196 ANATOMY FOR NUKSES. [CHAP. XVI. 



containing a certain quantity of mucus. It is said to have a 

 solvent action upon all the food-stuffs, but at best its powers 

 are slow and feeble, and we have no satisfactory reason for 

 supposing that the actual digestion of food in the intestine is 

 to any great extent aided by it. 



During the passage of the food through the small intestine 

 the remaining proteids, starch, and fats are converted into pep- 

 tones, sugar, and emulsified fats or soluble soaps, and these 

 products as they are formed pass either into the lymphatics, 

 or into the blood-vessels in the intestinal walls, so that the 

 contents of the small intestine, by the time they reach the ileo- 

 csecal valve, are largely deprived of their nutritious constitu- 

 ents. So far as water is concerned, the secretion of water into 

 the small intestine maintains such a relation to the absorption 

 from it that the intestinal contents at the end of the ileum, 

 though otherwise much 'changed, are about as fluid as in the 

 duodenum. 



Changes in the large intestine. We have no very definite 

 knowledge of the particular changes which take place in the 

 large intestine. The contents are acid, although the secretions 

 of the intestinal wall are alkaline, and certain acid fermenta- 

 tions must therefore take place in them. These are probably 

 due to the action of micro-organisms; but however this may 

 be, the chief work of the colon is absorption. 



By the abstraction of all the soluble constituents, and espe- 

 cially by the withdrawal of water, the liquid contents become, 

 as they approach the rectum, changed into a firm and solid 

 mass of waste matters, ready for ejection from the body, and 

 called feces. 



The feces. The feces consist of the undigested and indigesti- 

 ble substances of the food : among them are the elastic fibres of 

 connective tissue ; the cellulose, which is the chief constituent 

 of the envelopes encasing the cells of plants ; the indigestible 

 mucin of mucus. These three materials, together with some 

 water, some undigested food-stuffs, and some excretory sub- 

 stances found in the various secretions poured into the aliment- 

 ary canal, form the bulk of the material expelled from the body. 



To sum up the digestive processes : - 



The transformation of the food we take into our mouths into 

 products capable of absorption is mainly a chemical process. 



