Digestion is practically completed in the small intestine. The _colon_ consists of a proximal portion, consisting of the cecum, ascending colon, and one-half of the transverse colon; a mesial part, formed by the other half of the transverse colon and part of the descending colon and a distal portion, formed by the remainder of the descending colon and of the rectum. The contents of the proximal portion are fluid; food accumulates in this portion; it is gradually concentrated by the absorption of water. It has been shown by the _x_-rays that a meal reaches the cecum in four or five hours; the hepatic flexure in six or seven hours; the splenic flexure in nine hours; and the rectum in about eighteen hours. The feces consist of the residue of the food, digestive secretions, etc. The contents of the alimentary canal become altered in character as they descend its course. The mass attains a greater density as it descends, more and more nutrient being absorbed from it, and even in the colon it becomes more solid; its color is brownish and its odor characteristic. The amount of feces from an average mixed diet represents from one-seventh to one-eighth of the food consumed. _Defecation._—The waste matter which collects in the lower bowel must be evacuated every day. Allowed to remain longer than this, the digestive system is clogged by the non-removal of worn-out material, and the blood is constantly absorbing matter which is poisonous to the body. Decomposition goes on without being suspected by the sufferer. _Intestinal bacteria_ or _organized ferments_ are present throughout the entire intestinal canal and play a part in the food changes. They are most abundant in the lower part of the small intestine and the upper part of the large. They act upon the proteins, causing putrefaction, dissolve cellulose, and cause a decomposition of the carbohydrates. The products of these fermentations include indol and skatol, which have the characteristic fecal odor; volatile fatty acids and gases, some of which are carbon dioxid, hydrogen, marsh-gas, and hydrogen sulphid. Fermentations of this character up to a certain extent are normal and may be beneficial, but they may proceed so far as to be deleterious to health. Anything which retards digestion, such as imperfect mastication, excessive eating, abnormal amounts of meat in the diet, and failure of the organs secreting the digestive fluids to supply these secretions in sufficient quantity, gives these bacteria a better opportunity to act on the food residues and so increases their bad effects.