FOOD AND DIGESTION. 333 



as they are squeezed out from between them, until they have been suffi- 

 ciently chewed. 



Muscles. The simple up and down, or biting movements of the lower 

 jaw, are performed by the temporal, masseter, and internal pterygoid mus- 

 cles, the action of which in closing the jaws alternates with that of the 

 digastric and other muscles passing from the os hyoides to the lower jaw, 

 which open them. The grinding or side to side movements of the lower 

 jaw are performed mainly by the external pterygoid muscles, the muscle 

 of one side acting alternately with the other. When both external 

 pterygoids act together, the lower jaw is pulled directly forward, so that 

 the lower incisor teeth are brought in front of the level of the upper. 



Temporo-maxillary Fibro- cartilage. The function of the inter-articu- 

 lo-fibro-cartilage of the temporo-m axillary joint in mastication is to serve : 

 (1) As an elastic pad to distribute the pressure caused by the exceed- 

 ingly powerful action of the masticatory muscles. (2) As a joint-surface 

 or socket for the condyle of the lower jaw when the latter has been par- 

 tially drawn forward out of the glenoid cavity of the temporal bone by 

 the external pterygoid muscle, some of the fibres of the latter being at- 

 tached to its front surface, and consequently drawing it forward with the 

 condyle which moves on it. 



Nervous Mechanism. The act of mastication is partly voluntary and 

 partly reflex and involuntary. The consideration of such nervous actions 

 will come hereafter. It will suffice here to state that the afferent nerves 

 chiefly concerned are the sensory branches of the fifth and the tenth or 

 glosso-pharyngeal, and the efferent are the motor branches of the fifth and 

 the twelfth (hypoglossal) cerebral nerves. The nerve-centre through 

 which the reflex action occurs, and by which the movements of the vari- 

 ous muscles are harmonized, is situated in the medulla oblongata. In so 

 far as mastication is voluntary or mentally perceived, it is under the in- 

 fluence of the cerebral hemispheres. 



Insalivation. The act of mastication is much assisted by the saliva 

 which is secreted by the salivary glands in largely increased amount dur- 

 ing the process, and the intimate incorporation of which with the food, 

 as it is being chewed, is termed insalivation. 



The Salivary Glands. 



The glands which secrete the saliva in the human subject are the sal- 

 ivary glands proper, viz., the parotid, the sub-maxillary, and the sub-lin- 

 gual, and numerous smaller bodies of similar structure, and with sepa- 

 rate ducts, which are scattered thickly beneath the mucous membrane of 

 the lips, cheeks, soft palate, and root of the tongue. 



Structure. The salivary glands are compound tubular or tubulo-race- 



