Anatomy - Digestive System.

The mouth.

Digestion begins in the mouth, which is also called the "oral cavity." It is made up of the teeth, the hard palate, the soft palate, the tongue and related muscles, the cheeks and the lips. Horses also have three pairs of salivary glands, the parotid (largest salivary gland and located near the poll), submaxillary (located in the jaw), and sublingual (located under the tongue). Horses select pieces of forage and pick up finer foods, such as grain, with their sensitive, prehensile lips. The front teeth of the horse, called incisors, clip forage, and food is then pushed back in the mouth by the tongue, and ground up for swallowing by the premolars and molars.

The esophagus.

The esophagus is about 4 to 5 feet (1.2 to 1.5 m) in length, and carries food to the stomach. A muscular ring, called the cardiac sphincter, connects the stomach to the esophagus. This sphincter is very well developed in horses. This and the oblique angle at which the esophagus connects to the stomach explains why horses cannot vomit.The esophagus is also the area of the digestive tract where horses may suffer from choke.

The stomach.

Horses have a relatively small stomach for their size, and this limits the amount of feed a horse can take in at one time. The average sized horse (800 to 1,200 pounds (360 to 540 kg)) has a stomach with a capacity of around 4 US gallons (15 L), and works best when it contains about 2 US gallons (7.6 L). Because the stomach empties when 2/3 full, whether stomach enzymes have completed their processing of the food or not, and doing so prevents full digestion and proper utilization of feed, continuous foraging or several small feedings per day are preferable to one or two large ones. The horse stomach consists of a non-glandular proximal region (saccus cecus), divided by a distinct border, the margo plicatus, from the glandular distal stomach.

In the stomach, assorted acids and the enzyme pepsin break down food. Pepsin allows for the further breakdown of proteins into amino acid chains. Other enzymes include resin and lipase. Additionally, the stomach absorbs some water, as well as ions and lipid soluble compounds. The end product is food broken down into chyme. It then leaves the stomach through the pyloric valve, which controls the flow of food out of stomach.

The Small Intestine.

The horse’s small intestine is 50 to 70 feet (15 to 21 m) long and holds 10 to 12 US gallons (38 to 45 L). This is the major digestive organ, and where most nutrients are absorbed. It has three parts, the duodenum, jejunum and ileum. The majority of digestion occurs in the duodenum while the majority of absorption occurs in the jejunum. Bile from the liver aids in digesting fats in the duodenum combined with enzymes from the pancreas and small intestine . Horses do not have a gall bladder, so bile flows constantly. Most food is digested and absorbed into the bloodstream from the small intestine, including proteins, simple carbohydrate, fats, and vitamins A, D, and E. Any remaining liquids and roughage move into the large intestine.

The Large Intestine.

     The cecum is the first section of the large intestine. It is also known as the "water gut" or "hind gut." It is about 4 feet (1.2 m) long that holds 7 to 8 US gallons (26 to 30 L). It contains bacteria that digest cellulose plant fiber through fermentation. These bacteria feed upon digestive chyme, and also produce certain fat-soluble vitamins which are absorbed by the horse. The reason horses must have their diets changed slowly is so the bacteria in the cecum are able to modify and adapt to the different chemical structure of new feedstuffs. Too abrupt a change in diet can cause colic, as the new food is not properly digested.

     The large colon, small colon, and rectum make up the remainder of the large intestine. The large colon is 10 to 12 feet (3.0 to 3.7 m) long and holds up to 20 US gallons (76 L) of semi-liquid matter. It is made up of the right ventral (lower) colon, the left ventral colon, the left dorsal (upper) colon, the right dorsal colon, and the transverse colon, in that order. Three flexures are also named; the sternal flexure, between right and left ventral colon; the pelvic flexure, between left ventral and left dorsal colon; the diaphragmatic flexure, between left dorsal and right dorsal colon. The main purpose of the large colon is to absorb carbohydrates, which were broken down from cellulose in the cecum. Due to its many twists and turns, it is a common place for a type of horse colic called an impaction.

     The small colon is 10 to 12 feet (3.0 to 3.7 m) in length and holds only 5 US gallons (19 L) of material. It is the area where the majority of water in the horse's diet is absorbed, and is the place where fecal balls are formed. The rectum is about 1 foot (30 cm) long, and acts as a holding chamber for waste matter, which is then expelled from the body via the anus.