THE LIVER AND PANCREAS 255 



hydrolysis, for dogs may be kept in health when fed on the split 

 products of pancreatic digestion, with a sufficiency of carbo- 

 hydrates and fat, while the split products of acid hydrolysis 

 cannot be so utilised. The question will be considered again in 

 dealing with nutrition, but the matter is of interest here as show- 

 ing that the laboratory of the body reduces the complex protein 

 molecule into many simpler substances, which can be reproduced 

 outside the body by active chemical means, and yet are not quite 

 the same, for the material artificially produced by means of 

 boiling acid is of no subsequent use to the body, whereas that 

 produced in the system can be utilised. 



The amino-bodies resulting from tryptic digestion are mainly 

 organic acids containing either one amino (NH 2 ) group, or two 

 such groups in union with carbon. Among the mon-amino-bodies 

 are leucine, tyrosine, glycine, aspartic acid, glutaminic acid, and 

 tryptophan. Of the diamino-bodies, lysin, arginin, and histidin 

 are present. 



It should be stated that another view regarding the breaking 

 down of protein into simpler substances exists — viz., that the 

 whole protein molecule is not split up by tryptic digestion, but 

 that a nucleus remains which in chemical character comes 

 between a peptone and amino-bodies. It is described as a 

 peptid, or more generally as polypeptid, as it is probable that it 

 is not a simple body. The polypeptid may by acid hydrolysis 

 be converted into amino-bodies. It is suggested that the 

 production of polypeptids serves as a startingrpoint for synthesis, 

 for the protein substances taken in as food have to become 

 converted into the tissues of the living animal, reconstructed in 

 part from the amino-bodies, and the necessary synthesis, it is 

 suggested, occurs around, or is directed by, the polypeptids. 

 Whether this is necessary or no does not in any way affect the 

 important statement that all protein received as food is foreign 

 to the body, and that before it can be built into the tissues of the 

 living animal it has first to be pulled to pieces, and then again 

 built up. 



Should any protein or peptone have escaped the action of 

 pepsin and trypsin, it may be attacked by another enzyme found 

 in the intestinal mucous membrane, known as erepsin, which 

 also has the power of breaking down albumoses and peptones 

 into leucine and tyrosine. Erepsin is found in most of the 

 tissues of the body, so is not specific to the intestine. 



Under the influence of bacterial action numerous decompo- 

 sition products may be split off from protein ; among others 

 aromatic bodies are formed — phenol, indol, and skatol, the 

 latter being responsible for the faecal odour of a pancreatic 

 digestion mixture. These substances are produced from trypto- 



