ESOPHAGUS The esophagus is a muscular tube about 25 cm (10  inches)  long.  It  is  the  passageway  between  the pharynx and the stomach. By means of waves of muscular  contractions  (peristalsis),  food  is  pushed along this tube to the stomach. When peristalsis is  reversed,  vomiting  occurs. STOMACH The stomach is a saccular enlargement of the gastrointestinal tube, connecting the lower end of the esophagus and the first portion of the small intestine  (duodenum).  It  lies  in  the  left  upper quadrant  of  the  abdomen.  Muscular  rings,  or sphincters, at each end of the stomach form valves to  close  off  the  stomach  and  to  prevent  its  con- tents from escaping in either direction while they are being mixed by peristaltic muscular contrac- tions  of  the  stomach  wall.  The  sphincter  at  the esophageal  end  is  the  cardiac  sphincter;  at  the duodenal  end  it  is  the  pyloric  sphincter. The stomach acts as an initial storehouse for swallowed  material  and  helps  in  the  chemical breakdown  of  food  substances.  Small  glands  in the wall of the stomach secrete gastric juice, the principal  components  of  which  are  hydrochloric acid and pepsinogen. Hydrochloric acid activates pepsin from pepsinogen, kills bacteria that enter the  stomach,  inhibits  the  digestive  action  of ptyalin, and helps regulate the opening and clos- ing of the pyloric sphincter. The action of pepsin is confined to protein, which it splits. The stomach is half-empty within 1 hour of a normal meal and completely  empty  in  6  hours. Most food absorption takes place in the small intestine.  There  is  little  food  absorption  in  the stomach.  One  exception  is  alcohol,  which  is  ab- sorbed  directly  through  the  stomach  wall.  For  this reason  intoxication  happens  quickly  when  alcohol is  taken  on  an  empty  stomach. ABDOMINAL   CAVITY The stomach and intestines are enclosed in the abdominal   cavity, the   space   between   the diaphragm and the pelvis. This cavity is lined with serous   membrane,   the   PERITONEUM.   The peritoneum covers the intestines and the organs and, by secreting a serous fluid, prevents friction between  adjacent  organs.  The  MESENTERY (double  folds  of  peritoneum)  extends  from  the cavity walls to the organs of the abdominal cavity, suspending them in position and carrying blood vessels to the organs. 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. The mesentery is gathered together like a folding fan, permitting coiling of the intestine, allowing  this  long  organ  to  be  contained  in  a relatively small space. The small intestine is divided into three contin- uous  parts:  the  duodenum,  jejunum,  and  ileum. It  receives  digestive  juices  from  three  accessory organs   of   digestion:   the   pancreas,   liver,   and gallbladder. The DUODENUM is about 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  con- taining  the  enzymes  carbohydrate,  peptidase,  and lipase. The JEJUNUM is the middle part of the small intestine and is about 2.5 meters (7.5 feet) long. Its  enzymes  continue  the  digestive  process. The ILEUM is the last and longest part of the small  intestine.  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. 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. The  unabsorbed  food  or  waste  material  passes through   the   CECUM   into   the   COLON.   The cecum is a pouch at the beginning of the large in- testine, located in the lower right portion of the abdominal  cavity.  Twelve  hours  after  the  meal, most  of  the  waste  material  passes  through  the colon  slowly,  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 an outpouching of the cecum located near the junction of the ileum and the cecum. It has  no  known  function  but  frequently  becomes infected  and  an  inflammation  known  as  appen- dicitis  develops. The RECTUM is 12.5 cm (5 inches) long and follows  the  contour  of  the  sacrum  and  coccyx  until 3-44