460 DIGESTION. 



facilitating the act of swallowing. The saliva containing mucin is espe- 

 cially important in this regard, and PAWLOW'S school has shown that the 

 secretion also regulates itself in this regard. The saliva is also of import- 

 ance, as it serves in washing out the mouth and thereby acts as a pro- 

 tection against destructive substances or bodies foreign to the mouth. 

 The power of converting starch into sugar is not inherent in the saliva 

 of all animals, and even when it possesses this property the intensity 

 varies in different animals. In man, whose saliva forms sugar rapidly, 

 a production of sugar from (boiled) starch undoubtedly takes place in the 

 mouth, but how far this action proceeds after the morsel has entered the 

 stomach depends upon the rapidity with which the acid gastric juice mixes 

 with the swallowed food, and also upon the relative amounts of the 

 gastric juice and food in the stomach. The large quantity of water which 

 is swallowed with the saliva must be absorbed and pass into the blood, 

 and it must in this way go through an intermediate circulation in the 

 organism. Thus the organism possesses in the saliva an active medium 

 by which a constant stream, conveying the dissolved and finely divided 

 bodies, passes into the blood from the intestinal canal during digestion. 

 The relation of the saliva or the salivary glands to the secretion of gastric 

 juice will be mentioned in the next section. 



Salivary Concrements. The so-called tartar is yellow, gray, yellowish-gray, 

 brown or black, and has a stratified structure. It may contain more than 200 

 p. m. organic substances, which consist of mucin, epithelium, and LEPTOTHRIX- 

 CHAINS. The chief part of the inorganic constituents consists of calcium car- 

 bonate and phosphate. The salivary calculi may vary in size from that of a 

 small grain to that of a pea or still larger (a salivary calculus has been found 

 weighing 18.6 grams), and they contain variable quantities of organic substances 

 (50-380 p. m.), which remain on extracting the calculus with hydrochloric acid. 

 The chief inorganic constituent is calcium carbonate. 



H. THE GLANDS OF THE MUCOUS MEMBRANE OF THE STOMACH, AND 



THE GASTRIC JUICE. 



The glands of the mucous coat of the stomach have long been 

 divided into two distinct classes. Those which occur in the greatest 

 abundance and which have the greatest size in the fundus are called 

 fundus, rennin or pepsin glands, and the others, which occur only in 

 the neighborhood of the pylorus, have received the name of pyloric 

 glands, sometimes also, though incorrectly, called mucous glands. The 

 division of these two forms of glands in the mucous membrane of the 

 stomach is essentially different in various animals. The mucous coat- 

 ing of the stomach is covered throughout with a layer of columnar 

 epithelium , which is generally considered as consisting of goblet cells that 

 produce mucus by a metamorphosis of the protoplasm. 



