Anatomy

The small intestine is the longest part of the gastrointestinal tract and is divided into the duodenum (discussed previously), jejunum and ileum (Figure 4.1). It is primarily concerned with digestion and absorption of food. Together, the jejunum and ileum measure approximately 6 metres (jejunum 2.5 m/ileum 3.5 m) in the adult. The jejunum commences at the duodenojejunal junction (flexure) and the ileum ends at the ileocaecal valve (two horizontal folds of mucosa that project around the orifice of the ileum as it joins the caecum). A fan-shaped fold of peritoneum (the small intestinal mesentery) attaches the small intestine to the posterior abdominal wall. The long edge of the mesentery completely encloses the intestine, allowing it to be mobile, while the short "root", which is attached to the posterior abdominal wall, admits blood vessels, lymphatics and nerves which supply the intestine by traversing the mesentery.

Figure 4.1. Small intestine. Reproduced with permission from Hermanek P, Hutter RVP, Sobin, LH, Wagner, G, Wittekind, Ch. (Eds). TNM Atlas: Illustrated guide to the TNM/pTNM classification of malignant tumours, 4th edition. Springer-Verlag: Berlin and Heidelberg, 1997.

Duodenum

Duodenum

Jejunum

Figure 4.1. Small intestine. Reproduced with permission from Hermanek P, Hutter RVP, Sobin, LH, Wagner, G, Wittekind, Ch. (Eds). TNM Atlas: Illustrated guide to the TNM/pTNM classification of malignant tumours, 4th edition. Springer-Verlag: Berlin and Heidelberg, 1997.

Ileum

Jejunum

Ileum

Although there is a gradual change from jejunum to ileum, in general the jejunum tends to be located in the upper part of the abdominal cavity, is thicker-walled with more prominent plicae circulares (permanent mucosal folds) and has more numerous Peyer's patches (aggregations of lymphoid tissue).

Histologically, the mucosa of the small intestine projects into the lumen in the form of fingerlike structures covered by ciliated epithelium. These projections are called villi and increase the surface area for absorption. The circular and longitudinal muscle layers are continuous.

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