410 DIGESTIVE SYSTEM OF THE HORSE 



two layers of fibers arranged spirally or elliptically, which intercross dorsally and 

 ventrally. At the origin two bundles nearly an inch (ca. 2 cm.) wide arise from 

 the wide posterior part of the pharyngeal raphe and from a tendon common to 

 the crico-pharyngeus and thyro-pharyngeus. These bundles, which blend and 

 decussate at their origin, diverge and pass to each side of the oesophagus. In the 

 angle between them a deeper layer of circular fibers is visible. Two small ventral 

 bundles emerge from the depression between the lamina of the cricoid cartilage 

 and the arytenoid cartilages. These curve around to the side of the oesophagus 

 and blend with the dorsal bundles before described. In the terminal part there 

 is an external longitudinal layer and an internal circular layer, the latter being 

 extremely thick. The mucous membrane is pale, and is covered with squamous 

 stratified eisithelium. It is loosely attached to the muscular coat by an abundant 

 submucosa, and lies in longitudinal folds which obliterate the lumen except during 

 deglutition.^ 



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



Nerve-supply. — ^'agus, 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 cavitj^ by the diaphragm and is continuous behind 

 with the pelvic cavity. The line of demarcation between the abdominal and 

 pelvic cavities is known as the terminal line, or brim of the pelvis; it is formed by 

 the base of the sacrum dorsally, the ileo-pectineal lines laterally, and the anterior 

 borders of the pubic bones ventrally. 



The cavity is ovoid in form, but is 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 by the lumbar vertebrae, the lumbar 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 with the iliacus muscles, the 

 cartilages of the asternal ribs, and the parts of the posterior ribs which are below 

 the attachment of the diaphragm. 



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 deeply concave, thus 

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



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

 siderable part of the lateral walls, since its costal part 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 of contact 

 includes abput all of the costal part. This fact is of clinical importance, with reference to auscul- 

 tation and percussion and penetrating wounds. The cupola of the diaphragm extends as far 

 forward as a plane through the sixth rib to the right of the heart. 



The muscular walls are Uned by a layer of fascia, distinguished in different 

 parts as: (1) The diaphragmatic fascia; (2) the transversalis fascia; (3) the iliac 

 fascia; (4) the deep layer of the lumbo-dorsal fascia. 



The subserous tissue (Tela subserosa) unites the fascia and peritoneum. 



1 The potential lumen is difficult to determine at all accurately. When distended, its average 

 diameter (according to Rubeli) is a little more than 6 cm. The thickness of the wall varies (in- 

 versely as the lumen) from 0.4 to 1.2 cm. or more. The change in type of the muscular tissue is 

 gradual, and begins in the deep part; bundles of striped muscle are present even after the apparent 

 change to unstriped muscle, and may extend to the stomach. In the posterior part of the tube 

 many bundles intercross in an irregular plexiform manner. 



