230 HANDBOOK OF PHYSIOLOGY. 



leaving at length only the crown as a mere shell, which is shed to make 

 way for the eruption of the permanent teeth (Fig. 165). 



Each temporary tooth is replaced by a corresponding tooth of the 

 permanent set which is developed from a small sac set by, so to speak, 

 from the sac of the temporary tooth which precedes it, and called the 

 cavity of reserve. 



MASTICATION. 



The act of chewing or mastication is performed by the biting and 

 grinding movement of the lower range of teeth against the upper. The 

 simultaneous movements of the tongue and cheeks assist partly by crush- 

 ing the softer portions of the food against the hard palate and gums, and 

 thus supplementing the action of the teeth, and partly by returning the 

 morsels of food to the action of the teeth, again and again, as they are 

 squeezed out from between them, until they have been sufficiently 

 chewed. 



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

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

 muscles, 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 exter- 

 nal pterygoids act together, the lower jaw is pulled directly forwards, 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-artic- 

 ular fibro-cartilage of the temporo-maxillary joint in mastication is to 

 serve : (1) As an elastic pad to distribute the pressure caused by the ex- 

 ceedingly 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 partially drawn forward out of the glenoid cavity of the temporal 

 bone by the external pterygoid 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. 



Nervous Mechanism. The act of mastication is partly voluntary and 

 partly reflex and involuntary. The consideration of such sensori-motor 

 actions will come hereafter (see Chapter on the Nervous System). It 

 will suffice here to state that the afferent nerves chiefly concerned are 

 the sensory branches of the fifth and the glosso-pharyngeal, and the ef- 

 ferent are the motor branches of the fifth and the ninth (hypoglossal) 

 cerebral nerves. The nerve-centre through which the reflex action oc- 

 curs, and by which the movements of the various muscles are harmo- 

 nized, is situated 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 hemispheres. 



