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 anahs 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 intemus is the terminal thickening of the circular coat 

 of the bowel. 



2. The sphincter ani extemus 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 extemus. 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 coccygeal 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 vulvae. 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 vertebrae. 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 (Fades 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 caecus 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 (Fades ventralis) faces ventrally and backward; it is iq 

 general concave. It presents two impressions, separated by an oblique ridge. The 

 smaller of these (Impressio csecalis) lies to the right, and is caused by the pressure 

 of the base of the caecum; the larger one (Impressio colica) indicates the area of 



' A descriptive method based on that in use in human anatomy cannot be applied with 

 clearness to the organ in the horse. 



