10 BIOLOGY AND SOCIAL PROBLEMS 



those reflex operations of our bodies which 

 seem to us to exhibit the most primitive forms 

 of activity, we may well turn to the responses 

 of our internal organs. The heart of a normal 

 adult man in repose beats at a rate of about 

 seventy pulses per minute. If he rises and 

 stands, it usually increases its beats to about 

 eighty per minute; if he lies down, the rate 

 may fall to some sixty per minute. Unusual 

 exercise, as in running and other forms of 

 vigorous muscular work, calls forth a marked 

 increase in the rate which may last for some 

 time after the exercise has ceased. All these 

 changes in the action of the heart are con- 

 cerned with the appropriate supply of blood 

 to the working body, particularly its muscles, 

 and are of an obviously adaptive kind. Since 

 they follow with great precision and regular- 

 ity the changes in the state of the individual, 

 they might be looked upon as good examples 

 of simple reflexes. But a careful inspection 

 of them will show that this is not strictly 

 true. 



The heart of man, like that of other higher 

 animals, receives at least two kinds of nerve 

 fibers, sympathetic and vagus fibers. The 

 first of these on stimulation accelerates the 



