The horse esophagus is a muscular tube that carries feed from the rear of the mouth to the stomach. It synchronously contracts and relaxes in the swallow reflex to carry a bolus of feed downstream. In a normal esophagus the muscles are surrounded by a thin layer of mucous membrane. In some horses, however, the esophagus becomes distended and unable to contract normally. This is known as megaesophagus and can be caused by a number of different disorders.
The esophagus is a long muscular tube that varies in length from 125 to 200 cm (49 to 78 inches). It lies dorsal to the trachea in the thoracic portion of the neck and courses caudadly toward the heart in the peri-cardial mediastinum. In the thoracic girth it passes through the fifth rib. It then deviates to the left in the middle part of the neck, reaches the mediastinum at the level of the 2nd thoracic vertebra and continues between the left sub-clavian artery and the left lung lobes in the pulmonary intercostal space. In a small percentage of horses, it curves to the right in the thoracic plane and finally crosses to lie ventral to the trachea at the thoracic inlet in the cranial one third of the neck.
Digestion of food begins in the mouth with chewing, reducing the size of the feed particle and mixing it with saliva which provides lubrication to allow easier passage through the esophagus. This is followed by a chemical breakdown of carbohydrates in the stomach. The stomach is relatively small compared to the total digestive tract and is divided into glandular and non-glandular areas. It also contains a pylorus which prevents the esophagus from vomiting into the stomach.
In the intestines, the food is broken down into nutrient components by absorption and adsorption. It can take up to 3-4 hours for food to spend in the small intestines during healthy digestion. The large intestine, cecum and rectum function like the forestomach of a ruminant and allow continual microbial fermentation of dietary fibers.
The rectum is large to allow for the fermentation of a significant volume of grasses. It can become gas filled and this is why it needs to be inspected regularly. It can be enlarged by a procedure called cecal trocarization, in which a catheter is inserted through the rectum to decompress the sac.