524 VERTEBRATE LIFE AND ORGANIZATION 



digests starches to maltose (malt sugar). Maltose, and the double sugars 

 sucrose and lactose (cane and milk sugar) that may be in the ingested 

 food, are finally cleaved to single sugars by the intestinal enzymes 

 maltase, sucrose and lactase, respectively. Glucose is the most im- 

 portant single sugar, though lesser amounts ol fructose and galactose 

 are derived Irom the breakdown of sucrose and lactose. Most of the 

 final hydrolysis probably occurs in the lumen of the intestine, though 

 some double sugars may enter the mucosal cells and be digested intra- 

 cellularly. The digestive enzymes of man are summarized in Table 3. 



Digestion is completed in the small intestine and the products of 

 digestion are absorbed. Absorption results partly from the simple dif- 

 fusion of molecules from the lumen of the intestine through the mucosa 

 and into the blood and lymph vessels, and partly from the active 

 uptake of molecules by the mucosal cells. That the mucosal cells play 

 an active role is indicated by the fact that poisons which interfere with 

 their metabolism greatly reduce the rate and amount of absorption. 

 Most of the products of digestion are in solution and can be absorbed 

 easily, but the absorption of the fats and fatty acids presents a special 

 problem that is not completely understood. Apparently their uptake 

 is facilitated by combining with bile salts, for this makes a soluble 

 complex. Once they have passed through the cells lining the intestine, 

 the fatty acids recombine with glycerol to form fat and the bile salts 

 are freed. Most of the absorbed fats enter the lymph vessels, but the 

 sugars, amino acids and other absorbed materials enter the capillaries 

 of the blood vessels. 



The material left in the small intestine, which is still very fluid, 

 passes into the large intestine. Water and many of the salts are absorbed 

 as the residue passes through the colon. If too much water is absorbed, 

 the feces become very dry and hard and constipation may result. Many 

 bacteria reside in the colon and synthesize a variety of vitamins which 

 are absorbed from the colon. The bacteria reproduce very rapidly, and 

 many are eliminated. As much as 25 per cent of the feces may consist 

 of bacteria. 



222. The Control of Digestive Secretions 



Each of the various enzymes is secreted at an appropriate time: 

 We salivate when we eat, and gastric jince is produced when food 

 reaches the stomach. The control of these digestive secretions is partly 

 nervous and partly endocrine. The smell of food or its presence in 

 the mouth stimulates sensory nerves that carry impulses to a salivating 

 center in the medulla of the brain. From there the impulses are relayed 

 along motor nerves to the salivary glands, which then secrete. 



The control of gastric secretion is more complex. Years ago the 

 famous Russian physiologist, Pavlov, performed an experiment in 

 which he brought the esophagus of a dog to the surface of the neck 

 and severed it. When the dog ate the food did not reach the stomach, 

 yet some gastric juice was secreted provided that the vagus nerve, 

 which carries motor fibers to the stomach and other internal organs, 



