CHEMISTRY OF DIGESTION AND NUTRITION. 347 



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 

 paper by Pfliiger. Dog, weight 28.1 kilograms, fed at 11a. m. with 2070.7 

 grams of meat : 



2070.7 grams of meat contain G9.2 grams X. 



Total nitrogen eliminated in urine and feces in twenty-four 



hours (7 a.m. to 7 a.m.) 71. 2 " " 



Deficit of N 0.96 grams. 



The total nitrogen in the urine alone was 68.5 grams. 



In urine from 7 a.m. to 1 1 a. m., the fasting period 6.9 grams. 



In urine from 11 a.m. to 7 a. m., time after feeding 61.6 " 



Therefore in the four hours of fasting the animal eliminated in his urine 

 1.7 grams N per hour, while in the twenty hours after eating he excreted 

 3.1 grams N per hour. This experiment shows not only the completeness 

 with which an excessive proteid diet is handled by the tissues, but also the 

 rapidity with which the excess is destroyed. In so far as proteid food is burnt 

 in the body only as a source of energy and without being used to form new tis- 

 sue, its place can be supplied in part, but only in part, by non-nitrogenous food- 

 stuffs — carbohydrates and fats. The double use of proteid as a tissue-former and 

 an energy-producer would seem to imply that if, in any given case, sufficient pro- 

 teid were used in the diet to cover the tissue-waste, the balance of the diet might 

 Decomposed of fats and carbohydrates, and the animal thereby be kepi in aitrog- 



