5.2.12: Colon: ascending, transverse, descending, sigmoid (3:29)
Colon: ascending, transverse, descending, sigmoid
Here's the ascending colon. It's held in place by the peritoneum of the posterior abdominal wall, which covers it on the front and sides. The ascending colon ends a long way back at this sharp 90° turn, the right colic flexure, or hepatic flexure. The hepatic flexure lies just below the lowest part of the liver, and the gall bladder, and in front of the lower part of the right kidney, which is back here.
The transverse colon crosses the abdominal cavity from right to left. Here the transverse colon has been pulled upward, along with the greater omentum. Here's where it's normally located.
In its natural location it's partly hidden by the geater omentum that clings to its anterior surface. The omentum isn't he real attachment of the transverse colon: it's only loosely adherent to it. The real attachment of the transverse colon, which we can see when we pull it upwards, is this double sheet of peritoneum, the transverse mesocolon. We'll take the transverse colon out of the picture to see its attachment.
Here's the divided transverse mesocolon. It goes from here, to here. It crosses the head of the pancreas, which is here, and also the duodenum, which is here. This is the divided root of the mesentery. The transverse colon hangs down in a curve that's parallel to the greater curve of the stomach.
The two structures are connected, as we've seen, by the part of the greater omentum that's known as the gastro-colic ligament. The transverse colon ends higher and even further back than it started, at this sharp downward turn, the left colic flexure, or splenic flexure.
With the colon in its natural location the splenic flexure is out of sight, right up here. The splenic flexure lies just below the spleen, and in front of the left kidney, which is back here.
Below the splenic flexure is the descending colon. Just like the ascending colon, it's fixed to the posterior abdominal wall. The descending colon is quite short. A little below the iliac crest, which is here, the descending colon is continuous with the sigmoid colon.
The sigmoid colon forms a large freely mobile loop that's attached by this double sheet of peritoneum, the sigmoid mesocolon. The sigmoid colon passes down into the pelvic cavity: there's a lot of it down there, which we'll bring out.
As it passes into the pelvic cavity the sigmoid colon approaches the midline. The sigmoid mesocolon becomes shorter as it enters the pelvis, and ends altogether down here. The sigmoid colon ends at the level of S3, where it merges with the rectum.