874 



THE ORGANS OF DIGESTION. 



the third molar at the highest. The curve of the lower dental arch is the reverse, 

 the first molar at its deepest part, the third molar at its extremity. The greater 

 the depth to which the upper incisors overlap the lower, the more marked this 

 curve and the more pointed are the cusps of the grinding teeth. 



FIG. 472. View of teeth in situ, with the external plates of the alveolar processes removed (Cryer). 



The movement of the human mandible is forward and downward, the resultant 

 of these directions being an oblique line, upon an average 35 from the horizontal 



plane. 1 When the lower 

 jaw is advanced until the 

 cutting edges of the in- 

 cisors are in contact, the 

 jaws are separated, but 

 as the highest point of 

 the lower arch, its third 

 molar advances, it meets 

 and rests upon a high 

 point, second molar of the 

 upper arch, and thus un- 

 due strain of the incisors 

 is obviated. 



In the lateral move- 

 ments of the mandible but 

 one side is in effective action 

 at one time ; the oblique 

 positions of the cusps of 

 the opposite teeth are such 

 that when either side is in action the other is balanced at two or more points. 



1 W. E. Walker, Denial Cosmos, 1896. 



FIG. 473. Front and side views of the teeth and jaws. 



