192 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



LoctAol 



Goblet cell 



Crypt of 

 Lieberkijtin 



UCOSO 



usculoris mucosae 



ubmucoso 



Fig. 11.19. Diagrammatic view of a minute portion of the lining of the small 

 intestine showing the villi that are constantly dipped into the digesting food; two 

 of them are cut open to expose the blood and lymph vessels into which digested 

 food is absorbed. The continual springing up and down of the villi keeps the food 

 in motion. (Courtesy, Villee: Biology. The Human Approach, ed. 2. Philadelphia, 

 \V. B. Saunders Co., 1954.) 



proteins. On the other hand, when it is mixed with intestinal juice containing 

 the enzyme enterokinase secreted by the glands in the lining of the intestine, 

 it immediately becomes potent. 



As soon as food comes into the intestine, pancreatic juice begins to flow 

 into it. The reason for this was a puzzle for many years. The nerves leading 

 to the pancreas were cut but, in animals thus treated, the flow went on as 

 before. In 1903, the British physiologists Bayliss and Starling discovered 

 that the mysterious messenger was a fluid, the hormone, secretin. The partly 

 digested food from the stomach stimulates glands in the intestinal wall to 

 produce secretin. This is picked up by the blood, carried to the pancreas, 

 and immediately stimulates that gland to produce its digestive secretion. 



The Completion of Digestion. The walls of the small intestine contain 

 minute glands which secrete additional enzymes that complete the digestion 

 of proteins and carbohydrates and fats (Table 11.3). Peptidases (erepsin and 

 others) complete the breakdown of protein into amino acids that can be 

 absorbed into cells everywhere. Sucrase, maltase, and lactase are enzymes 



