LIGAMENTS OF THE SPINE. 265 



filled with blood like the latter, but is, probably, an arrange- 

 ment for giving great mobility forwards to the lower jaw. 



The movements of this bone may be simply hinge-like, by 

 its depression, in which the mouth is regularly opened ; or, by 

 the action of the pterygoid muscles, it may be slid forwards. 

 When the muscles of but one side act, a species of rotation is 

 communicated ; in which one condyle advances on the tubercle 

 of the temporal bone, while the other reaches to the back part 

 of the glenoid cavity. The looseness and length of the capsu- 

 lar ligament of the articulation, along with the extreme facility 

 of motion from the interposition of a moveable cartilage, con- 

 tribute very materially to this movement. The sliding back- 

 wards and forwards of the intermediate cartilage of this arti- 

 culation, during mastication, sometimes produces a cracking; 

 audible to the by-standers, and extremely annoying to the in- 

 dividual who is the subject of it, from the noise being so near 

 his ear. 



Some persons are liable to a spontaneous dislocation of this 

 bone, from yawning too widely. I am disposed to believe, that, 

 in such cases, the accident arises from the posterior boundary 

 of the glenoid cavity, (as established by that margin of the 

 temporal bone which is continuous with the vaginal process, 

 and forms a part of the meatus externus,) being more advanced 

 and higher than usual; in consequence of which, whenever the 

 bone is depressed to a certain point, its neck strikes against 

 this ridge, and not being able to go farther back, the ridge acts 

 as a fulcrum, and starts the condyle over the tubercle of the 

 temporal bone into the zygomatic fossa. The fact is certain, 

 that very strongly marked differences of the glenoid cavity, in 

 this particular, occur in different individuals. 



VOL. I. 23 



