Jejunum

The jejunum is the part of the small intestine between duodenum and ileum.

For Barone[1], the classical division into jejunum and ileum remains arbitrary, although the terminal portion, formed by the ileum, presents clear morphological and functional distinctions. However, these features appear progressively, and no precise boundary can be defined between the two segments of the tract. That is why the jejunum is described in common with ileum as the jejuno-ileum:

The jejuno-ileum is suspended by a large mesentery, and is much longer and more mobile than the duodenum. It begins at the duodenojejunal flexure and ends at the ileal orifice (ostium ileale).

EXTERNAL SHAPE AND DIMENSIONS:

The jejuno-ileum is a long cylindrical tube whose surface, covered by peritoneum, appears smooth and shiny. When empty, it is flattened by the relaxation of its walls. Moderately curved, it has an almost perfectly circular cross-section and a slight bend at the free end such that, when removed from the abdominal cavity and freed from serous attachments, it naturally coils into a spiral when insufflated. The concave portion, where the mesentery is attached along its entire length, is known as the mesenteric border, sometimes called the small curvature. Opposite lies the free border or antimesenteric side, which is convex. These two borders are connected by two smooth, convex surfaces.

The diameter of jejunum and ileum varies depending on the species, the segment examined, and its functional state. It is proportionally larger in carnivores than in herbivores, and generally smaller in animals with longer intestinal tracts. For example, in a medium-sized dog, the undistended jejunal diameter is about 20–25 mm; in humans, 25–30 mm; in horses, 35–45 mm; and in cattle, 25–30 mm. The average total length of the jejuno-ileum is approximately 3.5 meters in dogs, 7 meters in humans, 22 meters in horses, and 40 meters in cattle. The diameter can more than double when distended or shrink to half when contracted. Additionally, the lumen narrows progressively in the distal half of the tract, while the intestinal wall becomes thicker and firmer. Rabbits, however, are an exception: the end of their ileum is instead dilated, forming an ileal ampulla or sacculus rotundus.

Contraction or relaxation never occurs simultaneously throughout the entire jejuno-ileum. This results in a very irregular alternation of dilated and constricted segments caused by contractions. In living animals, or shortly after death during evisceration, one can observe contractions moving toward the cecum over more or less extensive segments of the small intestine. These waves of contraction disappear after a certain distance, while others arise in new areas. Their overall action characterizes peristalsis, the mechanism responsible for moving food along.

INTERNAL STRUCTURE

The entire cavity of the jejuno-ileum is lined with a soft, velvety mucosa due to the presence of countless villi. Gray-pink or yellowish in color, this mucosa forms folds whose arrangement varies with species and location. In humans, from the end of the descending part of the duodenum to the beginning of the ileum, there are circular folds (Plicae circulares) — formerly known as "valves of Kerckring" — measuring six to eight millimeters high, transverse, and rarely complete. They cannot be flattened by distension and serve to significantly increase the mucosal surface area. Similar folds, though less pronounced or differently arranged, are found in other mammals. They are less regular and less prominent in species with more developed villi or where the mucosal folds are more irregular and distensible.

At certain locations, the mucosa takes on a distinctive appearance due to the accumulation of lymphatic nodules and a partial absence of villi. These are known as aggregated lymph nodules (Peyer's patches, Lymphonoduli aggregati). Their number varies widely, from a few dozen in dogs to over a hundred in horses, and their size ranges from a few millimeters to several centimeters or even tens of centimeters: in pigs, a single patch more than one meter long exists in the terminal part of the small intestine. Generally, these structures are absent from the duodenum and become increasingly numerous toward the ileum; the terminal enlargement of the rabbit’s ileum is entirely filled with such nodules. The reverse pattern is observed in some species: in dogs, nodules can be found in the duodenum, and their number decreases in the second half of the jejunum until they disappear in the ileum.

The jejunal cavity continues seamlessly from that of the duodenum, but the end of the ileum is usually marked by a somewhat narrowed ileal orifice (Ostium ileale). This orifice is typically located atop a prominent ileal papilla in the large intestine cavity.