FIGURE 17.16 • Photomicrograph of a dividing cell in the isthmus of a pyloric gland. The gastric pits in this photomicrograph were sectioned in a plane that is oblique to the axis of the pit. Note that on this section, gastric pits (arrows) can be recognized as invaginations of surface epithelium that are surrounded by lamina propria. The lamina propria is highly cellular because of the presence of large numbers of lymphocytes. 240. Inset. This high magnification of the area indicated by the rectangle shows a dividing cell in the isthmus. 580.
The serosa of the stomach is as described above for the alimentary canal in general. It is continuous with the parietal peritoneum of the abdominal cavity via the greater omentum and with visceral peritoneum of the liver at the lesser omen-tum. Otherwise, it exhibits no special features.
The small intestine is the longest component of the digestive tract, measuring over 6 m, and is divided into three anatomic portions:  Duodenum (25 cm long) is the first, shortest, and widest part of the small intestine. It begins at the pylorus of the stomach and ends at the duodenojejunal junction (Plate 59, page 616).
 Jejunum (2.5 m long) begins at the duodenojejunal junction and constitutes the upper two fifths of the small intestine. It gradually changes its morphologic characteristics to become the ileum (Plate 60, page 618).
 Ileum (3.5 m long) is a continuation of the jejunum and constitutes the lower three fifths of the small intestine. It ends at the ileocecal junction, the union of the distal ileum and cecum (Plate 61, page 620).
The small intestine is the principal site for the digestion of food and absorption of the products of digestion.
Chyme from the stomach enters the duodenum, where enzymes from the pancreas and bile from the liver are also delivered to continue the solubilization and digestion process. Enzymes, particularly disaccharidases and dipeptidases, are also located in the glycocalyx of the microvilli of the enterocytes, the intestinal absorptive cells. These enzymes contribute to the digestive process by completing the breakdown of most sugars and proteins to monosaccharides and amino acids, which are then absorbed (Folder 17.4). Water and electrolytes that reach the small intestine with the chyme and pancreatic and hepatic secretions are also reabsorbed in the small intestine, particularly in the distal portion.
Plicae circulares, villi, and microvilli increase the absorptive surface area of the small intestine.
The absorptive surface area of the small intestine is amplified by tissue and cell specializations of the submucosa and mucosa.
 Plicae circulares (circular folds), also known as the valves of Kerckring, are permanent transverse folds that contain a core of submucosa. Each circular fold is circularly arranged and extends about one half to two thirds of the way around the circumference of the lumen (Fig. 17.17). The folds begin to appear about 5 to 6 cm beyond the pylorus. They are most numerous in the distal part of the duodenum and the beginning of the jejunum and become reduced in size and frequency in the middle of the ileum.
 Villi are unique, fingerlike and leaflike projections of the mucosa that extend from the theoretical mucosal surface for 0.5 to 1.5 mm into the lumen (Fig. 17.18). They completely cover the surface of the small intestine, giving it a velvety appearance when viewed with the unaided eye.