106 HUMAN PHYSIOLOGY. 



lowed by dilatation of the vessels, flushing of the mucous membrane, 

 and reestablishment of the secretion. It is evident, therefore, that 

 during digestion afferent impulses are passing up the pneumogastrics 

 to the medulla; efferent impulses, in all probability, pass through 

 the fibers of the sympathetic nervous system to the blood-vessels 

 and glands concerned in the elaboration of the gastric juice. After 

 all the nerve connections of the stomach are divided, the secretion 

 of a small quantity of juice continues for several days. This has 

 been attributed to the action of a local nervous mechanism and to 

 the direct action of the food upon the protoplasm of the secreting 

 cells. 



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 con- 

 dition and acquires a distinct acid odor. This semifluid mass will vary 

 in composition and appearance 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 conversion 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 and ruptures the cellulose walls so as to permit 

 the introduction of gastric juice and the solution of its nutritive 

 principles. 



The principal action of the gastric juice, however, is the trans- 

 formation of the different proteid principles of the food into peptones, 

 the intermediate stages of which are due to the influence of the acid 

 and pepsin respectively. As soon as any one of the albumins is pene- 

 trated by the acid it is converted into acid-albumin, a fact which 

 indicates that the first step in gastric digestion is the acidification of 

 the proteids. This having been accomplished, the pepsin becomes 

 operative and in a varying length of time transforms the acid-albu- 



