24 HUMAN PHYSIOLOGY. 



masseter and internal pterygoid muscles; moved laterally by the alternate 

 contraction of the external pterygoid muscles ; moved anteriorly by the 

 pterygoid and posteriorly by the united actions of the genio-hyoid, mylo- 

 hyoid and posterior fibres of the temporal muscle. 



The food is kept between the teeth by the intrinsic and extrinsic mus- 

 cles of the tongue from within, and the orbicularis oris and buccinator 

 muscles from without. 



The Movements of Mastication, though originating in an effort of 

 the will and under its control, are, for the most part, of an automatic or 

 reflex character, taking place through the medulla oblongata and induced 

 by the presence of food within the mouth. The nerves and nerve centres 

 involved in this mechanism are shown in the following table : 



NERVOUS CIRCLE OF MASTICATION. 



AFFERENT OR EXCITOR NERVES. EFFERENT OR MOTOR NEKVES. 



1. Lingual branch of 5th pair. I. 3d branch of 5th pair. 



2. Glosso-pharyngeal. 2. Hypo-glossal. 



3. Facial. 



The impressions made upon the terminal filaments of the sensory nerves 

 are transmitted to the medulla ; motor impulses are here generated which 

 are transmitted through motor nerves to the muscles involved in the move- 

 ments of the lower jaw. The medulla not only generates motor impulses, 

 but coordinates them in such a manner that the movements of mastication 

 may be directed toward the accomplishment of a definite purpose. 



Insalivation is the incorporation of the food with the saliva secreted 

 by fat parotid^ sub-maxillary and sub-lingual glands ; \\\t parotid saliva, 

 thin and watery, is poured into the mouth through Steno's duct ; the sub- 

 maxillary and sub-lingual salivas, thick and viscid, are poured into the 

 mouth through Wharton's and Bartholini's ducts. 



In their minute structure the salivary glands resemble each other. They 

 belong to the racemose variety, and consist of small sacs or vesicles, 

 which are the terminal expansions of the smallest salivary ducts. Each 

 vesicle or acinus consists of a basement membrane surrounded by blood 

 vessels and lined with epithelial cells. In the parotid gland the lining cells 

 are granular and nucleated ; in the sub-maxillary and sub-lingual glands the 

 cells are large, clear, and contain a quantity of mucigen. During and 

 after secretion very remarkable changes take place in the cells lining the 

 acini, which are in some way connected with the essential constituents of 

 the salivary fluids. 



In a living serous gland, e. g. y parotid, during rest, the secretory cells 



