646 PHYSIOLOGY 



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, in nitrogenous 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 containing no 

 protein, but in its place a mixture of gelatin with tyrosine and tryptophane. 

 It is doubtful, however, whether such experiments 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 digestion is not, 

 however, carried out by the action of juices secreted by the intestinal canal 

 itself. The solution of the cellulose is effected partly under the influence 

 of a cellulose or cytase present in the plant cell themselves, partly under the 

 influence of the micro-organisms living in the paunch or csecum. Under the 

 influence of these bacteria cellulose is dissolved -with the production of 

 carbon dioxide, methane, and butyric and acetic acids. In man the greater 

 part of the cellulose of the food is undigested, and its chief value appears to 

 be that of lending bulk to the indigestible material and so aiding the normal 

 movements of the intestines. In the case of young plant cells, such as those 

 of lettuce or carrots, a certain percentage of the cellulose undergoes solution 

 in the intestine. Here, again, the digestion is probably effected by the 

 agency of putrefactive organisms. 



ALCOHOL. When alcohol is taken by man in moderate quantities the 

 greater part of it undergoes oxidation, and leaves the body as carbon dioxide 

 and water. About 10 per cent, which escapes oxidation is excreted unaltered 



