GELATINE AND FAT. 37 



balance the increased absorption of oxygen (Pettenkofer). 

 The quantity of proteid required by the organism daily for 

 its maintenance is proportional to the weight of active 

 living material (protoplasm) that it contains, so that, in 

 general, those organisms are most vigorous which are 

 capable of producing the largest quantity of urea in pro- 

 portion to their weight ; but, in the case of muscle, the 

 proteid so used is not the source of the work done ; for 

 even if the whole of the proteid material which enters the 

 organism in a day, were devoted to this purpose, and em- 

 ployed in the most advantageous way, it would not afford 

 the material for a day's work. 



The facts above stated are most easily understood on the 

 hypothesis that the disintegration of food proteid, i.e., the 

 production of urea and other " nitrogenous metabolites," is 

 exclusively a function of " living material," and that this 

 process is carried on in the organism with an activity which 

 is dependent on the activity of the living substance itself, 

 and on the quantity of material supplied to it. No 

 evidence at present exists in favour of a "luxus con- 

 sumption " of proteid. 



Use of Gelatine. In carnivorous animals on a diet of 

 flesh and fat, nitrogen equilibrium can be maintained with 

 a much smaller daily allowance of proteid with than with- 

 out gelatine. Hence gelatine is capable of partly replacing 

 proteid. But normal nutrition cannot be maintained either 

 with gelatine alone or with any mixture of gelatine and 

 fat, or of gelatine and proteid. 



Relation of Fat to the Exchange of Material. Fat is stored 

 for the purposes of nutrition in the adipose tissue, which, 

 without any disturbance of its histological integrity, gives 

 or receives fat according to the requirements of the or- 

 ganism. Tissue fat is not, however, as a rule, derived from 

 food fat of the same kind, for even when animals previously 

 starved receive fatty food, the fat " laid on " is not neces- 

 sarily chemically identical with that given. In the fattening 



