Mesenteries are dual-layers of peritoneum that extend from the abdominal wall and attach the small intestines and other abdominal organs posteriorly to the wall. They are stringy connective tissue, and can be easily probed or cut away to isolate the intestines. The probe in the above picture is pointing to a mesentery.
The small intestines have three sections, in descending order: the duodenum, the jejunum, and the ileum. The duodenum is the only section that is not attached to the abdominal wall via mesenteries, as can be seen in the picture below with the probe pulling it out.
The jejunum, pointed at (in general) with the probe, takes up half the length of the small intestine, is mostly used for absorbing carbohydrates and proteins. It starts at the point where the intestine is attached to the mesenteries. The following section, the ileum, is also attached to mesenteries, but is slightly wider. Below, you can see the ileum, if you notice the coloration of the intestine beginning to change slightly it is probably the ileum. the ileum is located right before the cecum and spiral colon. to look for this, first find the spiral colon, and then try to find the point that switches from small intestines to large intestines. the small intestine side will be the ileum.