l.'^2 



THE A n Tin 7. A TIOXS 



Movements.— The chief movement of this joint is of (i) a ginglymoid or 

 hinge character, aceomi»anie(l h}- a slight gliding action, as in opening or sliutting 

 the mouth. In the opening movement the condyle turns like a hinge on the fibro- 

 cartilage, while at the same time the fibro-cartilage, together with the condyle, 

 glides forwards so as to rise upon the eminentia articularis; the fibro-cartilage 

 reaching as far as the anterior edge of the eminence, which is coated with articular 

 cartilage to receive it ; but the condyle never reaches quite so far as the summit of 

 the eminence. Should the condyle, however, by excessive movement (as in a con- 

 vulsive yawn), glide over the summit, it shps into the zygomatic fossa, the mandil)le 

 is dislocated, and the posterior portion of the capsule is torn. In the shutting 

 movement the condyle revolves back again, and the fibro-cartilage glides back, 

 carrying the condyle with it. This combination of the hinge and gliding motions 

 gives a tearing as well as a cutting action to the incisor teeth, without any extra 

 niuscular exertion. 



There is (ii) a horizontal gliding action in an antero-posterior direction, by 

 which the lower teeth are thrust forwards and drawn back again: this takes place 



Fig. 



193. — Veetical Section through the Condyle of Jaw to show the Two 

 Synovial Sacs and the Interaeticular Fibko-caetilage. 



Interarticular flbro-_4 ^ 



cartilage ^ ■? 



SECTION THROUGH CONDYLE > ;,_^ ■ !V i 



Posterior portion of tt' , /'^'^ L/ 



capsule 



Spheno-mandibular ligament 



Stylo-mandibular ligament 



almost entirely in the upper compartment, because of the closer connection of the 

 libro-cartilage with the condyle than with the squamosal bone, and also because of 

 the insertion of tlie crtenutl jderygoid into both bone and cartilage. In these two 

 sets of movements the joints of both sides are simultaneously and similarlv engaged. 

 The third form of movement is called (iii) the oblique rotatory, and* is that by 

 Avhich the grinding and chewing actions are performed. It consists in a rotation of 

 the condyle about the vertical axis of its neck in the lower compai-tment, while the 

 cartilage glides ol)li(iuely forwards and inwards on one side, and backwards and 

 inwards on the other, u])on the articular surface of the squamosal bones, each side 

 acting alternately. If the symphysis be simply moved from the centre to one 

 side and back again, and not from side tQ side as in grinding, the condyle of that side 

 moves round the vertical axis of its neck, and the opi)osite condyle and cartilage 

 glide forwards and inwards upon the glenoid fossa. But in the ordinary grinding 

 movement, one condyle advances and the other recedes, and then the first recedes 

 while the other advances, slight rotation taking place in each joint meanwhile. 



