10 APPLIED PHYSIOLOGY 



the total intake of energy. It varies, of course, with the 

 bulk of the diet, for the manipulation of large masses of 

 food materially increases the amount of muscular work 

 to be done by the stomach and intestine. This has to 

 be taken into consideration in appraising the value of 

 some forms of purely vegetable diet which are apt to be 

 very bulky. Thus, it has been calculated that in the 

 case of a horse fed entirely upon hay 48 per cent, of the 

 energy of the food is expended in its digestion and 

 absorption. It varies, too, with the chemical constitu- 

 tion of the food. The digestion of proteins seems to 

 demand most work, then that of carbohydrates, whilst 

 that of fats requires least of all. Of common articles of 

 food, milk is that which entails the least digestive work. 

 Anything which increases the frequency or force of 

 peristalsis also raises expenditure under this head, and 

 part at least of the wasting effect of purgation or 

 diarrhoea is thus to be explained. 



(iii.) By external (as distinguished from internal) 

 work is meant the work expended by the muscles in 

 performing not only the day's task of labour, but that 

 required for locomotion and all the thousand ways in 

 which the skeletal as opposed to the visceral muscles 

 are brought into play. It is inevitably, therefore, a 

 heavy item. 



It must be remembered, too, that all expenditure on 

 actual external work is unavoidably accompanied by an 

 increase of the work done by the heart and lungs as 

 well as by an increased production of heat. Physiolo- 

 gists differ as to exactly how much of the increased 

 expenditure of energy that muscular labour entails goes 



