832 NITROGENOUS METABOLISM. [BOOK n. 



only about 7 p.c. was laid up as dry proteid material during the 

 fattening period, though the amount of proteid food was low. 

 This contrasts strongly with the amount of fat stored up during 

 the same period (see 506). Similar observations carried out on 

 sheep shewed that in these animals the storing up of nitrogenous 

 material was even less, only about 4 p.c. of that given in the food. 



We are thus led to the conception that proteid material taken 

 into the alimentary canal affects proteid metabolism in two ways. 

 On the one hand a part excites a rapid proteid metabolism, giving 

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

 other hand, another part 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 become 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 alimen- 

 tary 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 metabolism of living 

 substance, and "circulating proteids" or "floating 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 manifestations but in the form 

 of heat only. We shall later on consider 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 body 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, this seemed to be a wasteful expenditure of precious 

 material; and the metabolism of this portion of proteid food 

 was accordingly spoken of as a " luxus-consumption," a wasteful 

 consumption. 



523. The Effects of Fatty and of Carbohydrate Food. We 

 cannot as we can in the case of proteid food study the effects 

 of fats or of carbohydrates when these are offered as the only food. 



