THE INTAKE OF MATERIALS AND ENERGY 



71 



Pancreatic juice is mildly to rather strongly alkaline; bile, though quite 

 alkaline in the liver, becomes neutral or weakly alkaline during a process 

 of concentration that it undergoes in the gall bladder. The intestinal 

 juice is definitely alkaline through the presence of sodium carbonate and 

 bicarbonate. The mixing of these juices with the chyme tends to neu- 

 tralize the acid of the latter and establishes a circumneutral or slightly 

 alkaline condition that persists, with some variation, throughout the 

 intestine. 



The work of the intestinal and pancreatic enzymes is greatly facili- 

 tated by the muscular activity of the intestinal wall. The mass of food 



peritoneum 



longitudinal muscles 

 circular muscles 



blood 

 capillaries 

 tunica propria I lymph vessel 



pithelium \ ^£531^^^/ (lacteal) 



submucoso 'y m P h duc,s 



lumen of intestine 



Fig. 4.5. A section of small intestine, with enlarged detail of a villus. 



received from the stomach is pinched into numerous separate portions 

 by local constriction of the intestinal wall, and these semifluid separate 

 portions are shunted backward and forward, reunited and again separated 

 by alternate contraction of the circular (constricting) and longitudinal 

 (dilating) muscle layers. In this way, the food is much more effectively 

 exposed to the action of the digestive fluids, and new portions are con- 

 tinually brought in contact with the absorptive walls of the intestine. 

 Peristaltic waves of contraction passing along the intestine keep the 

 food moving gradually toward the posterior end. 



Digestion is completed when the proteins, carbohydrates, and fats 

 have been broken down into amino acids, monosaccharides, and fatty 

 acids and glycerol. As soon as these end products of digestion are formed, 

 they, like water and the salts and vitamins, tend to be absorbed through 



