30 



MAINTENANCE RATIONS OF EAEM ANIMALS. 



kinetic form in the body, but since it takes at once the form of heat, it is not 

 available energy in the sense in which the term is here used, 

 v The same general considerations, of course, apply to the other fermentations 

 and putrefactions which occur in the digestive tract, but their amount in 

 herbivora is probably small compared with that of the methane fermentation, 

 and we have relatively little knowledge regarding them. 



DIGESTIVE CLEAVAGES. 



It is well known that extensive cleavages of the feed ingredients occur in the 

 digestive tract. The nutrients, by the action of the digestive ferments, are split 

 up into simpler atomic groupings — the so-called building stones of the mole- 

 cule — out of which the proteins, carbohydrates, and fats peculiar to the animal 

 body are built up. One argument which has been brought forward in the past 

 against the extensive occurrence of such cleavages in natural digestion, espe- 

 cially of the proteins, has been the teleological one that the splitting up into 

 these comparatively simple compounds was a waste of valuable nutritive mate- 

 rial. On the other hand these processes have been invoked to explain the 

 striking effect of the proteins in stimulating the metabolism — their large specific 

 dynamic effect, to use Rubner's terminology. So far as the peculiar use of pro- 

 tein in the body is concerned, it is well established that its crystalline cleavage 

 products can be resynthesized to form protein. It is of special interest, there- 

 fore, to learn that these cleavages and resyntheses are apparently nearly isother- 

 mic processes. Some of the cleavage products of protein contain more potential 

 energy per gram than protein itself, as, for example, leucin, with 6.525 calories 

 per gram, and tyrosin, with 5.916 calories per gram. Others, like alanin, with 

 a heat of combustion of 4.356 calories, contain but little less energy than the 

 protein from which they are derived. Even the simplest amino-acid, glycocol, 

 resulting from this cleavage has a heat of combustion of 3.129 calories per 

 gram. The impression which these figures give — that but little energy is lost 

 in the cleavage of the proteins — is confirmed by direct experiments. Loewi 1 

 found the dry residue of the tryptic digestion of meat to have an energy value of 

 4.6 calories per gram. Tangl, Lengyel, and Hari 2 found the products of the 

 peptic or tryptic digestion of egg albumin and serum albumin to contain nearly 

 or quite as much potential energy as the original protein. Grafe 3 has made arti- 

 ficial digestions of protein in a calorimeter, and found no noticeable evolution 

 or absorption of heat. It seems safe, therefore, to regard the digestive cleavage 

 of protein as at least a nearly isothermic process, causing little loss of energy in 

 digestion. 



Substantially the same thing is true of the digestive cleavage of carbohy- 

 drates and fats. Thus 1 gram of starch yields 1.111 grams of dextrose, and 

 the heats of combustion of these quantities are, respectively, 4.183 calories and 

 4.159 calories, showing a loss of less than 0.6 per cent. One gram of sucrose 

 yields 0.5264 gram each of dextrose and levulose, and the energy values are, 

 respectively, 3.955 calories and 3.947 calories, or a loss of less than 0.2 per cent 

 So, too, 1 gram of tristearin with a heat of combustion of 9.43 calories yields 

 by hydrolysis 0.9573 gram of stearic acid, equivalent to 9.026 calories, and 

 0.1033 gram of glycerin, equivalent to 0.424 calorie, or a total of 9.45 calories. 



1 Leathes. Problems in Animal Metabolism, p. 129. 



2 Archiv fur die gesammte Physiologie des Menschen und der Thiere (Pfliiger), vol. 

 115, p. 1. 



3 .Tahresbericht Tiber die Fortschritte der Tier Chemie, vol. 37, p. 917. 



