226 HAND-BOOK OF PHYSIOLOGY. 



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

 lower incisor teeth ^e brought in front of the level of the upper. 



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

 articular fibro-cartilage of the temporo-maxillary joint in mastication 

 may be here mentioned. (1) As an elastic pad, it serves well to distrib- 

 ute the pressure caused by the exceedingly powerful action of the masti- 

 catory muscles. (2) It also serves as a joint-surface or socket for the 

 condyle of the lower jaw, when the latter has been partially drawn for- 

 ward out of the glenoid cavity of the temporal bone by the external ptery- 

 goid muscle, some of the fibres of the latter being attached to its front 

 .surface, and consequently drawing it forward with the condyle which 

 moves on it. 



Nerve-mechanism of Mastication. As in the case of so many 

 other actions, that of mastication is partly voluntary and partly reflex 

 and involuntary. The consideration of such sensor i-mot or actions will 

 come hereafter (see Chapter 011 the Nervous System). It will suffice here 

 to state that the nerves chiefly concerned are the sensory branches of the 

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

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

 the reflex action occurs, and by which the movements of the various 

 muscles are harmonized, is situate in the medulla oblongata. In so far 

 as mastication is voluntary or mentally perceived, it becomes so under 

 the influence, in addition to the medulla oblongata, of the cerebral hemi- 

 spheres. 



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

 which is secreted by the salivary glands in largely increased amount 

 during the process, and the intimate incorporation of which with the 

 food, as it is being chewed, is termed insaUvation. 



THE SALIVARY GLANDS. 



The salivary glands are the parotid, the sub-maxillary, and the siib- 

 lingual, and numerous smaller bodies of similar structure, and with sep- 

 arate 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 usually described as compound 

 tubular glands. They are made up of lobules. Each lobule consists 

 of the branchings of a subdivision of the main duct of the gland, which 

 are generally more or less convoluted toward their extremities, and some- 

 times, according to some observers, sacculated or pouched. The convo- 

 luted or pouched portions form the alveoli, or proper secreting parts of 

 the gland. The alveoli are composed of a basement membrane of flattened 

 cells joined together by processes to produce a fenestrated membrane, the 

 spaces of which are occupied by a homogeneous ground-substance. With- 

 in, upon this membrane, which forms the tube, the nucleated salivary 



