346 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



"the circulating albumin or proteid." According to Voit, a well-fed animal 

 has in its lymph and tissues always a certain excess of proteid which is to 

 undergo the fate of the circulating proteid, and this supposition is used to 

 explain the fact that for the first day or so a starving animal metabolizes 

 more proteid, as determined by the nitrogenous excreta, than in the subse- 

 quent days, after the supply of the circulating proteid has been destroyed. 

 A portion of the proteid food, however, before its final destruction is utilized 

 to replace the nitrogenous waste of the tissues ; it is built up into living proto- 

 plasm to supply the place of organized tissue that has undergone disassimi- 

 lation or to furnish new tissue in growing animals. To the proteid that is 

 built up into tissue Voit gives the name of " organeiweiss," the best translation 

 of which, perhaps, is " tissue-proteid." It should be stated that this division 

 of the proteid into circulating proteid and tissue-proteid has been severely 

 criticised by some physiologists, but it has the merit at least of furnishing. 

 a simple explanation of some curious facts with regard to the use of proteid 

 in the body. To avoid misunderstanding, it is well to say that the sepa- 

 ration into circulating proteids and tissue-proteids does not mean that the 

 proteid that is absorbed from the alimentary canal is of two varieties. 

 The terms refer to the final fate of the proteid in the body: a certain 

 portion is utilized to replace protoplasmic tissue, and it then becomes " tis- 

 sue-proteid," while the balance is metabolized in various ways and con- 

 stitutes the "circulating proteid." Any given molecule of proteid, as far 

 as is known, may fulfil either function. With regard to the general nutri- 

 tive value of piot eids, it has been demonstrated clearly that they are abso- 

 lutely necessary for the formation of protoplasmic tissue. An animal fed only 

 on non-nitrogenous food such as fats and carbohydrates will inevitably starve 

 to death in time : this has been shown by actual experiments, and, besides, it 

 follows from a priori considerations. Protoplasm contains nitrogen; fats and 

 carbohydrates are non-nitrogenous, and therefore cannot be used to make new 

 protoplasmic material. It is requisite, moreover, not only that the food shall 

 contain some nitrogen, but that this nitrogen shall be in the form of proteid. 

 If an animal is fed upon a diet containing fats and carbohydrates and nitrog- 

 enous material other than proteids, such as amido-acids or gelatin, nitrogenous 

 equilibrium cannot be maintained. There will be a steady loss of nitrogen in 

 the excreta, due to a breaking-down of proteid tissue within the body, and the 

 final result of maintaining such a diet would be the death of the animal. It 

 may be said, then, with regard to animal metabolism that proteid food is 

 absolutely necessary for the formation of new protoplasm; its place in this 

 respect cannol betaken by any other element of our food. lint, in addition to 

 this use. proteid, as has been described above, may be oxidized in the body with- 

 out being first constructed into protoplasmic material. According to an older 

 theory in physiology, advanced by Liebig, food-stuffs were either plastic or 

 respiratory; by plastic foods he meant those that are built into tissue, and he 

 supposed that the proteid- belonged to this class ; by respiratory foods he meant 

 those that are oxidized or burnt in the body to produce heat : the fats and 



