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 em.) wide arise from 
the wide posterior part of the pharyngeal raphé 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 cesophagus. 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 cesophagus 
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 epithelium. 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.— 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 pelvie cavity. The line of demarcation between the abdominal and 
pelvie 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 vertebre, 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 about 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 lined by a layer of fascia, distinguished in different 
parts as: (1) The diaphragmatic fascia; (2) the transversalis fascia; (3) the iliae 
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 Rubel) is a little more than 6 em. The thickness of the wall varies (in- 
versely as the lumen) from 0.4 to 1.2 em. 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. 
