316 THE HUMAN BODY 



cation of that drug to the nerve-cells of the heart abolishes the influ- 

 ence of the vagi, but does not affect the sympathetic control at all. 



Physiologically the vagus fibers are inhibitory; their stimula- 

 tion slows and weakens the heart-beat. When very strongly 

 stimulated they may bring the heart to a complete standstill, 

 although in mammals the standstill is maintained for a few 

 seconds only, the heart soon "breaking through" the inhibition. 

 The sympathetic fibers have precisely the opposite function, be- 

 ing augmentor; their stimulation accelerates and strengthens the 

 beat of the heart. 



In addition to the efferent sympathetic innervation just de- 

 scribed the heart is provided with a set of afferent nerve-fibers. 

 These reach the central nervous system either by way of the 

 vagus nerves, or in some species of animals, rabbits for example, 

 as separate nerve-trunks known as the depressor nerves. The 

 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-inhibitory center and the cardio-aug mentor 

 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 

 scattered 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. 



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 



