230 UNCONSCIOUS NERVOUS OPERATIONS 



325. Ferments. The ptyalin belongs to a kind of sub- 

 stances called ferments. Of these there are two classes, 

 the organized and the unorganized. The organized 

 ferments are living vegetable organisms. Yeast is one 

 of them, the vinegar plant is another, and the various 

 kinds of bacteria, some of which cause disease, are other 

 examples. The unorganized ferments are chemical sub- 

 stances resulting from the activity of living cells, and 

 capable of effecting certain changes in particular sub- 

 stances. The ptyalin is one of these. The ferments are 

 the agents for effecting most of the changes which the 

 food undergoes in digestion. 



326. Gastric Juice. The epithelium of the lining of the 

 stomach consists of a single layer of cells, and the mucous 

 membrane is almost entirely composed of simple tubelike 

 glands closely packed together. When food reaches the 

 stomach, more blood is sent into the dilated blood vessels, 

 and the glands make from the blood a colorless fluid called 

 gastric juice, which flows into the cavity of the stomach. 



Chemical analysis shows that gastric juice contains, 

 besides water, a small amount of salts, a little free hydro- 

 chloric acid, and two of the ferments, called pepsin and 

 rennin. Rennin is that element in the gastric juice which 

 causes milk to curdle. The use of rennet (which is ob- 

 tained from the stomach of a calf) in the making of cheese 

 depends upon the presence of this ferment. Pepsin is 

 the ferment which is able to change proteids into soluble 

 form, and to make diffusible such as are not already so. 

 The ferments of the gastric juice act only in the presence 

 of an acid, and to assist their action seems to be the func- 

 tion of the hydrochloric acid. . 



327. The Food in the Stomach. The effect of the gastric 

 juice upon the food in the stomach is to make the whole 



