Chap, v.] NUTRITION. 627 



on sheep shewed that in these animals the storing up of nitro- 

 genous material was even less, only about 4 p.c. of that given 

 in the food. 



Every quantity of proteid material taken into the alimen- 

 tary canal thus appears to affect proteid metabolism in two ways. 

 On the one hand it excites a rapid proteid metabolism giving 

 rise to an immediate, and generally large, increase of urea; on 

 the other hand, it serves to maintain the more regular normal 

 proteid metabolism continually taking place in the body, and so 

 contributes to the normal regular discharge of urea. It seems 

 very natural to suppose that the proteid which plays the first of 

 these two parts is not really built up into the tissues, does not be- 

 come actual living substance, but undergoes the changes which 

 give rise to urea outside the actual living substance in the blood 

 or elsewhere ; and we have seen that under the influence of the 

 pancreatic juice some of the proteid food may undergo the 

 greater part of such a change while it is as yet within the ali- 

 mentary canal. Hence has arisen the very natural distinction 

 to which we have already alluded between " tissue proteids " or 

 " morphotic proteids " which are actually built up into the living 

 substance of the tissues and give rise to urea through the metab- 

 olism of living substance, and " circulating proteids " or " float- 

 ing proteids " which do not at any period of their career within 

 the body become an integral part of the living substance and by 

 their metabolism set free energy not in the way of vital mani- 

 festations but in the form of heat only. We shall later on con- 

 sider what is the exact meaning which we ought to attach to the 

 words " becoming part of the living substance ; " and hence 

 shall defer until then any discussion of the appropriateness of 

 these phrases and of the validity of the distinction which they 

 formulate. 



It was once thought, as we shall presently see erroneously, 

 that the exclusive purpose of proteid food was to supply the 

 proteid tissues, and that all the energy set free in the bociy in 

 vital manifestations, such as movement and the like as distin- 

 guished from heat, had its origin in proteid metabolism, the 

 metabolism of fats and carbohydrates giving rise to heat only. 

 Hence when it first became known that a certain proportion of 

 proteid food apparently underwent a metabolism giving rise to 

 heat only, without becoming part of the tissues, this seemed to 

 be a wasteful expenditure of precious material ; and the metab- 

 olism of this portion of proteid food was accordingly spoken of 

 as a " luxus-consumption," a wasteful consumption. 



§ 417. The Effects of Fatty and of Carbohydrate Food. Un- 

 like those o"f proteid food, the effects of fats and carbohydrates 

 cannot be studied alone. When an animal is fed simply on 

 non-nitrogenous food, death soon takes place ; the food rapidly 

 ceases to be digested, and starvation ensues. We can there- 



