closes  the  opening  to  the  larynx  when  food  is  being swallowed down the pharynx. Food is deflected away from   the   trachea   to   prevent   particle   aspiration (inhalation). Esophagus The esophagus is a muscular tube about 25 cm (10 inches) long (fig. 1-53). It is the passageway between the pharynx and the stomach. By means of peristalsis, food is pushed along this tube to the stomach. When peristalsis is reversed, vomiting occurs. Stomach The  stomach  acts  as  an  initial  storehouse  for swallowed   material   and   helps   in   the   chemical breakdown  of  food  substances.  The  stomach  is  a saccular enlargement of the gastrointestinal tube and lies  in  the  left  upper  quadrant  of  the  abdomen  (fig. 1-53). It connects the lower end of the esophagus with the first portion of the small intestine (the duodenum). The stomach is divided into the cardiac, fundic, body, and  pyloric  regions  (fig.  1-54).  At  each  end  of  the stomach, muscular rings (or sphincters) form valves to close  off  the  stomach.  The  sphincters  prevent  the stomach's  contents  from  escaping  in  either  direction while food substances are being mixed by peristaltic muscular   contractions   of   the   stomach   wall.   The sphincter   at   the   esophageal   end   is   the   cardiac sphincter;  at  the  duodenal  end  it  is  the   pyloric sphincter. The chemical breakdown of food in the stomach is accomplished  through  the  production  of  digestive juices (enzymes) by small (gastric) glands in the wall of  the  stomach.  The  principal  digestive  enzymes produced by the gastric glands are hydrochloric acid and  pepsinogen.  Hydrochloric  acid  activates  pepsin from pepsinogen, kills bacteria that enter the stomach, inhibits  the  digestive  action  of  amylase,  and  helps regulate   the   opening   and   closing   of   the   pyloric sphincter. Pepsin is a protein-splitting enzyme capable of beginning the digestion of nearly all types of dietary protein. Most  food  absorption  takes  place  in  the  small intestine.  In  general,  food  is  not  absorbed  in  the stomach. An exception is alcohol, which is absorbed directly through the stomach wall. It is for this reason that intoxication occurs 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  called  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 (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. 1-51 HM3F0154 FUNDIC REGION OF STOMACH BODY OF STOMACH ESOPHAGUS CARDIAC SPHINCTER CARDIAC REGION OF STOMACH PYLORIC SPHINCTER (PYLORUS) DUODENUM PYLORIC CANAL PYLORIC REGION OF STOMACH Figure 1-54.—Major regions of the stomach.