of any 

 kind, may be referred to a reflex effect upon either the cardio- 

 inhibitory or the accelerator center. While physiology has demon- 

 strated the general properties of the regulating nerves of the heart, 

 the inhibitory, on the one hand, and the accelerator and augmen- 

 tor on the other, it is necessary for much more work to be done in 

 order to explain satisfactorily how these nerves participate in the 

 various normal and pathological changes of rate and force of beat. 



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. 



* Hooker, "American Journal of Physiology," 19, 417, 1907, 



