174 ANATOMY FOR NUKSES. [CHAP. XV. 



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 ilium, 

 though otherwise mucja 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 become distinctly acid, although 

 the secretions of the intestinal wall are alkaline, and certain 

 acid fermentations 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 a meal : 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. 

 The mechanical subdivision, bruising, and crushing of the food, 

 accomplished by the teeth and the muscular contractions of the 

 walls of the alimentary canal, is merely a process of preparation 

 for the solvent action of the digestive juices. Of these juices 

 there are five, each having a special action. 



(1) The saliva, containing the digestive ferment ptyalin, 

 transforms starch into sugar. 



(2) The gastric juice, containing pepsin (a ferment acting in 

 the presence of an acid), transforms proteids into peptones. 



