THE PANCREAS 371 



The anus is the terminal orifice 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. The mucous lining is 

 pale, glandless, and covered with a thick, squamous, stratified epithelium. 



There are three muscles of the anus. 



1. The sphincter ani intemus is merely a terminal thickening of the circular 

 coat of the bowel. 



2. The sphincter ani extemus is a broad band of striped muscle-fibers outside 

 the internal sphincter. Some fibers are attached to the coccygeal fascia alcove, 

 others to the perineal fascia below. Its action is to close the anus. 



3. The retractor ani (M. levator ani) is a flat muscle which lies between the 

 rectum and the sacro-sciatic ligament. It arises from the superior ischiatic spine 

 and the sacro-sciatic ligament, and ends under the external sphincter. Its action 

 is to reduce the partial prolapse which the anus undergoes during defecation. 



The suspensory ligament of the anus is a band of unstriped muscle which arises 

 from the first coccygeal vertebra, passes downward over the retractor, and unites 

 with its fellow below the anus. In the male it is largely continued bj' the retractor 

 penis muscle; in the female it blends with the constrictor vulvae. It may act as 

 an accessory sphincter of the anus. 



Blood-supply. — Internal pudic artery. 



Nerve-supply. — Hiemorrhoidal and perineal nerves (for the sphincter ani exter- 

 nus and retractor ani). 



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 faces upward 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 cceliac 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 looks downward and backward ; it is in general concave. 

 It presents two impressions, separated by an oblique ridge. The smaller of 

 these (Impressio caecalis) Ues to the right, and is caused by the pressure of the 

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

 tact with the terminal part of the great colon and its junction with the small colon. 

 It has usually no peritoneal covering except over a small area at the anterior angle. 



The right border is nearly straight; it is related to the second part of the duo- 

 denum and the caudate lobe of the liver. 



The left border is slightly concave, and is related to the first part of the duode- 

 num, the left sac of the stomach, and the splenic vessels. 



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

 clearness to the organ in the horse. 



