CHEMISTRY OF DIGESTION AND NUTRITION. 287 



paper by Pfliiger. Dog, weight 28.1 kilograms, fed at 11 A. M. with 2070.7 

 grams of meat : 



2070.7 grams of meat contain 69.2 grams N. 



Total nitrogen eliminated in urine and feces in twenty- four 



hours (7 A. M. to 7 A. M.) 71.2 " " 



Deficit of N 0.90 grams. 



The total nitrogen in the urine alone was 68.5 grams. 



In urine from 7 A. M. to 11 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 

 be composed of fats and carbohydrates, and the animal thereby be kept in nitrog- 

 enous equilibrium. Apparently this is not the case, as is seen from experiments 

 of the following character : When an animal is allowed to starve, the nitrogen in 

 the urine, after the first few days, becomes practically constant, and represents 

 the amount of oxidation of proteid tissue taking place in the body. If, now, the 

 animal is given an amount of proteid just equal to that being destroyed in the 

 body, nitrogenous equilibrium is not established ; some of the body-proteid con- 

 tinues to be lost, and to get the animal into equilibrium a comparatively large 

 excess of proteid must be given in the food. The same result holds if carbo- 

 hydrates and fats are given along with the proteid, with the exception that upon 

 this diet nitrogen equilibrium is more readily established that is, less proteid 

 is required in the food. Upon the theory of circulating proteids and tissue- 

 proteids, this fact may be accounted for by saying that of the proteid given as 

 food, a part always undergoes destruction as circulating proteid without going 

 to form tissue, so that to cover tissue-waste a larger amount of proteid must be 

 taken as food than would be necessary if it could all be used exclusively for the 

 repair of tissue. Carbohydrates and fats diminish the amount of proteid 

 destroyed as circulating proteid, and thereby enable us to keep in nitrogen 

 equilibrium on a smaller proteid diet. With albuminoid food (gelatin) the 

 facts seem to be different. If albuminoids be given in the food together with 

 proteids or with proteids and a non-nitrogenous food-stuff (fats or carbo- 

 hydrates), nitrogen equilibrium may be established upon a much smaller 

 amount of proteid than in the case of a diet consisting of proteid alone or of 

 proteid together with fats and carbohydrates. It seems probable that albu- 

 minoids can take the place entirely of circulating proteids, so that only enough 



