52 ANATOMY AND PHYSIOLOGY. 



The articulation of the lower jaw with the temporal bone is 

 almost completely a hinge-joint. The upper end, or condyle of the 

 jaw-bone, is covered with cartilage, and so is the socket ; and the 

 two bones are tied together by side ligaments. Besides, there is a 

 movable cartilage in the joint, which accompanies the condyle of 

 the jaw in its motion. Notwithstanding all these appliances, the 

 jaw is sometimes dislocated, slipping forward off the eminence 

 upon which it gets when the mouth is opened. When this acci- 

 dent takes place, which generally happens from a violent yawn, 

 the patient is left with his mouth wide open, and has not the power 

 of closing it, presenting a very ludicrous figure, though his situa- 

 tion is to himself sufficiently uncomfortable. After this accident 

 has happened, it is exceedingly liable to be reproduced, in conse- 

 quence of the torn ligaments never properly uniting. 



The shoulder-joint is of all others the most frequently dislocated. 

 This results partly from its form, and partly from its being more 

 exposed to violence, since every fall, whether upon the shoulder, 

 elbow, or hand, has a tendency to displace it. The cavity on the 

 shoulder-blade is so small and shallow, that the round head of the 

 arm-bone is not laid in it but on it ; its capsular ligament is strong 

 but loose, so that the bone depends for being retained in its place 

 upon the muscles which surround it ; and if these^afe overcome, or 

 taken by surprise, particularly when the arm is raised above the 

 head, the head of the bone is dislocated down into the armpit. It 

 is in general replaced without much difficulty, but is exceedingly 

 liable to be thrown out again. I recollect one man, says Douglas, 

 who fell into fits occasionally, wjiose shoulder-joint I saw dislocated 

 and assisted to reduce three times, on three successive days. 



The elbow-joint is more complex than the shoulder-joint. It is 

 double in its motions, admitting of the flexion and extension of the 

 forearm on the arm, and the rolling of the head of the radius. For 

 the first motion it has two strong lateral ligaments, which render it 

 a hinge-joint, and the neck of the radius is confined to the srde of 

 the ulna by a collar in which it rolls. A synovial membrane covers 

 the ends of the bones, and lines the different ligaments. It may 

 be dislocated in many directions. Both bones of the forearm are 

 most commonly thrown backward ; then the arm is nearly straight, 

 and cannot be bent. Sometimes the forearm is thrown sideways, 

 either outward or inward, and sometimes the radius is dislocated 

 alone, either backwards or forwards. From its complexity, it is 

 also subject to disease. 



