156 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



sive application of the mind in intellectual pursuits ; 

 the reception of an excessive number of emotional 

 stimuli through the special senses of sight and hear- 

 ing. To this list might be added the overuse of the 

 muscles, but it is questionable if this is often an 

 important element in the absence of the other factors. 

 The improper and excessive use of food is one of 

 the commonest factors in bringing about physical 

 disorders which determine mental and nervous ones. 

 They can be avoided only through the use of excep- 

 tional judgment and self-restraint. The tendency 

 to eat in excess is almost universal at present, for 

 people are guided by appetite rather than by their 

 needs. Every human machine has a definite and 

 ascertainable food requirement, and to exceed this 

 is as unwise and as uneconomical as to exceed the 

 coal requirements of a locomotive engine. In time, 

 educated people will take the trouble to learn their 

 individual nutritional requirements and will take 

 pains not to exceed them. The ill effects of exces- 

 sive sexual excitement are even less understood by 

 people than the effects of improper diet. The central 

 fact is that excessive sexual excitement exhausts 

 the nervous system. This causes a depression of 

 the secretions necessary for digestion in the stomach 

 and intestine, and this, in turn, permits the over- 

 growth of undesirable bacteria in the digestive tract. 

 This overgrowth causes erroneous decompositions 

 in the food, permanent alterations in the mucous 

 membranes, and the formation ^nd absorption of 



