DIGESTION AND ABSORPTION 165 



These are the chemical reactions which occur through the 

 agency of enzymes in the conversion of carbohydrate food, not 

 already in the form of monosaccharoses, into this form. 



The Mouth 



Saliva, its Properties and Action. — Beginning with the 

 mouth let us follow the course of the digestion of carbohydrates 

 until they have all reached the assimilable form of one of the 

 monosaccharoses. Digestion of carbohydrates in the mouth 

 consists in the conversion of polysaccharoses into maltose by 

 the action of the two enzymes, ptyalin and maltase. These 

 enzymes are present in the digestive juice known as saliva. 

 The saliva is secreted by glands near the mouth known as 

 salivary glands, and in man amounts to about 1000 to 1500 c.c. 

 per twenty-four hours. The mechanical operation of mastica- 

 tion causes the salivary glands to discharge into the mouth the 

 juice saliva. There are several salivary glands each discharg- 

 ing a saliva of somewhat different physical character. The 

 drier and larger the amount of food mass the more watery is 

 the saliva sent to the mouth, while a compact mass of food 

 causes the flow of a juice more mucousy in its character. The 

 stimulation of the salivary glands to discharge saliva may be 

 brought about by a variety of means. The stimulation may be 

 (a) mechanical, as is proven by the flow of saliva caused by 

 putting pebbles or sand into a dog's mouth, (h) chemical, as is 

 produced when a small amount of acid placed in the mouth 

 causes a flow of watery saliva for the purpose of dilution or 

 ejection, (c) psychical, as is produced when the mere sight of 

 food causes the flow of saliva or, as the expression is, " makes 

 the mouth water." In addition to these more common stimuli 

 the flow of saliva may be brought about by either {d) electrical 

 or {e) thermal stimulation. It is not possible to separate sharply 

 these different kinds of stimulation or to determine which is 

 the particular stimulation most active. In most cases where 

 normal food acts, the stimulation will be a combination of 

 several of those given. This stimulation of the flow of saliva 



