DIGESTION 207 



through the intestine and hence the extent to which absorption of water in 

 the large intestine takes place. On a meat diet the consistency is firm; on a 

 vegetable diet it is apt to be soft. The amount discharged from day to day 

 on a mixed diet varies from 120 to 170 grams containing from 30 to 42 grams 

 of dry matter. On a meat diet alone the quantity diminishes; on a vege- 

 table diet, especially if the articles of food are rich in cellulose, the quantity 

 will increase considerably beyond the customary amount. 



The color on a mixed diet varies from a light yellow to black. The 

 usual brown color is due to the pigment urobilin or stercobilin, a derivative 

 of the pigments of the bile. On a meat diet the color deepens until it becomes 

 quite black due to the presence of sulphid of iron, the result of the union 

 of sulphuretted hydrogen with the iron derived from hematin contained in 

 the meat. On a vegetable diet the color lightens and may become slightly 

 yellow. If the contents of the intestine are carried forward too rapidly, the 

 time may be insufficient for a complete reduction of the bile pigments, hence 

 they appear in the feces imparting to them a green color. If there is an 

 obstruction to the discharge of bile into the intestine the feces may become 

 yellow or clay-colored. 



The odor is characteristic and due to the presence of skatol and allied 

 bodies produced by the putrefaction of proteins by bacterial action. Sul- 

 phuretted hydrogen also contributes to the odor. 



The chemic composition of the feces is complex. They consist of water, 

 mucin, an indigestible residue of food, decomposition products, excretions 

 from the intestinal glands, and inorganic salts. The residue of the food 

 usually consists of the denser portions of the connective tissue of meats and 

 the cellulose of vegetables and cereals. When the latter are eaten in large 

 amounts the cellulose residue is increased and by its mechanic stimulation 

 increases the peristalsis and hastens the transfer of the feces through the 

 intestine. The decomposition products are derived from protein, fat, and 

 carbohydrate food by bacterial action and include skatol, indol, fat acids, 

 soaps, xanthin, ammonia, sulphuretted hydrogen, etc. The excretion from 

 the intestine itself contributes a considerable portion to the fecal mass. The 

 inorganic salts excreted include phosphates of calcium and magnesium 

 together with various sodium and potassium compounds. 



Defecation. Defecation is the final act of the digestive process and 

 consists in the expulsion of the indigestible residue of the food and its asso- 

 ciated compounds 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 passage of the feces 

 into the rectum there is a relaxation of the sphincter muscles and a contrac- 

 tion of the longitudinal and circular muscle-fibers, in consequence of which 

 the feces are expelled. These complex muscle actions are also aided by the 

 voluntary contractions of the diaphragm and abdominal muscles. 



Nerve Mechanism of Defecation. The act of defecation is primarily 

 reflex though somewhat influenced by voluntary efforts. The reflex charac- 

 ter of the act is especially noticeable in young children in whom by reason of 



