THE DIGESTIVE SYSTEM 



This system consists of the organs directly concerned in the reception and 

 digestion of the food, its passage through the body, and the expulsion of the un- 

 absorbed portion. These organs are conveniently grouped under two heads, 

 viz.: (1) the alimentary canal; (2) the accessory organs. 



The alimentary canal (Tractus alimentarius) is a tube, about 100 feet (ca. 

 30 m.) in length in the horse, and extends from the lips to the anus. It has a com- 

 plete lining of mucous membrane, external to which is an almost continuous muscu- 

 lar coat. The abdominal portion of the tube is largely covered with a serous 

 membrane — the visceral peritoneum. The canal consists of the following consecu- 

 tive segments: 



1. Mouth. 4. Stomach. 



2. Pharynx. 5. Small intestine. 



3. ffisophagus. 6. Large intestine. 



The accessory organs are the teeth, tongue, salivary glands, liver, and pancreas. 



DIGESTIVE SYSTEM OF THE HORSE 



THE MOUTH 



The mouth' (Cavum oris) is the first part of the alimentary canal. In the 

 horse it is a long cylindrical cavity, and when closed, it is almost entirely filled up 

 by the contained structures. The entrance to it (Rima oris) is closed by the lips. 

 Laterally it is bounded by the cheeks; above, by the hard palate; below, by the 

 body of the mandible and the mylo-hyoid muscles; behind, by the soft palate. 



The cavity of the mouth is subdivided into two parts by the teeth and alveolar 

 processes. The space external to these and inclosed externally by the lips and 

 cheeks is termed the vestibule of the mouth (Vestibulum oris). In the resting 

 state of the parts the walls of this cavity are in contact, and the space is practically 

 obliterated. Its existence becomes very evident in facial paralysis, when the food 

 tends to collect in it laterally, pouching out the cheeks. The space within the 

 teeth and alveolar processes is termed the mouth cavity proper (Cavum oris pro- 

 prium). When the teeth are in contact, it communicates with the vestibule only 

 by the interdental spaces and the intervals behind the last molar teeth. Poste- 

 riorly it communicates with the pharynx through the isthmus of the fauces. 



The mucous membrane lining the mouth (Tunica mucosa oris) is continuous 

 at the margin of tiic lips with the connnon integument, and behind with the mucous 

 lining of the pharynx. During life it is chiefly of a pink color. 



The lips (La])ia oris) arc two musculo-membranous folds which surround the 

 orifice of the mouth. Their angles of union (Anguli oris s. commLssura? labiorum) 

 are situated near the first chec^k tooth and are rounded. Each lip presents two 

 surfaces and two bordcTs. The outer surface is covered by the skin, which pre- 

 sents long tactile hairs in addition to the ordinary fine hair. The upper lip shows 



1 The term "mouth" is eonmionly used to sifiuify either the oral cavity (Cavum oris) or the 

 entrance to it (Rima oris). 



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