This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The ascending colon lies in contact with the anterior abdominal wall from its lower end to above the iliac crest; here it dips down to lie on the kidney and form the hepatic flexure above (Fig. 427). At this point some of the coils of the small intestine may lie in front of the hepatic flexure, between the beginning of the transverse colon above and the ending of the ascending colon below. The ascending colon lies on the quadratus lumborum muscle and kidney behind and has the psoas to its inner side. It has no mesentery or peritoneum on its posterior surface in 64 . per cent, of the cases (Treves) and in tumors of the kidney it may be pushed forward and across their anterior surface. This is a point to be remembered in diagnosis.
Fig. 427. - View of the interior of the abdomen: the mesentery has been cut, the small intestines removed, and the transverse colon turned upward. The pelvic colon and iliac colon together form the sigmoid flexure.
The transverse colon passes diagonally up and to the left across the abdomen. It starts at the hepatic flexure on the under surface of the liver to the outer side of the gall-bladder. It runs parallel with the lower edge of the liver and stomach and its lower border may reach nearly or quite to the level of the umbilicus. The great omentum passes over the transverse colon, so that to see the latter it is necessary to raise the omentum and look on its under surface. The omentum as it passes from the colon to the stomach forms the gastrocolic omentum and the two organs may be either close together or some distance apart. The transverse colon instead of running upward and to the left may form a large curve downward, reaching almost to the pelvis. In cases of dilatation and descent (ptosis) of the stomach the transverse colon descends with it. The transverse mesocolon passes backward and one layer goes up and covers the pancreas while the other goes down to the mesentery. Its importance in gastroenterostomy has been pointed out (page 406). Tumors and cysts of the pancreas may push forward above it, or below it, or it may cross directly over the surface of the growth.
The descending colon at its commencement at the splenic flexure is much higher and more deeply situated than is the hepatic flexure. It follows the stomach upward and backward and lies against the spleen. From here it descends and is entirely covered by small intestine, the sigmoid flexure coming to the front in the left iliac fossa. The descending colon is much smaller in size than the ascending colon, and like it in the majority (two-thirds) of cases has no mesentery. In doing a colostomy through the loin, the external border of the quadratus lumborum muscle is the guide to the descending colon. It lies 1.25 cm. (1/2 in.) behind the middle of the crest of the ilium.
The sigmoid flexure is composed of two parts: one in the iliac fossa, called the iliac colon, and the other in the pelvis, called the pelvic colon, or omega loop of Treves.
The iliac colon is about 12.5 to 15 cm. (5 to 6 in.) long, and runs from the crest of the ilium to the inner edge of the iliopsoas muscle. It has no mesentery in 90 per cent. of the cases (Jonnesco), and usually comes into contact with the abdominal wall to the inner side of the anterior superior spine sometimes as far down as the middle of Poupart's ligament. In doing an inguinal colostomy this is the portion of the colon it is desired to find. It is then followed down until a part is reached which has sufficient mesentery to allow of its being drawn out of the wound.
The pelvic colon is about 40 to 42.5 cm. (16 to 17 in.) long and runs from the edge of the psoas muscle to the level of the third sacral vertebra. It makes a large horseshoe-shaped loop, from which it was named by Treves the omega loop, and has a mesentery from 3 to 8 cm. (1 1/4 to 3 1/2 in.) long. The length of the loop as well as its mesentery and its position all vary considerably. Its terminal portion usually runs longitudinally down to end in the rectum, but its intervening portion may pass over the bladder to the right side, or high above the symphysis, or even extend well up in the abdominal cavity. On the under or left side of the loop between its branches is the inter sigmoid fossa (see Fig. 422, page 410); sometimes it forms a constricted pouch in which a knuckle of intestine has been known to become strangulated.