The Mouth and Pharynx
The process of digestion begins at the mouth with the lips and incisors grasping food. The molars then grind the food while mixing it with saliva. This alkaline saliva is produced from the paratoid, submaxmillary and sublingual glands. The average mature horse produces about 10 gallons of saliva per day. This saliva acts as a digestive juice on the sugars and starches in the feed and lubricates it for swallowing. The tongue flips balls of food back to the pharynx once it has thoroughly chewed. The pharynx is a muscular passage to the esophagus.  TOP

The Esophagus
The esophagus is a 5-foot long tube with circular muscles, which force the food and water down to the stomach. These muscular waves of constriction are known as peristalsis. The angle at which the esophagus meets the stomach is such that excessive gastric buildup pushes the membrane flaps of the opening closed. This makes vomiting in the horse extremely difficult. A horse that is vomiting may have ruptured is stomach lining.  TOP

The Stomach
The stomach is J-shaped organ with the openings to the esophagus and small intestine close together. Digestion is best achieved when the stomach is only 2/3 full, thus the importance of several smaller meals, as opposed to one or two large feeding. Cells in the lining primarily secrete hydrochloric acid and the enzyme pepsinogen. The hydrochloric acid acts to change pepsinogen to pepsin, a protein digester, and it dissolves mineral matter. Food enters the stomach in layers and stays until the 2/3 full. At this point, is pushed out to the small intestine. During a large meal, some of the food being pushed out will not have had the chance to be totally digested.  TOP

The Small Intestine
The small intestine is about 70 feet long and 2 to 3 inches in diameter. It is divided into three parts for identification, the duodenum, the jejunum, and the ileum. The inner lining is comprised of specialized mucosal cells arranged into villi, which vastly increases the absorptive surface area. These cells perform the major part of the enzymatic breakdown of feed. Proteins, sugars, fats are absorbed through the villi and enter the blood stream.
      The three main types of enzymes in the small intestine are: protein-digesting, carbohydrate-digesting, and fat-digesting. The protein-digesting enzymes, trypsin and chrymrotrypsin, are created in a non-active form in the pancreas and become active protein-digesters in the small intestine. Trypsin and chymrotrypsin are not totally effective unless preceded by the pepsin enzyme in the stomach. These enzymes produce the final breakdown of proteins to amino acids. The carbohydrate-digesting enzyme amylase is formed in the pancreas and acts on starches, changing them to maltose, a sugar. This maltose and other 2 part sugars (disaccarides) are hydrolyzed, or broken down by inserting water molecules, single part sugars (monosaccarides) and absorbed. Fat-digesting enzymes are also formed in the pancreas, as well as the small intestine. The affect lipids (fats) and break them down into fatty acids and monoglycerides by hydrolysis.
      Muscular contractions mix the food in four different variations in the small intestine. The first type of contraction is the contraction and relaxation of various short segments. The second type is pendular: the swaying back and forth of the small intestine. The third type of muscular contraction occurs in short propulsive movements; weak waves of contraction, which move about 2 inches before stopping. The contractions then pause for a moment before starting up where they left off. The final type of contraction is paristalsis, which is not normal in the small intestine and results in diarrhea-like symptoms.  TOP

The Large Intestine
The large intestine is divided into four parts; the cecum, the large colon, the small colon, and the rectum. The cecum is 4 feet long and contains mostly liquid. The food and water ingested by the horse are well mixed here. The bacterial population breaks down the food, resulting in the formation of fatty acids and some vitamins. The large colon is 10 to 12 feet long and 8 to 18 inches in diameter. Some digestion, mostly bacterial fermentation and absorption occur here. The small colon is also about 10 to 12 feet in length, but is only 3 to 4 inches in diameter. Most of the moisture is absorbed in the colons. The remaining food matter is formed into balls at the end of the small colon. The last foot of the large intestine is the rectum, where the waste is stored until its is passed.  TOP