THE ACTION OF THE HEART 317 



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. We may illus- 

 trate all these effects by specific cases. The augmentor center 

 seems to be to a peculiar degree subject to impulses of muscle 

 sense. Running, therefore, or any other form of violent muscular 

 exercise, stimulates this center strongly and so the heart-rate is 

 increased. When one lies down the stream of muscle sense im- 

 pulses is greatly diminished; the augmentor center is therefore 

 less active, and the heart beats more slowly. Successive swallow- 

 ing, as in sipping water, increases the heart-rate by depressing 

 the inhibitory center. A blow over the stomach {(the solar plexus) 

 gives rise to afferent impulses which stimulate the inhibitory 

 center; the heart-rate is therefore diminished. 



These are specific illustrations of the general rule that the heart- 

 beat may be modified by all sorts of sensory stimulations. It is 

 a matter of ordinary observation that many experiences, particu- 

 larly those involving sensory impressions of high intensity, are 

 accompanied by marked changes in heart-rate. In common with 

 the structures of the body generally that are innervated by the 

 sympathetic system the heart-rate is also much affected by 

 emotional states. Excitement, fear, almost any strong emotion, 

 is reflected in the conduct of the heart. 



In connection with this analysis of the control of the heart- 

 beat the importance of obtaining the proper viewpoint for con- 

 sidering physiological processes may well be emphasized. If one 

 who has not studied the subject particularly be asked why run- 

 ning makes the heart beat faster he will probably answer that 

 exercising muscles require more blood than resting ones, and that 

 the heart beats faster to furnish this extra amount. A moment's 

 thought shows that this statement, though quite true, does not 

 really answer the question. It implies that the heart has knowl- 

 edge of the needs of the tissues, which, of course, it cannot have. 

 The increased heart-rate which accompanies exercise is undoubtedly 

 an adaptive response, as are most reflex responses, but its explana- 

 tion resides, not in the adaptation, but in the^ reflex mechanism 

 which brings it about. We should be continually on guard against 

 the tendency to explain physiological processes by their results 

 rather than by the means by which the results are accomplished. 



