94 HUMAN PHYSIOLOGY. 



After digestion they rapidly diminish in size and return to their former condi- 

 tion. The pyloric glands secrete pepsin only. 



Mechanism of Secretion. In the intervals of digestion, the mucous 

 membrane of the stomach is covered with a layer of mucus. As soon as the 

 food passes from the esophagus into the stomach, the blood-vessels dilate, the 

 circulation becomes more active, and the membrane assumes a bright red 

 appearance. Coincidentally, small drops of gastric juice begin to exude from 

 the glands, which as they increase in number, run together and trickle down 

 the sides of the stomach. This pouring out of fluid continues during the 

 presence of food in the stomach. 



The secretion of gastric juice is mediated by nerve centers in the medulla 

 oblongata. From these centers, nerve impulses descend the vagus and 

 vaso-motor nerves to the blood vessels and epithelium of the gastric gland 

 and excite them to action. 



The nerve centers in the medulla are aroused to action primarily by nerve 

 impulses descending from the brain in consequence of psychic states devel- 

 oped by the sight and odor of food and secondarily by nerve impulses reflected 

 from the mouth during the act of mastication. 



After a meal has been swallowed the continued secretion of gastric juice is 

 attributed to the development in the pyloric end of the stomach, by the action 

 of certain articles of food, e. g., dextrin, meat broths, soups, or by the first 

 products of digestion, some chemic agent that is absorbed into the blood and 

 in due time reaches the gland cells and stimulates them. Such an agent is 

 termed secretin. 



Chemic Action of the Gastric Juice. By the combined influence of the 

 contraction of the muscular walls, the action of the gastric juice, and the 

 temperature, the food is reduced to a semiliquid condition and acquires a 

 distinct acid odor. This semifluid mass will vary in compositionXnd appear- 

 ance according to the nature of the food. To this matter the/term chyme has 

 been given. 



Meat is rapidly disintegrated by the solution of its connective tissue. The 

 fibers thus separated are readily broken up into particles by solution of the 

 sarcolemma. Well-cooked meat is more easily digested, owing to the con- 

 version of the connective tissue into gelatin. 



Connective tissues in the raw or imperfectly gelatinized condition are very 

 slowly dissolved. Cartilage, tendons, and even bones will in time be corroded 

 and liquefied. 



Vegetables are not easily digested unless thoroughly prepared by sufficient 

 cooking. The nutritive principles are inclosed by cellulose walls, which are 

 not affected by gastric juice. The influence of heat and moisture softens 



