Now we'll move on to look at the jejunum and the ileum. They're the sites of absorbtion of digested foodstuffs. Jejunum and ileum are names given to the proximal and distal parts of one continuous tube. There's no distinct boundary between them, and they're often spoken of together as the jejuno-ileum.
We'll look at a dissection in which all the organs are present, with only the dependent part of the greater omentum removed. All of this seemingly haphazard arrangement of loops and coils is the jejuno-ileum. To make its orientation clearer, we'll re-arrange it like this.
The jejuno-ileum starts up here to the left of the mid-line. It runs downward and to the right, ending here. The jejuno-ileum lies within a space that's bounded by the ascending colon to the right, the descending colon to the left, and the transverse colon and its mesentery above.
Along its six metre length the jejuno-ileum changes gradually: it becomes narrower, thinner walled, and less vascular. The loops of jejuno-ileum are attached to the posterior abdominal wall by this peritoneal sheet, the mesentery.
Since the attachment of the mesentery to the intestine is about thirty times longer than its attachment to the body wall, the mesentery is arranged like a richly folded fan. The mesentery carries the blood vessels of the jejuno-ileum, and its nerves and lymphatics.
The blood vessels are arranged in arcades, which we can see when we hold the mesentery up to the light. Here in the proximal jejuno-ileum there's a single arcade. Here more distally there are multiple arcades: there's one here, another one here. The mesentery contains fat between its peritoneal layers, more so distally than proximally.
We'll take the jejuno-ileum out of the picture again, to look at the attachment of the mesentery to the posterior abdominal wall. It begins here on the left in front of the last part of the duodenum, and runs downward and to the right, ending here close to the cecum.
Here's part of the jejunum that's been divided longitudinally. The mucosal lining is thrown into folds that project into the lumen. The folds are more pronounced here in the jejunum, than in the ileum.
Seen in close-up, the mucosal surface is a carpet of minute projections, villi, which vastly increase its absorbtive surface area. The jejuno-ileum ends down here. We'll remove some fat to see that better. The ileum joins the large intestine at the ileo-cecal valve, which is here.