174 TEXT-BOOK OF PHYSIOLOGY 



been called the gastric hormone 1 or the gastric secretin. Whatever the agen 

 or the mechanism may be, there is not only an increase in the quantity but 

 a change in the quality of the juice in accordance with the character of the- 

 food; in other words, there is an adaptation of the juice to the kind of food 

 to be digested. Thus the protein of bread causes a secretion of five times more 

 pepsin than the same amount of the protein of milk, while the protein of 

 meat causes a secretion of 25 per cent, more pepsin than milk. Meat extract 

 and bouillon have a very stimulating effect on the quantity of juice produced, 

 while alkalies have an inhibitor effect. 



Histologic Changes in the Gastric Cells during Secretion. During 

 the periods of rest and secretor activity the cells of the gastric glands underg 

 changes in histologic structure which are believed to be connected with t v 

 production of the enzymes, pepsin and rennin, and the acid. In the resti% 

 period the protoplasm of the chief or central cells of the cardiac glands be-" 

 comes crowded with large and well-defined granules, which during the period 

 of secretory activity largely disappear, so much so that only the lumina 

 border of the cell is occupied by them, the outer border being clear and hya- 

 line in appearance. The parietal cells during rest are large and finely 

 granular, but after secretion they are smaller in size though still granular. 



The cells of the pyloric glands, though containing granules, do not show 

 any marked difference between the resting and active conditions. According 

 to some observers they contain pepsinogen; according to others, mucin. 

 The epithelial cells lining the ducts of the pylorus and cardiac glands, if not 

 identical with the epithelial cells on the surface of the mucous membrane, 

 pass by transitional forms into them. Among these cells are found many 

 goblet cells which secrete a portion of the mucin found in the stomach and 

 gastric juice. In the period of rest the protoplasm of the epithelial cells 

 absorbs and assimilates from the surrounding lymph-spaces material which 

 eventually makes its reappearance as a product of metabolism in the form of 

 granules and hydrochloric acid. With the onset of digestive activity there 

 is a dilatation of the blood-vessels, an increase in the blood-supply, a stimu- 

 lation through the^nerve-supply of the cells, and an output of a fluid to which 

 the name gastric juice is given. 



The Physiologic Action of Gastric Juice. In the study of the physi- 

 ology of gastric digestion as it takes place under normal conditions it is 

 important to bear in mind that the foods introduced into the stomach are 

 heterogeneous compounds consisting of both nutritive and non-nutritive 

 materials, and that before the former can be digested and utilized for nutri- 

 tive purposes they must be freed from their combinations with the latter. 

 This is accomplished by the solvent action of the gastric juice, which in 

 virtue of the chemic activity of its constituents on proteins, gradually disinte- 

 grates the food and reduces it to the liquid or semiliquid condition. 



The nature of this change and the respective influence which the acid 

 and pepsin exert can be studied with almost any form of protein. A most 

 convenient form, however, is fibrin obtained from blood by whipping and 

 thoroughly freed from corpuscles by washing under a stream of water. The 



1 A hormone (from fynaw, "I excite") may be defined as an agent of known or unknown 

 composition which is secreted by some one organ, is discharged into and carried by the blood stream 

 to some correlated organ near or remote, on the functional activities of which it exerts an excitator 

 or stimulating influence. (See Chapter on Internal Secretion.) 



