100 HUMAN PHYSIOLOGY. 



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

 presence of the organic ferment, ptyalin. Ptyalin is an amorphous 

 nitrogenized substance, which can be precipitated from the saliva 

 by calcium phosphate. 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 important of the 

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

 granule swells up 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 com- 

 plete, with the formation of soluble starch. If the action of saliva 

 be continued, a number of substances intermediate between starch 

 and sugar are formed, to which the name dextrin has been given. 

 Among these may be mentioned : 



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



iodin. As the digestion continues and sugar is formed, the 



erythrodextrin disappears, giving way to 

 (fr) Achroodextrin, 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^H^On. A small quantity of dextrose is also formed. 



NERVE MECHANISM OF INSALIVATiON. 



Afferent Nerves. Efferent Nerves. 



1. Lingual branch of 5th pair. i. Auriculotemporal branch of 5th 



2. Glossopharyngeal. pair, for parotid gland. 



2. Chorda tympani, for submaxil- 

 lary and sublingual glands. 



The centers regulating the secretion are located in the medulla 

 oblongata. The nerve mechanism is excited to action by nerve im- 

 pulses developed by the contact of the food with the terminal fila- 

 ments of afferent nerves in the mucous membrane of the mouth. 

 These nerve impulses are transmitted by afferent nerves to the med- 



