114 



MANUAL OF PHYSIOLOGY. 



is inclosed in a kind of modified bone tissue called crusta 

 petrosa. 



The two rows of grinding teeth, molars and premolars (one 

 on each side) of the lower jaw are made to rub against the 

 corresponding teeth in the upper fixed jaw by the combined 

 vertical and horizontal movements induced by the action of 

 the powerful muscles of mastication, the temporal muscles, 

 together with the masseters and internal pterygoids, all tending 



FIG. 48. 



Section through a portion of the Fang of a Tooth. (a) Dentine tubules near the surface 

 of the fang ; (ft) Granular layer ; (c) Crusta petrosa. 



by their contraction to elevate the lower jaw and bring the 

 teeth forcibly together. This action is opposed by the digastric, 

 the genio- and mylohyoid muscles, which by their combined 

 force depress the jaw and separate the teeth. The horizontal 

 movements are in the main accomplished by the external 

 pterygoid muscles, which, acting together, pull the lower jaw 

 forward so as to make the lower teeth protrude beyond the 



