28 FEEDS AND FEEDING 



These proteoses and peptones have resulted from the cleavage or 

 splitting of the very complex egg protein into simpler molecules. Upon 

 such cleavage these molecules 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 in a similar 

 manner, 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 been acted upon further by trypsin 

 and erepsin. Trypsin, an enzyme of the pancreatic juice, not only 

 attacks protein directly and converts 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 intestinal secretion, 

 also acts on proteoses and peptones and splits them into simpler com- 

 pounds. By the action of these last two enzymes the proteoses and pep- 

 tones are finally cleaved into amino acids, which, as we have learned, 

 are the "building stones " from which proteins are formed. 



The amino acids are soluble in the juices of the small intestine and 

 are ready for transference thru the intestinal walls into the blood to be 

 carried to various parts of the body. These amino acids are still rela- 

 tively complex in structure, but are much simpler than the proteoses 

 and peptones. They form the great primary nitrogenous building mate- 

 rial out of which the protein tissues of the animal body are built. So 

 far as known, protein compounds taken as food cannot be broken apart 

 further than into amino acids and remain useful in body building. 



50. Tissue building. The process of protein digestion is the breaking 

 down of complex nitrogenous bodies into simpler ones. A good picture 

 of what takes place can be had by likening the protein molecule to a 

 house being taken down by a builder in order that he may construct 

 another from the materials. An animal eating protein compounds cannot 

 use the protein molecules in the form in which the plant has built them 

 up into its own substance, but must first take them apart to a greater or 

 less extent, and from the parts reconstruct another kind of protein mole- 

 cule suitable for its own use. In others words, its protein molecules must 

 have a different architecture from those of the plants which serve as its 

 food. The proteoses and peptones may be likened to the roof and walls 

 of the house. These walls and the roof can be broken down into bricks 

 and tiles, which are represented by the amino acids; and from these 

 the animal, beginning anew, can construct new proteins of the specific 

 architecture its body may require. 



51. Bacteria. In the stomach bacteria find unfavorable conditions 

 for growth, because of the free acid of the gastric juice, and in the 

 small intestine the presence of bile rapidly causes the death of bacteria. 

 Consequently bacteria play little or no part in digestion in either the 



