354 THE HUMAN BODY 



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 undoubt- 

 edly an adaptive response, as are most reflex responses, but its 

 explanation resides, not in the adaptation, but in the reflex mech- 

 anism 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. 



