28 Feeds and Feeding. 



these last two enzymes the proteoses and peptones have their mole- 

 cules further cleaved into simpler but still complex molecules, 

 water being again taken up as in the first cleavage. The simplest 

 products of such cleavage of the proteins of food substances are 

 the amino acids. 



The amino adds are the common final nitrogenous nutritive ma- 

 terials of the digestive tract, resulting from the cleavage of the 

 complex molecules of the food proteins. They are soluble in the 

 juices of the small intestine and are ready for transference thru 

 the intestinal walls into the body proper. These acids are still 

 relatively complex in structure, but are much simpler than the pro- 

 teoses and peptones from which they are derived. The amino 

 acids, derived from the nitrogenous portion of foods, constitute the 

 great primary nitrogenous building material out of which the pro- 

 tein 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. The amino acids 

 are now obtained by the physiological chemist as a laboratory 

 product. The mixture of amino acids secured by completely di- 

 gesting a protein and then evaporating the water is a syrup-like 

 substance. 



41. 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 pro- 

 tein molecule to a house being taken down by a builder in order 

 that he may construct another from the materials. An animal eat- 

 ing 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 molecule suitable for 

 its own use. In other words, its protein molecules must have a 

 different architecture from those of the plants w^hich serve as its 

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

 walls of the house. These Avails 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 pro- 

 teins of the specific architecture its body may require. 



It is possible that in certain cases portions of the protein mole- 

 cule which are more complex than the amino acids may be of such 

 structure that they can be directly utilized in the reconstruction of 

 body protein, without first being broken down into amino acids. 



