62 BIOLOGY AND SOCIAL PROBLEMS 



humor. Every one is familiar with the feeling 

 of utter exhaustion after protracted strenuous 

 physical exercise. Probably most of you know 

 that if the blood of a dog in this state of 

 overwork is transfused into the body of a 

 rested dog, the second animal shows all the 

 signs of lassitude that the first one did. Thus 

 excessive muscular exercise must change the 

 composition of the blood in such a way that 

 when it acts upon the conscious centers, it 

 generates those vague sensations that we de- 

 scribe as utter tiredness, a condition which can 

 be easily interpreted from the standpoint of 

 the hormones. These substances, then, not 

 only interrelate distant parts and activities of 

 the human body as in its simpler phases the 

 nervous system does, but they play the part 

 of the ancient humors, and color momentarily 

 or even permanently that aspect of our nerv- 

 ous systems which we call temperament. 



