562 



DIGESTION. 



the stylo-hyoideus is attached, is anterior to the articulation; and its 

 effect ought to be to depress the upper jaw. The view of Professor 

 Chaussier is the most probable. He ascribes the slight elevation of the 

 upper jaw to the mechanical arrangement of the joint of the lower. 

 The temporo-maxillary articulation is not formed by a single condyle, 

 but by two, which are so disposed, that the lower cannot roll downwards 

 during the depression of the lower jaw without causing the upper con- 

 dyle to roll upwards, and, consequently, to elevate slightly the upper 

 jaw. Under ordinary circumstances, then, the jaws cannot be at all 

 separated without both participating ; but if we determine to fix the 

 upper jaw we can make the lower the sole agent in the movement. 



As soon as the food is introduced into the mouth, the jaws are closed 

 to retain it, and subject it to mastication. Frequently, however, they 

 assist in the act of prehension, as when we bite a fruit, to separate a 

 portion from it; the incisor teeth acting, in such case, like scissors. 

 This is chiefly produced by the contraction of the muscles that raise 

 the lower jaw; and it is probable, that the action of the stylo-hyoideus 



Fig. 247. 



Action of the Lower Jaw in Prehension. 



A. Frontal bone. B. Temporal. C. Parietal. D. Occipital. E. Coronoid process of the lower 

 jaw, to which the temporal muscle is attached. F. Condyloid process or head of the lower jaw. G. 

 Lower jaw. H. Mastoid process. I. Upper jaw. J. Cheekbone. K. Orbit. L. Meatus auditoriua 

 externus. L*. Coronal suture. M. Squamous suture. N. Lambdoidal suture, g. Lower jaw de- 

 pressed. 



is concerned in the movement; drawing the head and upper jaw with it 

 downwards and forwards. The levator muscles of the jaw act here with 



