The irregularly shaped tube (the alimentary canal) through which the food passes on its way into and out of the horse's body, is formed by the mouth, gullet, stomach, and intestines (small and large intestine), all of which organs are lined by mucous membrane. This membrane also lines the air passages (nostrils, windpipe, and bronchial tubes), bladder, urethra (tube that carries the urine outwards from the bladder), ureters (two tubes which convey urine from the kidneys to the bladder), and interior of the eyes and eyelids. It is continuous with the skin, and the skin is continuous with the membrane that secretes the hoof. We may regard the mucous membrane as a modification of the skin, from which it differs with respect to its structure only in its being thinner and softer. The mucous membrane of the stomach and intestines is provided with a great number of minute vessels, called absorbents. The membrane which secretes the hoof may also be looked upon as a modification of the skin. The structure known as skin is composed of a secreting surface (the true skin) and a covering surface (epidermis, page 18), which consists of more or less horny scales secreted by the true skin, and which is more or less impermeable to fluids. The mucous membrane secretes mucus, which is a slimy and semi-fluid substance, as we may see from an inspection of phlegm, or of discharge from the nose. The three surfaces in question possess strong mutual sympathy. Thus, in catarrh for example, the coat loses its polish, and the intestines become irritable; during an attack of indigestion, the condition known as "hidebound" is often present; and laminitis (inflammation of the feet) may arise from superpurgation. On the other hand, a soft, glossy coat is an indication that the organs of breathing and digestion are in good order.

At the beginning of the alimentary canal we have the lips, mouth, and gullet, which, at its entrance into the stomach, is constructed so as to render vomition extremely difficult.

The horse's stomach has an average capacity of about 3 1/2 gallons, and is provided with a large number of glands which supply it with gastric juice, and which are found only on about one-half of the interior surface of that organ; the other half being merely a continuation of the gullet. Hence the size of the functional stomach is only about half that of the apparent one.

The small intestine begins at an opening on the right side of the stomach. It is about 72 ft. long, and about 1 1/2 in. in diameter. It terminates at the coecum, which is the first portion of the large intestine, and which forms a cul-de-sac of about 3 ft. in length and of an average capacity of about 7 1/2 gallons. The large intestine is about 20 ft. long, and ends at the anus.

According to Colin, the capacity of the large intestine is about double that of the small one, and their united capacity is more than ten times as great as that of the stomach. In cattle the capacity of the stomach, of which there are four compartments, is about 2 1/2 times greater than that of the intestines.

Under the heading of "organs of digestion" must be placed certain glands that secrete fluids which flow into the alimentary canal, and which are respectively concerned in the digestion of food. The principal glands in question are as follows : - The salivary glands, the most important of which in the horse are the two parotid glands that respectively occupy, on each side, the space where the head is joined to the neck. The office of the salivary glands is to secrete saliva for mastication. The gastric glands secrete gastric juice, which flows into the stomach. The pancreas (sweetbread) secretes pancreatic juice, which is carried out of the gland by a tube that opens into the small intestine close to the stomach. This tube also serves to convey into the small intestine bile, which is secreted by the liver. The intestines are supplied with a large number of small glands (intestinal glands) that secrete a fluid which aids in digestion.