796 NITROGENOUS METABOLISM. [BOOK n. 



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. 



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 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 alimentary 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, without becoming part of the tissues, 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. 



Before leaving this subject we may call attention to a possible 

 analogy between the history of proteids and that of fats and 

 carbohydrates. The uniform composition of the blood, which the 

 body seems ever striving to maintain, probably applies to its 

 proteids as well as to its other constituents. We have seen that 

 a surplus of non-nitrogenous materials in the blood is withdrawn 

 from the circulation and stored up as fat or glycogen, and it 



