ARTICULATIONS OF THE HEAD. 189 



125, 2), this ligament becomes greyish-coloured and elastic for the remainder of 

 its extent, and greatly diminishes in thickness, especially in front. Its inner 

 face IS covered by the synovial capsules, and adheres to the circumference of the 

 interarticular fibro-cartilage. Its external face is related, in front, to the temporal 

 and masseter muscles ; behind, to the parotid gland ; inwardly, to the external 

 pterygoid muscle ; and outwardly, to a fibrous expansion which separates it from 

 the skin. (Leyh mentions a lateral external and a posterior ligament for this 

 articulation, but Chauveau and Bigot evidently look upon these as portions of 

 the capsular.) 



Synovial membranes. — This articulation has two synovial sacs — one above the 

 other — which are separated by the fibro-cartiiaginous disc. 



Movements. — The temporo-maxillary articulation is the centre of all the 

 movements performed by the lower jaw. These are : depression, elevation, lateral 

 motion, and horizontal gliding. 



The lower jaw is depressed when it separates from the superior one, and is 

 elevated when it approaches this. These two opposite movements are executed by 

 a mechanism of such great simplicity, that it need not be 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 fibro-cartilage, is brought into contact with the temporal condyle, 

 while the other is embedded in tiie glenoid cavity of the opposite side. The 

 horizontal gliding is effected from behind to before, or vice veisd. 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 supra-condyloid 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 supra-condyloid eminence. 



In the Dog and Cat, the maxillary condyle is exactly fitted to the temporal cavity. This 

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

 a singular manner the lateral and horizontal gliding motions. The interarticular fibro-cartilage 

 is extremely thin in these animals. 



In the Rabbit, the narrow condyle of the maxilla moves from before to behind and to 

 each side, on the temporal bone, the articular surface of which is very elongated and destitute 

 of a subglenoid eminence. 



5. Hyoideal Aeticulations. 



(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 

 tempo ro-hyoideal articulations ; to the second belong the joints which unite the 

 different pieces of the hyoid bone — the interhyoideal articulations. 



Temporo-hyoideal Articulations. — These are two amphiarthrodial joints, 

 in the formation of which each great cornu 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 -^q to i%- 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. 

 15 



