304 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



interchangeable in dietetics in the ratio of their combustion equivalents* 

 Such, in fact, is the case. The ratio of interchange is known as the " iso- 

 dynamic equivalent," and it is given usually as 1 : 2,4 or 2.2 ; that is, fats 

 may replace over twice their weight of carbohydrate in the diet. It follows 

 from the general principles just stated that if we wished to know the amount 

 of heat produced in the body in a given time, say twenty-four hours, we might 

 ascertain it in one of two ways : In the first place, the animal might be placed 

 in a calorimeter and the heat given off in twenty-four hours be measured 

 directly. This method, which is that of direct calorimetry, is described more 

 completely in the section treating of Animal Heat. Secondly, one might 

 feed the animal upon a diet containing known quantities of proteid, fats, and 

 carbohydrates, and by collecting the total N and C excreta determine how much 

 of each of these had been destroyed in the body. Knowing the combustion 

 equivalent of each, the total quantity of heat liberated in the body could be 

 ascertained. This latter method is known as indirect calorimetry. The two 

 methods, if applied simultaneously to the same animal, should give identical 

 results. It is very interesting to know that an experiment of this character 

 has been successfully performed by Rubner ; l his experiments were made with 

 the greatest accuracy and with careful attention to all the possible sources of 

 error, and it was found that the quantities of heat as determined by the two 

 methods agreed to within less than 0.5 per cent. These experiments are note- 

 worthy because they furnish us with the first successful experimental demon- 

 stration of the accuracy of the general principles, stated above, upon which 

 the available energy of foods is calculated. 



Dietetics. The subject of the proper nourishment of individuals or col- 

 lections of individuals armies, inmates of hospitals, asylums, prisons, etc. 

 is treated usually in books upon hygiene, to which the reader is referred for 

 practical details. The general principles of dieting have been obtained, how- 

 ever, from experimental work upon the nutrition of animals. These principles 

 have been stated more or less completely in the foregoing pages, but some 

 additional facts of importance may be referred to conveniently at this point. 

 In a healthy adult who has attained his maximum weight and size the main 

 object of a diet is to furnish sufficient nitrogenous and non-nitrogenous food- 

 stuffs, together with salts and water, to maintain the body in equilibrium 

 that is, to prevent loss of proteid tissue, fat, etc. In speaking of the nutritive 

 value of the food-stuffs it was shown that in carnivora (dogs) this condition 

 of equilibrium may be maintained upon proteid food alone, putting aside all 

 consideration of salts and water, or upon proteids and fats, or upon proteids and 

 carbohydrates, or upon proteids, fats, and carbohydrates. When proteids alone 

 are used, the quantity must be increased far above that necessary in the case of 

 a mixed diet, and it is doubtful whether, in the case of man or the herbivora, 

 a healthy nutritive condition could be maintained long upon such a diet, owing^ 

 to the largely increased demand upon the power of the alimentary canal to 

 digest and absorb proteids, to the greater labor thrown on the kidneys, etc. 

 1 Zeitschrift fur Biologie, 1893, vol. xxx. p. 73. 



