Small Intestine
The small intestine is a muscular, convoluted, coiled tube, about 7 meters (23 feet) long and attached to the posterior abdominal wall by its mesentery (fig. 1-53).

The small intestine is divided into three contiguous parts: the duodenum, jejunum, and ileum. It receives digestive juices from three accessory organs of digestion: the pancreas, liver, and gallbladder.

DUODENUM.-The duodenum is approxi- mately 25 cm (10 inches) long and forms a C-shaped curve around the head of the pancreas, posterior to the liver. It is lined with a mucous membrane that contains small glands. These glands secrete intestinal juices containing the enzymes carbohydrase, peptidase, and lipase.

JEJUNUM.-The jejunum is the middle part of the small intestine and is approximately 2.5 meters (8.2 feet) long. Its enzymes continue the digestive process.

Figure 1-54.-Major regions of the stomach.

ILEUM.-The ileum is the last and longest part of the small intestine. It is approximately 3.5 meters (11.5 feet) long. Most of the absorption of food occurs in the ileum, where fingerlike projections (villi) provide a large absorption surface. After ingestion, it takes 20 minutes to 2 hours for the first portion of the food to pass through the small intestine to the beginning of the large intestine.

Large Intestine
The large intestine is so called because it is larger in diameter than the small intestine (fig. 1-53). It is considerably shorter, however, being about 1.5 meters (5 feet) long. It is divided into three distinct parts: the cecum, colon, and rectum.

CECUM AND COLON.-The unabsorbed food or waste material passes through the cecum into the ascending colon, across the transverse colon, and down the descending colon through the sigmoid colon to the rectum. Twelve hours after the meal, the waste material passes slowly through the colon, building in mass and reaching the rectum 24 hours after the food is ingested.

The appendix, a long narrow tube with a blind end, is a pouchlike structure of the cecum located near the junction of the ileum and the cecum (fig. 1-53). There is no known function of this structure. Occasionally, the appendix becomes infected, causing inflammation to develop. This inflammation of the appendix is known as appendicitis.

RECTUM.-The rectum is approximately 12.5 cm (5 inches) long and follows the contour of the sacrum and coccyx until it curves back into the short (2.5 to 4 cm) anal canal. The anus is the external opening at the lower end of the digestive system. Except during bowel movement (defecation), it is kept closed by a strong muscular ring, the anal sphincter.