ARTICULATIONS OF THE HEAD. 139 



described here. Lateral movements take place when the inferior extremity of 

 the jaw is carried alternately to the right and left. It then happens that 

 one of the maxillary condyles, taking with it the tibro-cartilage, is brought 

 into contact with the temporal condyle, while the other is imbedded in the 

 glenoid cavity of the opposite side. The horizontal gliding is eifected from 

 behind to before, or vice versa. In the first case, the two maxillary condyles 

 are carried at the same time under the temporal condyles, bearing with 

 them the fibro-cartilages. In the second case, they are drawn into the 

 glenoid cavities, and rest against the supraoondyloid eminence, which 

 prevents their going further. It will be understood, after this brief 

 description, that the presence of the fibro-cartilages singularly favours the 

 lateral movements and horizontal gliding of the lower jaw. 



In the Pig the temporo-maxillary articulation is formed after the same type as that 

 of rodents, and allows very extensive movements from before to behind ; a circumstance 

 due to the complete absence of the supraoondyloid eminence. 



In the Bog and Gat the maxillary condyle is exactly fitted into the temporal cavity. 

 This disposition, in giving great precision to the movements of depression and elevation, 

 restrains in a singular manner the lateral and horizootal gliding motions. The inter- 

 articular fibro-cartilage is extremely thin in these animals. 



5. Hyoideal Articulations. 



(^Preparation. — Disarticulate the lower jaw, and dissect away from the right of each 

 articulation the muscles that may conceal the view.) 



These are of two kinds : extrinsic and intrinsic. The first comprise the 

 two temporohyoideal articulations ; to the second belong the joints which 

 unite the different pieces of the hyoid bone — the interhyoideal articulations. 



Tbmpoeo-htoideal Aeticulations. — These are two amphiarthrodial 

 joints, in the formation of which each great branch of the hyoid bone 

 opposes its upper extremity to the hyoideal prolongation lodged in the 

 vaginal sheath of the temporal bone. An elastic cartilage, from 4-lOths to 

 6-lOths of an inch in length, unites the two bones in a solid manner ; and 

 it is owing to the flexibility of this cartilage that the hyoid bone can move 

 entirely on the temporal bones. 



Intbbhtoideal Aeticulations. — A. The great branch articulates with 

 the small one by an amphiarthrosis analogous to the preceding. To form 

 this articulation, these two pieces of bone are joined at an acute angle 

 through the medium of a more or less thick cartilaginous band, in the 

 centre of which there is often a little bony nucleus. This cartilage is elastic 

 and flexible, and permits the opening and closing of the articular angle at 

 the summit of which it is placed. 



J3. Each small branch is raiited to the body of the hyoid bone by an 

 arthrodial articulation. The articular surfaces are : for the hyoideal branch, 

 the small cavity terminating its inferior extremity ; for the body, the 

 rounded lateral facet situated at the origin of the cornu. These surfaces 

 are covered by cartilage, and enveloped by a small synovial sac and a 

 peripheral fibrous capsule. They can glide on each other in nearly every 

 direction. (Median and superior hyoideal capsular ligaments are described 

 by Leyh as sometimes present. The latter unites the upper and middle 

 branches, and the former the middle with the inferior branches. They are 

 absent when these branches are confounded with the superior ones.) 



