DIGESTION. 211 



of the hard parts of the cereals, vegetable seeds, cellulose, etc., the 

 quantity and variety of which depend on the nature of the food. 

 These substances, passing into the large intestine along with the 

 excrementitious matters of the bile, become incorporated with the 

 mucous secretions and assist in the formation of the feces. Under 

 the influence of a peristaltic movement similar to that witnessed in 

 the small intestine, all this excrementitious matter is deprived by 

 absorption of the excess of contained water and is gradually car- 

 ried downward to the sigmoid flexure, where it accumulates prior to 

 its extrusion from the body. 



Intestinal Fermentation. Owing to the favorable conditions 

 in the intestine for fermentative and putrefactive processes e. g., 

 heat, moisture, oxygen, and the presence of various microorganisms 

 the food, when consumed in excessive quantity or when acted on 

 by defective secretions, undergoes a series of decomposition changes 

 which are attended by the production of gases and various chemic 

 compounds. -^Dextrose and maltose are partially reduced to lactic 

 acid; this to butyric acid, carbon dioxid, and hydrogeix^Fats are 

 reduced to glycerol and fatty acids j^the glycerol, according to the 

 organisms present, yields succinic acid, carbon dioxid, and hydrogen. 

 XThe proteids under the prolonged action of the pancreatic juice are 

 decomposed, with the production of leucin and tyrosin.XThese 

 crystalline compounds are in turn reduced to simpler forms. The 

 former yields valerianic acid, ammonia, and carbon dioxid; the 

 latter gives rise..to indol, which is the antecedent of indican, found in 

 the urine. Skatol, another derivative of the proteid molecule, due 

 to bacterial action, gives the characteristic odor to the feces. 



Feces. The feces consist of water, mucin, the indigestible resi- 

 due of the food, decomposition products, and inorganic salts. The 

 consistency of the fecal matter varies from fluid to semi-fluid, depend- 

 ing largely on the length of time it remains in the intestine and the 

 extent to which absorption of its watery portion has taken place. <2The 

 odor is due to the presence of sulphuretted hydrogen and sk 

 The c^lpxiixdiie partly to the altered coloring-matter of the b]le, hydro- 

 bilirubin or stercobilin, and partly to the character of the food. The 

 total quantity discharged daily varies from four to six ounces. 



Defecation. Defecation is the final act of the digestive process 

 and consists in the expulsion of the indigestible residue of the food 

 from the intestine. This act usually takes place in the human being 

 but once in twenty-four hours, as the diet contains but a minimum 

 quantity of indigestible matter. Previous to their expulsion the feces 

 which have accumulated in the sigmoid flexure must pass downward 

 into the rectum. In so doing they develop the sensation which leads 

 to the act of defecation. The descent of the feces is accomplished by 

 the peristaltic contraction of the intestinal wall. Coincident with the 



