THE .SIGNIFICANCE OF THE FOOD-STUFFS 723 



such as elastin and keratin, reappearing for the greater part in the 

 fgeces. As we have seen, gelatin, the first product of hydration of 

 collagen, represents, so to speak, an imperfect protein. When it is 

 hydrolysed by acids its disintegration products include the ordinary 

 araino-acids of the fatty series, including a considerable amount 

 of glycine and also a certain amount of phenyl alanine and pro line. 

 The oxy- phenyl group which occurs in all the food-proteins in the form 

 of tyrosine, as well as the indol-containing group tryptophane, are 

 absent. On this account gelatin does not give either Millon's 

 test or the Hopkins-Adamkiewicz test with glyoxylic acid. As 

 might be expected from its composition, gelatin cannot entirely 

 replace the proteins of the food, but is able to take the place 

 of part of the proteins. If nitrogenous equilibrium has been 

 attained on a certain amount of protein together with a mixed 

 diet, a considerable proportion of the protein, but not all, can 

 be replaced by gelatin. In an experiment on a dog, m nitro- 

 genous equilibrium on a mixed diet containing 0-6 grm. protein 

 per kilo, it was found that fully five- sixths of the protein could be 

 replaced by gelatin without any disturbance of the nitrogenous 

 metabolism. Physiologists have succeeded in maintaining animals 

 for a short time in a state of nitrogenous equilibrium on a diet con- 

 taining no protein, but in its place a mixture of gelatin with tyrosine 

 and tryptophane. It is doubtful, however, whether such experi- 

 ments could be continued indefinitely. In most cases the animal 

 after a time refuses to eat the gelatin. A somewhat similar behaviour 

 is found in the case of zein, the crystallised protein from maize, which 

 yields no tryptophane or tyrosine on hydrolysis. Hopkins has shown 

 that animals fed with zein, together with a small proportion of 

 tryptophane, live longer than those fed with zein alone. But in no 

 case could he maintain life on this diet for a period greater than 

 forty-five days. There are evidently other groups in the protein 

 molecule which are essential for the maintenance of life and which were 

 not represented in the mixture of zein and tryptophane. 



Experiments have been carried out in order to ascertain whether asparagine, 

 which forms so important a nitrogenous constituent of young plants, can be 

 directly utilised by animals. There is evidence that this substance has a real 

 nutritive value for certain herbivora. The utilisation is not, however, a direct 

 one The asparagine appears to be taken up by the bacteria which swarm in 

 the paunch or csecum of these animals. It is built up by these micro-organisms 

 into protein, and it is the protein of the micro-organisms and not the asparagine 

 itself which is digested, absorbed, and utilised by the mammal. 



OTHER CONSTITUENTS OF THE FOOD 



CELLULOSE. This substance, which forms the cell walls of 

 plants, furnishes an important food-supply to the herbivora. Its 



