DIGESTION. 89 



corpuscles and epithelial cells. Chemically it is composed of water, proteid 



matter, a ferment (ptyaliri), and inorganic salts. The amount secreted in 



twenty hours is about 2$ Ibs. Its function is twofold: 



i. Physical. Softens and moistens the food, agglutinates it, and facilitates 



swallowing. 



2. Chemic. Converts starch into sugar. This action is due to the presence 

 of an enzyme, ptyalin. Its power of converting starch into sugar is 

 manifested most decidedly at the temperature of the living body and in a 

 slightly alkaline medium. The conversion of starch into sugar takes place 

 through several stages, the nature of which depends upon the structure of 

 the starch granule. This consists of two portions, a stroma of cellulose 

 and a contained material, granulose, which is the more abundant and im- 

 portant of the two. When subjected to the action of boiling water, the 

 starch granule swells and bursts, forming a viscid, opalescent mass of starch 

 paste. If saliva be now added to this paste and kept at a temperature 

 of 104 F. for a few minutes, the paste becomes clear and limpid. The 

 first stage in the digestion is now complete, with the formation of soluble 

 starch. If the action of saliva be continued, a number of substances in- 

 termediate between starch and sugar are formed, to which the name 

 dextrin has been given. 



(a) Erythrodextrin, which gives the reddish-brown color with iodin. 

 As the digestion continues and sugar is formed, the erythrodextrin 

 disappears, giving way to 



(b) Ackroddeoctrin, which yields no coloration with iodin, but which may 

 be precipitated by alcohol. 



The sugar formed by the action of saliva is maltose the formula for which is 

 C 12 H 22 O n . A small quantity of dextrin is also formed. 



The successive stages in the conversion of starch into sugar may be repre- 

 sented by the following schema: 



f erythrodextrin / achroodextrin 

 Starch = soluble starch = < J = < 



( maltose. \ maltose. 



NERVE MECHANISM OF INSALIVATION. 



Afferent Nerves. Efferent Nerves. 



1. Lingual branch of 5th pair. i. Auriculo temporal branch of 5th 



2. Taste fibers in the glosso- pair, for parotid gland. 



pharyngeal. 2. Chorda tympani, for submaxil- 



3. Taste fibers in the lary and sublingual glands, 

 chorda tympani. 3. Sympathetic for all the glands. 



