66 



THE INDIVIDUAL ORGANISM 



stomach are kneaded, and the food is mixed with the gastric juice. The 

 effect of the mechanical processes is to break up and liquefy the solid 

 food. The chemical part of gastric digestion consists chiefly of breaking 

 down the proteins into intermediate products (peptones and proteoses) 

 by the pepsin 1 of the gastric juice. 



The food next to the stomach walls is digested and liquefied first, and 

 only gradually does the acid gastric juice penetrate to the center of the 



Qtewtd 



SALIVARY 



ii 



4t£UMfUl 



ptyalin 



maltase 



pepsin 



rennin 



steapsin 



amylopsin 



trypsin 



^Qodaf^cied 



starch 



maltose 



protein 



milk protein 



fats 



starches and 

 glycogen 



protein 



erepsin 



maltase 



lactase 



invertase 



peptones, proteoses, 

 polypeptids 



maltose 



lactose 



sucrose 



Product 



maltose 



monosaccharides 



peptones and 

 proteoses 



coagulated 

 milk 



fatty acids and 

 glycerol 



disaccharides 



polypeptids and 

 amino acids 



amino acids 



monosaccharides 



Fig. 4.3. A summary of digestion in man. Gastric lipase is omitted. 



food mass. Parts of the mass remain slightly alkaline for half an hour or 

 more, on the average, and so long as this condition persists, carbohydrates 

 continue to be digested by ptyalin. As gastric digestion proceeds, the 

 contractions of the stomach squeeze the outer and more fluid portion of 

 the stomach contents toward the pylorus. Each wave of contraction 

 sweeps a jet of the liquefied food {chyme) through the relaxed pylorus 

 into the small intestine; the pyloric valve then closes briefly and opens 

 again at the approach of the next wave. 



1 This digestive enzyme is secreted as -pepsinogen (an inactive precursor of pepsin, 

 which does not attack the proteins of the gland cells) ; pepsinogen is changed to active 

 pepsin by the hydrochloric acid in the cavity of the stomach. The stomach walls, as 

 noted above, are protected by their coating of mucin. 



