The Animal Body — Digestion — Metabolism. 27 



stance has considerable nutritive value, tho the manner of its diares- 

 tion is not yet understood. 



40. Protein digestion. — In the process of digestion the protein 

 compounds in the food are attacked first by pepsin in the stomach, 

 and later by trypsin and erepsin in the small intestine. The action 

 of these enzymes is to cleave the very complex protein molecules 

 into simpler ones, during which process the split molecules take up 

 water and become soluble. Proteoses and peptones are products 

 of the cleavage of proteins, an example of which may be seen in 

 the following experiment : If a fragment of the white part of a 

 hard-boiled egg, which is a protein substance, is placed in a dish 

 with dilute hydrochloric acid, a little pepsin added, and the whole 

 kept at body temperature, in a short time the edges of the opaque 

 egg mass will become swollen and transparent, the change gradu- 

 ally extending thru the whole fragment. After a time the mass will 

 have entirely disappeared, and in its stead there will remain a clear 

 solution. If this peptone solution is evaporated to dryness there 

 will be left a yellowish, transparent mass resembling the dried 

 white of an unboiled egg. This dry digested material, now a mix- 

 ture of proteoses and peptones, is soluble in water the same as the 

 white of egg; but if dissolved in water it will not solidify on heat- 

 ing, as does ordinary white of egg. This shows that the substance 

 has been changed to something other than protein, which always 

 coagulates or solidifies on heating. These proteoses and peptones 

 have resulted from the cleavage or splitting of the very complex 

 egg protein into simpler molecules, which upon such cleavage have 

 taken up chemically a large amount of water and become soluble. 

 When a piece of lean meat or hard-boiled egg is taken into the 

 human stomach, the pepsin, acting in the presence of hydrochloric 

 acid, gradually dissolves such meat or egg, changing it to soluble 

 peptones and proteoses. If it escapes solution in the stomach, it is 

 usually dissolved later in the small intestine. 



The soluble proteoses and peptones are not yet in suitable form 

 for use in. the body of the animal, and so are not absorbed, but are 

 retained in the small intestine until they have undergone further 

 enzyme action. This is effected by trypsin, which can not only at- 

 tack protein directly and convert it into proteoses and peptones, 

 as does pepsin in the stomach, but can also attack the peptones and 

 proteoses and cleave them further. Erepsin, an enzyme of the small 

 intestine, is of poM^erful action. It attacks nitrogenous substances 

 after they have become proteoses and peptones. By the action of 



