Part 125

It is attached to the sublumbar region by the =colic mesentery=, and to the termination of the duodenum by a short peritoneal fold (Lig. duodeno-colicum). The great omentum is also attached to the origin of the bowel. The colic mesentery is narrow at its origin, but soon reaches a width of about three feet (ca. 80 to 90 cm.). Its parietal border is attached along a line extending from the ventral surface of the left kidney to the sacral promontory; it is continuous in front with the root of the great mesentery, and behind with the mesorectum.

There are two =tæniæ= and two rows of sacculations. Of the tæniæ, one is free, the other concealed by the mesentery. When the bowel is hardened in situ its lumen between the pouches is reduced to a narrow slit.

THE RECTUM

The =rectum= (Intestinum rectum) is the terminal part of the bowel; it extends from the pelvic inlet to the anus. Its length is about one foot (ca. 30 cm.). Its direction may be straight or oblique. The first or =peritoneal part= of the rectum is like the small colon, and is attached by a continuation of the colic mesentery termed the =mesorectum=. The second or =retroperitoneal part= forms a flask-shaped dilatation termed the =ampulla recti=; it is attached to the surrounding structures by connective tissue and muscular bands.

The first part of the rectum is related to the small colon and the pelvic flexure of the great colon. It is frequently deflected to the left by the latter. The second part of the rectum is related dorsally and laterally to the pelvic wall. Ventrally the relations differ in the two sexes. In the male they are the bladder, the terminal parts of the vasa deferentia, the vesiculæ seminales, the prostate, the bulbo-urethral (Cowper’s) glands, and the urethra. In the female they are the uterus, vagina, and vulva.

=Structure of the Large Intestine.=—The =serous coat= covers the different parts in varying degrees. It does not cover (a) the opposed surfaces of the cæcum and colon which are between the layers of the cæco-colic fold and mesocolon; (b) the areas of attachment of the cæcum and colon to the pancreas, right kidney, and sublumbar region; (c) the second part of the rectum.

The =muscular coat= consists of longitudinal and circular fibers. The bulk of the former is in the bands already described. Some of the circular fibers pass from one part of the colon to another, where they are attached to each other, forming the fibræ transversæ coli. The muscular coat of the second part of the rectum presents special features. The longitudinal layer of fibers is very thick and consists of large bundles, loosely united. A large band, the =recto-coccygeus=, is detached from it on either side, and passes upward and backward to be inserted into the fourth and fifth coccygeal vertebræ.

The =submucous tissue= is abundant in the wall of the rectum, so that the mucous membrane is loosely attached to the muscular coat, and forms numerous folds when the bowel is empty.

The =mucous membrane= of the large intestine is thicker and darker in color than that of the small intestine. It forms large crescentic or semilunar folds corresponding to the external constrictions. It has no villi, Brunner’s glands, or Peyer’s patches. The =intestinal glands= (of Lieberkühn) are large and numerous. =Solitary glands= are also numerous, especially at the apex of the cæcum and in the left dorsal part of the colon.

=Blood-supply.=—Greater and lesser mesenteric and internal pudic arteries. The veins go to the portal vein.

=Nerve-supply.=—Mesenteric plexus of the sympathetic nerves.

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 internus= is merely a terminal thickening of the circular coat of the bowel.

2. The =sphincter ani externus= is a broad band 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= (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 by the retractor penis muscle; in the female it blends with the constrictor vulvæ. It may act as an accessory sphincter of the anus.

=Blood-supply.=—Internal pudic artery.

=Nerve-supply.=—Hæmorrhoidal and perineal nerves (for the sphincter ani externus 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 vertebræ.

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 cœliac artery and its divisions, the gastro-phrenic ligament and the saccus cæcus of the stomach, the right and caudate lobes of the liver, and the gastro-pancreatic fold. There are grooves for the divisions of the cœliac 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 cæcalis) lies to the right, and is caused by the pressure of the base of the cæcum; the larger one (Impressio colica) indicates the area of contact 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 duodenum and the caudate lobe of the liver.

The =left border= is slightly concave, and is related to the first part of the duodenum, the left sac of the stomach, and the splenic vessels.

The =posterior border= presents a deep notch to the right of the median plane for the portal vein, which passes through the gland very obliquely. There is a thin bridge of gland tissue dorsal to the vein, thus forming the =portal ring= (Annulus portarum). The root of the great mesentery is in contact with the border just to the left of the median plane.