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. ACCESSORY ORGANS OF DIGESTION The accessory organs of digestion include the salivary glands, pancreas, liver, and gallbladder. As stated earlier, during the digestive process, the accessory organs produce secretions that assist the organs of the alimentary canal. Salivary Glands The salivary glands are located in the mouth (fig. 1-53). Within the salivary glands are two types of secretory cells, serous cells and mucous cells. The serous cells produce a watery fluid that contains a digestive juice called amylase. Amylase splits starch and glycerol into complex sugars. The mucous cells secrete a thick, sticky liquid called mucus. Mucus binds food particles together and acts to lubricate during swallowing. The fluids produced by the serous a n d m u c o u s c e l l s c o m b i n e t o f o r m s a l i v a . Approximately 1 liter of saliva is secreted daily. Pancreas The pancreas is a large, elongated gland lying posteriorly to the stomach (fig. 1-53). As discussed earlier in “The Endocrine System,” the pancreas has two functions: It serves both the endocrine system and the digestive system. The digestive portion of the pancreas produces digestive juices (amylase, proteinase, and lipase) that are secreted through the pancreatic duct to the duodenum. These digestive juices break down carbohydrates (amylase), proteins (proteinase), and fats (lipase) into simpler compounds. Liver The liver is the largest gland in the body. It is located in the upper abdomen on the right side, just under the diaphragm and superior to the duodenum and pylorus (fig. 1-53). Of the liver's many functions, the following are important to remember:  It metabolizes carbohydrates, fats, and proteins preparatory to their use or excretion.  It forms and excretes bile salts and pigment from bilirubin, a waste product of red blood cell destruction.  It stores blood; glycogen; vitamins A, D, and B 12 ; and iron.  It detoxifies the end products of protein digestion and drugs.  It produces antibodies and essential elements of the blood-clotting mechanism. Gallbladder The gallbladder is a pear-shaped sac, usually stained dark green by the bile it contains. It is located in the hollow underside of the liver (fig. 1-53). Its duct, the cystic duct, joins the hepatic duct from the liver to form the common bile duct, which enters the duodenum. The gallbladder receives bile from the liver and then concentrates and stores it. It secretes bile 1-52