520 VERTEBRATE LIFE AND ORGANIZATION 



Reiinin has been extracted tor centuries irom the stoinachs of calves and 

 used to cmdle milk; this is an important step in the manufacture of 

 cheese. 



In view of the strong proteolytic action of pepsin, one might wonder 

 why it does not digest the wall of the stomach. A major factor preventing 

 sucli autodigestion is the secretion of copious amounts of mucus by other 

 multicellular glands in the stomach and by scattered cells throughout 

 the stomach lining. The mucus forms a coating which protects the 

 stomach walls from the action of pepsin. Furthermore, the amounts of 

 pepsin and acid in the stomach are very small except when food is 

 present to be digested. Sometimes, however, these safeguards break down, 

 pepsin digests away part of the stomach lining, and a peptic ulcer results. 



When the food is reduced to a creamy consistency and most of the 

 micro-organisms that entered the stomach with it have been killed by 

 the action of the gastric juices, the pyloric sphincter opens and the food 

 passes into the small intestine. The most fluid food passes first. Indeed, 

 upon entering the stomach, water passes almost immediately into the 

 intestine. The food enters the intestine in spurts and is quickly neu- 

 tralized by the alkalinity of secretions flowing into the intestine from 

 the liver and pancreas. 



220. The Liver and Pancreas 



The liver and pancreas are large glandular outgrowths from the 

 anterior part of the intestine. The liver, in fact, is the largest organ of 

 the body. Its cells continually secrete bile, which passes through hepatic 

 ducts into the common bile duct and then up the cystic duct into the 

 gall bladder. Bile does not enter the intestine immediately, for a 

 sphincter at the end of the bile duct is closed until food enters the intes- 

 tine. Contraction of the wall of the gall bladder forces the bile out. The 

 bile that is finally poured into the intestine is concentrated, for a con- 

 siderable amount of water is absorbed from the bile in the gall bladder. 



Although bile contains no digestive enzymes it nevertheless has a 

 twofold digestive role. Its alkalinity, along with that of the pancreatic 

 secretions, neutralizes the acid food entering the intestine and creates a 

 pH favorable for the action of pancreatic and intestinal enzymes. Its 

 bile salts emulsify fats, breaking them up into smaller globules and 

 thereby providing more surfaces on which fat-splitting enzymes can act. 

 These salts are also essential for the absorption of fats and fat-soluble 

 vitamins (A, D, K). Most of the bile salts are not eliminated with the 

 feces, but are absorbed in the intestine along with the fats and are car- 

 ried back to the liver by the blood stream to be used again. 



The color of bile (green, yellow, orange or red in different species) 

 is due to the presence of bile pigments, excretory products derived from 

 the breakdown of hemoglobin in the liver. The bile pigments undergo 

 further chemical reactions by the intestinal bacteria and are converted 

 to the brown pigments responsible for the color of the feces. If their 

 excretion is prevented by a gall stone or some other obstruction of the 

 bile duct, they are reabsorbed by the liver and gall bladder, the feces 



