294 THE MAINTENANCE OF THE INDIVIDUAL 



The chewing process theoretically inixes food with saliva thoroughly 

 but in man the bolus is invariably swallowed before the ptyalin has 

 completed its action. Recent studies indicate that salivary digestion 

 continues in the stomach for some time until stopped by the hydro- 

 chloric acid of the stomach. 



The Gastric Glands. The inner surface of the stomach is 

 covered with cells producing mucus, the entire region being dotted 

 with thousands of tiny gastric glands secreting gastric juice. Most 

 of the lumen of each gland is lined by columnar epithelial cells called 

 chief cells, while between the basement membrane and the chief cells of 

 the glands lie scattered parietal cells. The chief cells of the neck 

 of the gland secrete mucus while those lower down secrete an in- 

 activated enzyme or zymogen, called pepsinogen. Oval parietal cells 

 secrete hydrochloric acid, which activates the pepsinogen, converting 

 it into an active enzyme (pepsin), that, in the presence of this acid, 

 breaks down proteins to the intermediate products, peptones and 

 proteoses. Gastric juice is slightly acid in its chemical reaction, 

 containing about 0.2-0.4 per cent of free hydrochloric acid together 

 with another enzyme called rennin. The latter curdles or coagulates 

 casein, a protein found in milk, which is the basis of cheese. After 

 milk is curdled pepsin is able to act upon it. "Junket" tablets, 

 which contain rennin, are used for this purpose in the preparation of 

 a dessert which has milk as a basis. 



The stomach is the place where the digestion of proteins is initiated 

 and where digestion of carbohydrates may be continued. Some 

 investigators believe that emulsified fats such as cream are digested 

 by a gastric lipase. However, since saponification and emulsification 

 must take place before absorption, and after the fats reach the intes- 

 tine, it appears probable that fats undergo no digestive changes in 

 the stomach. 



Although little or no absorption takes place in the stomach, under 

 certain conditions water, salts, alcohol, and drugs may be absorbed. 

 There appears little evidence at present to support the contention that 

 sugars and peptones are appreciably absorbed in this organ. 



Food, after being mixed with gastric juice, becomes increasingly 

 liquid and is known as chyme, in which state it passes through the 

 pylorus. The next step is facilitated by the muscular movements of 

 the small intestine, which are primarily of two kinds. The first, 

 peristalsis, helps pass the food slowly along the intestine. The second, 

 rhythmical contractions or segmentation, may be described as a series of 



