432 DIGESTIVE SYSTEM OF THE HORSE 
iliac and lumbar glands. The nerves are derived from the mesenteric and pelvic 
plexuses of the sympathetic. 
THE ANUS 
The anus is the terminal part of the alimentary canal. It is situated below the 
root of the tail, where it forms a round projection, with a central depression when 
contracted. It is covered externally by an integument which is thin, hairless, and 
provided with numerous sebaceous and sweat glands. Its lumen, the anal canal 
(Pars analis recti), is about two inches (ca. 5 cm.) long; except during defecation 
it is closed by the contraction of the sphincter muscles and folds of the mucous 
lining. The mucous membrane is pale, glandless, and covered with a thick, squa- 
mous, stratified epithelium. 
There are three muscles of the anus. 
1. The sphincter ani internus is the terminal thickening of the circular coat 
of the bowel. 
2. The sphincter ani externus is a broad ring of striped muscle-fibers outside 
the internal sphincter. Some fibers are attached to the coccygeal fascia above, 
others to the perineal fascia below. Its action is to close the anus. 
3. The retractor ani is a flat muscle which lies between the rectum and the 
sacro-sciatic ligament; its fibers are directed backward and somewhat upward. 
It arises from the superior ischiatic spine and the sacro-sciatic ligament, and ends 
under the sphincter ani externus. Its action is to reduce the partial prolapse which 
the anus undergoes during defecation. 
The suspensory ligament of the anus (Lig. suspensorium ani) is a band of 
unstriped muscle which arises from the first coecygeal vertebra, passes downward 
under cover of the retractor ani, and unites with its fellow below the anus. In the 
male it is largely continued by the retractor penis muscle; in the female it blends 
with the constrictor vulve. It may act as an accessory sphincter of the anus. 
Vessels and Nerves.—The blood-supply comes from the internal pudic arteries, 
and the veins go to the internal pudic vein. The lymph vessels go to the anal 
lymph glands. The nerves come from the pudic nerve. 
THE PANCREAS 
The pancreas is situated transversely on the dorsal wall of the abdomen, the 
greater part being to the right of the median plane. Its central part lies under the 
sixteenth and seventeenth thoracic vertebree. When fresh, it has a reddish cream 
color, but if left in the unpreserved cadaver, it rapidly decomposes and becomes 
dark. It resembles the salivary glands in appearance, but is softer, and its lobules 
are more loosely united. Its average weight is about twelve ounces (ca. 350 g.). 
When hardened in situ its shape is very irregular. It is triangular in outline, and 
presents for description two surfaces, three borders, and three angles.' 
The dorsal surface (Facies dorsalis) faces dorsally and forward. It is partially 
covered by peritoneum. It is related chiefly to the ventral surface of the right 
kidney and adrenal, the posterior vena cava, the portal vein, the coeliac artery and 
its divisions, the gastro-phrenic ligament, and the saccus cecus of the stomach, the 
right and caudate lobes of the liver, and the gastro-pancreatic fold. There are 
grooves for the divisions of the cceliac artery and a large one for the splenic vein. 
The ventral surface (Facies ventralis) faces ventrally and backward; it is in 
general concave. It presents two impressions, separated by an oblique ridge. The 
smaller of these (Impressio czecalis) lies to the right, and is caused by the pressure 
of the base of the cecum; the larger one (Impressio colica) indicates the area of 
1A descriptive method based on that in use in human anatomy cannot be applied with 
clearness to the organ in the horse. 
