86 THE VERTEBRATE ANIMAL: METABOLISM 



material for their own nutrition, and it is only incidentally that it 

 affords nourishment for the animal harboring the bacteria. Bac- 

 teria in the large intestine of man also cause the putrefaction of 

 protein elements, forming products many of which are toxic, or 

 poisonous, and which, when absorbed into the body, cause head- 

 aches, drowsiness, and a general feeling of depression, or auto- 

 intoxication. Some idea of the great number of bacteria present 

 may be had when it is stated that from one-quarter to one-half of 

 the dry weight of the faeces consists of bacteria. 



In summarizing the essential processes of digestion, it may 

 be recalled that in some animals, which possess salivary glands, the 

 digestion of starchy carbohydrates is started in the mouth by 

 the action of ptyalin. This action on starch is continued in the 

 small intestine by the amylopsin of the pancreatic juice, with the 

 result that the starch is converted into maltose, a compound or 

 disaccharide sugar. The digestion of the maltose, together with 

 other compound sugars, is completed by the sugar-splitting 

 enzjones of the intestinal juice, with the formation of the simple 

 sugars or mono-saccharides. Protein digestion, started by pepsin 

 in an acid medium in the stomach, is continued in the small intes- 

 tine in an alkaline medium, by trypsin of the pancreatic juice, and 

 completed by erepsin of the intestinal juice, with the formation 

 of amino acids. Emulsified fats and lipins are split by gastric 

 lipase; and, in the small intestine, after the emulsification of 

 fats by bile, the steapsin from the pancreas is effective in producing 

 glycerol and fatty acids as the end products of fat and lipin diges- 

 tion. The entire result of digestion is the decomposition of com- 

 plex food materials into their structural units, which are simple 

 sugars, amino acids, and fatty acids and glycerol. These com- 

 ponents are thus made available for absorption and assimilation 

 in the maintenance of the protoplasmic system. 



Absorption. — By the process of digestion, complex food mate- 

 rials are chemically changed into the simpler compounds that can 

 be assimilated by the cells. Before assimilation can occur, the 

 simple nutrients must be absorbed from the alimentary canal into 

 the circulating fluids and be distributed to all cells. Absorption 

 may be defined as the passing of simple food compounds through 

 the mucous membrane of the digestive tract into the blood or 

 lymph. Examination of the wall of the digestive tract reveals 

 that the cavity is lined with a single layer of compactly placed 



