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