I 



CHAPTER XII. 



NUTRITION. 



Income and Expenditure. — A constant and fairly regular 

 waste is daily occurring in the bodies of animals, due to 

 the internal work of the system, the production of heat, 

 and, if labour be performed, to muscular movement. It is, 

 therefore, best to describe the work of the body as of two 

 kinds : viz., internal work, such as the movement of the 

 heart, lungs, bowels, and the production of animal heat ; 

 and external work, due to muscular movement. 



If the food an animal receives is neither greater nor less 

 than its requirements, the body- weight remains unaltered ; 

 if, on the other hand, the food is deficient in amount the 

 body loses weight, whereas if in excess it gains weight. By 

 physiological equilibrium we understand that the income 

 is equal to the expenditure, and the body- weight remains 

 unchanged. 



It is necessary, however, that the food an animal receives 

 should supply the various tissues of which the body is 

 composed, for it is quite possible to conceive a food which, 

 though equalling in weight a diet which has produced 

 physiological equilibrium, is yet one on which a loss of 

 body-weight is occurring, for the reason that it is deficient 

 in some of the elements out of which the tissues arc 

 repaired. 



The animal body consists of proteids, fats, salts, water, 

 and ;i wry small proportion of carbo-hydrates. Every food 

 must cither contain these principles, or must be capable of 

 conversion into them after having entered the body. Ac- 

 cording to Bischoff, the animal body consists of — 



