286 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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 certainty of animal metabolism that proteid food is abso- 

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

 cannot be taken by any other element of our food. But, in addition to this use, 

 proteid, as has been described above, may be oxidized in the body without 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 which were built into tissue, and he sup- 

 posed that the proteids belonged to this class ; by respiratory foods he meant 

 those which were oxidized or burnt in the body to produce heat : the fats and 

 carbohydrates constituted this class. We now know that proteids are respi- 

 ratory as well as plastic in the terms of this theory ; they serve as sources of 

 energy as well as to replace tissue, and Liebig's classification has therefore 

 fallen into disuse. Our present ideas of the twofold use of proteid food may 

 be supported by many observations and experiments, but perhaps the most 

 striking proof of the correctness of these views is found in the fact that a car- 

 nivorous animal can be kept in both nitrogen and carbon equilibrium upon a 

 meat diet only, excluding for the time a consideration of the water and inorganic 

 salts. Pettenkofer and Voit kept a dog weighing 30 kilograms in nitrogen 

 and carbon equilibrium upon a diet of 1500 grams of lean meat per day, and 

 by increasing the diet to 2500 grams per day the animal even gained in weight, 

 owing to an increase in fat. Pfliiger states also that he was able to keep a dog 

 in body-equilibrium as long as eight months upon a meat diet. Facts like 

 these demonstrate that the animal organism may get all its necessary energy 

 from proteid food alone, although, as we shall see later, it is more econom- 

 ical and more beneficial to get a part of it at .least from the oxidation of 

 fats and carbohydrates. Adopting the theory of "circulating proteids," we 

 may say that any excess of proteid above that utilized for tissue-repair 

 or tissue-growth will be metabolized in the body, with the liberation of 

 energy. It makes no difference how much proteid material we consume : 

 the excess beyond that used to replace tissue is quickly destroyed in some 

 way, and its nitrogen appears in the urine as urea or one of the related 

 compounds. A good example of the power of the tissues to oxidize large 

 amounts of proteid is given in the following experiment, selected from a 



