An enema is a rectal injection. The capacity of the rectum, it need hardly be stated, varies with the age of the individual. For an infant, half an ounce to an ounce; for a child of two to five years, two to six ounces; for five to fifteen years, six ounces to a pint; for an adult, a pint to a quart of fluid may be considered as an approximation to the capacity of the rectum at these ages respectively. Habitual use of injections no doubt increases the tolerance, as also the capacity of the rectum.

Injections may consist of water cold, tepid, warm, or hot; of medicated solutions—emollient, anodyne, laxative, cathartic, or anthelmintic. Under this head are to be considered only enemata administered with the view to cause an evacuation from the intestinal canal.

Enemata act either by a reflex irritation or by absorption. When a cold injection is thrown into the rectum, or this organ is distended, an action is set up for the expulsion of the offending substance, and the muscular fiber of the bowel more or less vigorously contracts according to its functional condition. The quantity, as well as the temperature of the fluid injected, must therefore be taken into consideration, when it is proposed to empty the bowels by a merely reflex irritation. On the other hand, when it is the intention to procure the absorption of the medicated fluid, the quantity injected must be relatively small, and its temperature should, as nearly as possible, be that of the rectum. In order to secure absorption, it is necessary also to regard the laws of diffusion. As the secretions of the rectum are alkaline, it is obvious that acidulated solutions will diffuse into the rectal veins with the greatest facility. It is doubtful whether colloidal substances of themselves are taken up in the rectum. (See Nutrient Enemata, p. 55.)

Irrigation of the intestines, or forced injections of a large quantity of water, is a modern expedient of great practical utility. The apparatus required for the performance of this operation consists of a rectal tube, a flexible rubber pipe three or four feet in length, and a funnel-shaped vessel to contain the fluid to be injected. The decubitus on either side, the hips being elevated, may be sufficient; but, to insure gravitation of the fluid to the ileo-caecal valve, the female patient should be placed in Sims's position, and the male patient on his hands and knees. The rectal tube should be inserted, and passed up to the sigmoid flexure; the flexible tube should then be attached. The height to which the reservoir is raised will regulate the hydrostatic pressure, and the flow of fluid through the flexible tube can be lessened or increased at the pleasure of the operator by compression with the fingers.

In administering rectal injections the utmost gentleness is requisite, especially when a large amount of fluid is to be introduced. Rude thrusting of the pipe into the rectum may injure the mucous membrane, and rapid and forcible dilatation of the bowel will excite an imperious desire to go to stool. Too great pressure, in the process of irrigation, may cause a rupture of the intestine, especially if its coats are softened by disease or penetrated by ulceration. It is possible that sudden and forcible distention of the bowel may produce dangerous cardiac syncope in susceptible subjects.

The experiments on the cadaver have demonstrated that, although the large intestine may be filled with water, no fluid can be made to pass the ileo-caecal valve. Notwithstanding these experiments, it has been claimed that in the living subject, by the irrigation method, water can be forced through the whole length of the intestine. If these observations are correct, it is probable that a pathological state of the ileo-caecal valve must have existed.