352 DIGESTIVE SYSTEM OF THE HORSE 



The principal relations of the oesophagus at its origin are: to the cricoid 

 cartilage below; to the guttural pouches and the ventral straight muscles above; 

 and to the carotid arteries laterally. In the middle of the neck the relations are: 

 to the leftlongus colli muscle above; to the trachea internally; to the left carotid 

 artery, vagus, sympathetic, and recurrent nerves externally. At its entrance 

 into the thorax it has the trachea on its inner side; the first rib, the roots of the 

 brachial plexus of nerves and the inferior cervical ganglion externally. After 

 gaining the upper surface of the trachea, it has the aorta on its left and the vena 

 azygos and right vagus nerve on its right side. In its course through the posterior 

 mediastinum the oesophageal trunks of the vagus nerves lie above and below it, 

 and the oesophageal artery is dorsal to it. 



Structure. — The wall is composed of four coats: (1) A fibrous sheath; (2) the 

 muscular coat; (3) a submucous layer; (4) the mucous membrane. The muscular 

 coat is of the striped variety as far as the l)ase of the heart, where it rapidly changes 

 to the unstriped type. In addition to this change, the muscular coat becomes 

 much thicker and firmer, while the lumen is diminished.^ The outer fibers are 

 arranged longitudinally, beginning in two bundles attached in the interval be- 

 tween the arytenoid and cricoid cartilages. The inner fibers run in two spiral 

 strata to the terminal part of the tube, where the arrangement is an outer longi- 

 tudinal and an inner circular layer.^ The mucous membrane is pale, and is covered 

 with squamous stratified epithelium. It is loosely attached to the muscular coat 

 by an abundant sul)mucosa, and lies in longitudinal folds which oliliterate the lumen 

 except during deglutition. 



Blood-supply. — Carotid, broncho-oesophageal, and gastric arteries. 



Nerve-supply. — Vagus, glosso-pharyngeal, and sympathetic nerves. 



THE ABDOMINAL CAVITY 



The abdominal cavity (Cavum abdominis) is the largest of the body cavities. 

 It is separated from the thoracic cavity by the diaphragm and is continuous behind 

 with the pelvic cavity. 



It is ovoid in form but somewhat compressed laterally. Its long axis extends 

 obliquely from the center of the pelvic inlet to the sternal part of the diaphragm. 

 Its dorso-ventral diameter is greatest at the first lumbar vertebra, while its greatest 

 transverse diameter is a little nearer the pelvis. 



The dorsal wall or roof is formed l^y the lumbar vertebrae, the luml)ar muscles, 

 and the lumbar part of the diaphragm. 



The lateral walls are formed by the oblique and transverse abdominal muscles, 

 the abdominal tunic, the anterior parts of the ilia, the cartilages of the asternal 

 ribs, and the parts of the posterior ribs which are below the attachment of the dia- 

 phragm. 



The ventral wall or floor consists of the two recti, the aponeuroses of the oblique 

 and transverse muscles, the abdominal tunic, and the xiphoid cartilage. 



The anterior wall is formed by the diaphragm, which is very deeply concave, 

 thus greatly increasing the size of the abdomen at the expense of the thorax. 



It should be noted that tlie cHaphragm also concurs practically in the formation of a con- 

 siderable part of the lateral walls, since its costal portion even during ordinary- inspiration lies 

 directly on the ribs over a width of four or five inches (ca. 10 to 12 cm.); in expiration this area 



' The potential lumen is difficult to determine at all accurately. When distended, its diam- 

 eter (according to Rubeli) i^aries from 5.7 cm. at its origin to 4 cm. at its cardiac end. The 

 thickness of the wall varies (inversely as the lumen) from 4 mm. to 1.2 cm. or more. 



- At the origin of the tube muscular l)undles arise on the raphe pharyngis and blend with the 

 crico-pharyngeus. \'(>ntrally fil)ers come from the depression between the cricoid and arytenoid 

 cartilages. Bundles of striped fibers may be continued in the superficial part of the muscular 

 coat as far as the cardia. 



