352 THE HUMAN BODY 



function of these afferent fibers will be discussed in Chap. XXII 

 in connection with the nervous control of the blood-vessels. 



The Inhibitory and Augmentor Centers. The control of the 

 heart-beat is reflex in its nature, and like most other " vital" 

 processes which are subject to reflex control is vested in certain 

 ''centers" of the medulla. Two heart-regulating centers are 

 recognized, the cardio-mhibitory center and the cardio-augmentor 

 center. The inhibitory center is in the nuclei of the tenth nerve. 

 It is bilateral, each side containing half of it. The exact position 

 of the augmentor center has not been determined. It is probably 

 not a compact mass of cells as is the inhibitory center, but is scat- 

 tered diffusely through the medulla. 



Both these centers are in the path of all incoming impulses, 

 and there is evidence that both of them are kept in constant 

 " tonic" activity through the incessant play of stimuli upon them. 

 Of recent years the view has been gaining ground that the tonic 

 activity of the "vital" centers is maintained, in part at least, by 

 chemical influences exerted through the blood. This influence 

 has long been known to exist in the case of the respiratory center. 

 That it is a factor in the regulation of the heart is only now com- 

 ing to be believed. 



The heart is thus constantly receiving both inhibitory and 

 augmentor impulses, the former tending to diminish its activity, 

 the latter to increase it. The actual heart-beat is the expression, 

 therefore, of the balance between two opposing tendencies, and 

 its increase or decrease indicates that one or the other has gained 

 the advantage. 



In attempting to analyze the causes of changes in the heart- 

 rate it must be remembered that an increase in rate may mean 

 either an increase in the activity of the augmentor center, or a 

 depression of the inhibitory center. Conversely, a decrease in 

 rate may mean either a depression of the augmentor center or an 

 increase in the activity of the inhibitory center. An observation 

 that helps us in deciding which of the centers may have been re- 

 sponsible for any observed change is that the inhibitory mechanism 

 acts much more promptly than does the augmentor. Any change 

 that follows quickly an exciting cause is, therefore, to be attrib- 

 uted to the inhibitory mechanism. Since the heart, as a matter 

 of fact, responds almost instantly to most influences we are in 



