this arrangement persists throughout life (see C. Toldt, “Die Formbildung d. menschl. Blinddarmes,” Sitz. der Wiener Akad. Bd. ciii. Abteil. 3, p. 41) .
The ascending colon runs up from the caecum at the level of the ileo-caecal valve to the hepatic flexure beneath and behind the right lobe of the liver; it is about 8 in. long and posteriorly is in contact with the abdominal wall and right kidney. It is covered by peritoneum except on its posterior surface (see fig. 1).
The transverse colon is variable in position, depending largely on the distension of the stomach, but usually corresponding to the subcostal plane (see Anatomy: Superficial and Artistic). On the left side of the abdomen it ascends to the splenic flexure, which may make an impression on the spleen (see Ductless Glands), and is bound to the diaphragm opposite the eleventh rib by a fold of peritoneum called the phrenico-colic ligament. The peritoneal relations of this part are discussed in the article on the coelom and serous membranes.
The descending colon passes down in front of the left kidney and left side of the posterior abdominal wall to the crest of the ilium; it is about 6 in. long and is usually empty and contracted while the rest of the colon is distended with gas; its peritoneal relations are the same as those of the ascending colon, but it is more likely to be completely surrounded.
The iliac colon stretches from the crest of the ilium to the inner border of the psoas muscle, lying in the left iliac fossa, just above and parallel to Poupart’s ligament. Like the descending, it is usually uncovered by peritoneum on its posterior surface. It is about 6 in. in length.
The pelvic colon lies in the true pelvis and forms a loop, the two limbs of which are superior and inferior while the convexity reaches across to the right side of the pelvis. In the foetus this loop occupies the right iliac fossa, but, as the caecum descends and enlarges and the pelvis widens, it is usually driven out of this region. The distal end of the loop turns sharply downward to reach the third piece of the sacrum, where it becomes the rectum. To this pelvic colon Sir F. Treves (Anatomy of the Intestinal Canal, London, 1885) has given the name of the omega loop. Formerly the iliac and pelvic colons were spoken of as the sigmoid flexure, but Treves and T. Jonnesco (Le Colon pelvien pendant la vie intra-utérine, Paris, 1892) have pointed out the inapplicability of the term, and to the latter author the modern description is due.
The rectum, according to modern ideas, begins in front of the third piece of the sacrum; formerly the last part of the Ω (or omega) loop was described as its first part. It ends in a dilatation or rectal ampulla, which is in contact with the back of the prostate in the male and of the vagina in the female and is in front of the tip of the coccyx. The rectum is not straight, as its name would imply, but has a concavity forward corresponding to that of the sacrum and coccyx.
When viewed from in front three bends are usually seen, the upper and lower of which are sharply concave to the left, the middle one to the right. At the end of the pelvic colon the mesocolon ceases, and the rectum is then only covered by peritoneum at its sides and in front; lower down the lateral covering is gradually reflected off and then only the front is covered. About the junction of the middle and lower thirds of the tube the anterior peritoneal covering is also reflected off on to the bladder or vagina, forming the recto-vesical pouch in the male and the pouch of Douglas in the female. This reflexion is usually about 3 in. above the anal aperture, but may be a good deal lower.
The anal canal is the termination of the alimentary tract, and runs downward and backward from the lower surface of the rectal ampulla between the levatores ani muscles. It is about an inch long and its lateral walls are in contact, so that in section it appears as an antero-posterior slit (see J. Symington, Journ. Anat. and Phys. vol. 23, 1888).
Structure of the Intestine.—The intestine has four coats: serous, muscular, submucous and mucous. The serous or peritoneal coat has already been described wherever it is present. The muscular coat consists of unstriped fibres arranged in two layers, the outer longitudinal and the inner circular (see fig. 2). In the large intestine the longitudinal fibres, instead of being arranged evenly round the tube as they are in the small, are gathered into three longitudinal bands called taeniae (see fig. 1); by the contraction of these the large intestine is thrown into a series of sacculi or slight pouches. The taeniae in the caecum all lead to the vermiform appendix, and form a useful guide to this structure. In the rectum the three taeniae once more become evenly arranged over the whole surface of the bowel, but more thickly on the anterior and posterior parts. The circular layer is always thicker than the longitudinal; in the small intestine it decreases in thickness from the duodenum to the ileum, but in the large it gradually increases again, so that it is thickest in the duodenum and rectum.
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| From A. Birmingham; Cunningham’s Text-Book of Anatomy. |
| Fig. 2.—Diagram to show the structure of the small and large intestine and the duodenum. |
The submucous coat is very strong and consists of loose areolar tissue in which the vessels break up.
The mucous coat is thick and vascular (see fig. 2); it consists of an epithelial layer most internally which forms the intestinal glands (see Epithelial, Endothelial and Glandular Tissues). External to this is the basement membrane, outside which is a layer of retiform tissue, and this is separated from the submucous coat by a very thin layer of unstriped muscle called the muscularis mucosae. In the duodenum and jejunum the mucous membrane is thrown into a series of transverse pleats called valvulae conniventes (see fig. 3); these begin about an inch from the pylorus and gradually fade away as the ileum is reached. About 4 in. from the pylorus the common bile and pancreatic ducts form a papilla, above which one of the valvulae conniventes makes a hood and below which a vertical fold, the frenulum, runs downward. The surface of the mucous membrane of the whole of the small intestine has a velvety appearance, due to the presence of closely-set, minute, thread-like elevations called villi (see fig. 2). Throughout the whole length of the intestinal tract are minute masses of lymphoid tissue called solitary glands (see fig. 2); these are especially numerous in the caecum and appendix, while in the ileum they are collected into large oval patches, known as agminated glands or Peyer’s patches, the long axes of which, from half an inch to 4 in. long, lie in the long axis of the bowel. They are always found in that part of the intestine which is farthest from the mesenteric attachment. In the interior of the rectum three shelf-like folds,
