. Comparative anatomy. Anatomy, Comparative. THE DIGESTIVE SYSTEM 313 by the presence of numerous villi which cover its inner surface and give it a velvety appearance. Somewhat arbitrarily thiee regions are dis- tinguished, duodenun, jejunum, and ileum. The duodenum, the anterior portion of the small intestine, averages about nine inches in length,, and is characterized by the presence of tubulo-acinous glands located in the submucosa and known as duodenal or Brunner's glands. The duodenal glands secrete an alkaline mucus which neutralizes the acidity of the food which enters the duodenum from
. Comparative anatomy. Anatomy, Comparative. THE DIGESTIVE SYSTEM 313 by the presence of numerous villi which cover its inner surface and give it a velvety appearance. Somewhat arbitrarily thiee regions are dis- tinguished, duodenun, jejunum, and ileum. The duodenum, the anterior portion of the small intestine, averages about nine inches in length,, and is characterized by the presence of tubulo-acinous glands located in the submucosa and known as duodenal or Brunner's glands. The duodenal glands secrete an alkaline mucus which neutralizes the acidity of the food which enters the duodenum from the stomach. Zymogenic cells are also found in the duodenal mucosa. PLEXUS OF AUERBACH^ PLEXUS OF MEISSNERIk. PERITONEUM Fig. 262.—A stereogram of a portion of the small intestine, showing the arrangement of sympathetic neurons in the plexuses of Meissner and Auerbach. Motor cells are shown in black, sensory cells with white nuclei. (Redrawn after Kahn's "Das Leben Des Menschen," W. Keller & Co.) The jejtmimi, which forms two-fifths of the remainder of the small intestine, contains numerous transverse crescentic folds, the valvulae conniventes, covered with large villi. These serve to retard the passage of food and also to increase the absorptive surface. In the ileum, the crescentic folds disappear, and villi become smaller and more scattered. Lymph nodules are abundant in the tunica propria. The four layers of tissue characteristic of the alimentary canal are present in the small intestine. The mucous epitheHum is of the simple columnar sort, and each cell has a striated border on its inner free surface. Throughout the entire length of the intestine are numerous tubular mucus-secreting glands, perpendicular to the surface of the intestine, the intestinal glands or crjrpts of Lieberkuehn. Goblet-shaped cells distended with mucus are abundant in the walls of these glands. The secretions of these glands are said to stimulate peristalsis of the intestine as well
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