The Digestive System
(1) Anus. (2) Rectum. (3) Base of caecum. (4) Small intestine.
(5) Kidney. (6) Liver. (7) Diaphragm. (8) Oesophagus. (9) Large colon.
(10) Caecum. (11) Small colon
The horse’s digestive system consists of those organs concerned with digestion, or the turning of complex food material such as hay, grass and corn, into simple substances such as carbohydrate, protein ( amino acids ), fatty acids, etc, which can be used by the body for energy, storage or body building processes. The organs consist of the alimentary tract which is the tube extending from the mouth to the anus and known also as the gut, intestines or alimentary canal, and the accessory organs such as the teeth, tongue, salivary glands, liver and pancreas.
The special characteristics of the horse’s mouth are the highly prehensile lips for gathering food which work in conjunction with the sharp front teeth when cropping grass, and the labile tongue which conveys the food to the back teeth. These have table-like surfaces crossed by ridges that form an ideal grinding surface between the upper and lower jaws. An adult horse has 40 teeth arranged as follows: in each left and right, upper and lower jaw there are three incisors, one canine (present only in colts and geldings), and six cheek teeth (three premolars and three molars). A young horse has temporary teeth, which are replaced by the permanent teeth by the time it is five years old. Ducts which discharge digestive juices from the parotid mandibular and sublingual salivary glands open into the mouth. The roof of the mouth is formed by the hard plate in front, which continues into the soft plate behind. The soft plate forms part of the pharynx where the air passages and digestive tract cross one another.
As a horse swallows, the food crosses the pharynx and enters the gullet or oesophagus, from where it is conveyed to the stomach and thence to the small intestines, large colon, small colon and rectum.
The alimentary tract, from the stomach to the rectum, together with the pancreas and liver (glands which contribute more digestive juices and bile) are contained in the abdominal cavity. This can be described as a large ‘box’, the sides of which are the diaphragm in front, the muscles below the spine forming the top, and the muscles of the ‘belly’, the bottom. The back part of the ‘box’ is closed by the pelvic outlet through which the rectum, urinary and reproductive tract reach the outside. The abdominal cavity also contains, in the female, the ovaries and the uterus; and in both male and female, the urinary organs, comprising the kidney, ureters and bladder.
The abdominal cavity is lined by the peritoneum and all the organs are suspended by reflections ( mesenteries and ligaments) of the peritoneum. A special free fold of the peritoneum is known as the omentum.
The anatomical peculiarities of the horse’s digestive system compared with other mammals are:
(i) that the greatest volume of the tract is in the hind end, namely the caecum, and colon, where the major process of digesting fibre occurs by bacterial fermentation
(ii) the relatively small stomach
(iii) the absence of a gall bladder (probably associated with the need for a continual supply of bile in an animal which is a continuous feeder)
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