FOOD INGESTION AND ENERGY TRANSFORMATIONS 

 WITH SPECIAL REFERENCE TO THE STIMU- 

 LATING EFFECT OF NUTRIENTS. 



INTRODUCTION. 



During the period of gestation the fetus is supplied with nourish- 

 ment from the mother through the placenta and no muscular movement 

 or exertion of any kind is required to secure food. After birth much 

 of the muscular activity of infants is a direct or indirect effort to secure 

 nourishment. This includes not only the act of suckling but also the 

 muscular activity and crying due to hunger. Thus the apparently 

 anomalous condition exists of an expenditure of a considerable amount 

 of energy for the purpose of obtaining energy for vital processes. 



With animals of prey there is usually a period of intense muscular 

 activity prior to feeding. The chase, the attack, the act of killing 

 and tearing apart of the prey, all make demands upon the energy supply 

 of the animal. After feeding, there is usually a relatively long period 

 of muscular repose, although, as will be seen later, immediately after 

 eating there is invariably an increase in the internal cellular activity 

 incident to the process of digestion. 



Even with non-predatory animals and birds a considerable amount 

 of energy is necessary to secure food. The trails to the feeding-grounds, 

 "salt-licks," and watering-places often lead over considerable distances. 

 Birds fly enormous distances to special feeding-grounds, while with birds 

 of prey the chase and attack are comparable to those of animals of prey. 



An exception to this general activity in securing food is the serpent, 

 which, instead of having to chase its prey, lies in wait for it. When the 

 victim is captured, the serpent kills it either by poison, which of itself 

 requires no muscular activity other than the act of striking, or by con- 

 striction, which usually continues but a short time. Probably no living 

 organism secures its food with so economical a consumption of energy 

 as the serpent does, not only because of its extraordinarily low metab- 

 olism, which permits it to live for a long time without food, but also 

 because of the minimum amount of muscular activity expended in 

 obtaining the food. With the human infant the muscular activity 

 incidental to securing food plays a very important role. To what 

 extent irritation and discomfort, accompanied by vigorous muscular 

 exercise and crying, may be directly charged to a desire for nourishment 

 is problematical, but in any event it is certain that a large part of the 

 physical activity of an infant is due to an effort to secure food. 



In human civilization it is rarely that an individual must pursue, 

 attack, gather, and prepare his food prior to eating, as the food materials 



