210 SKELETON. 



latter place ; both on account of the greater extent of motion 

 there, and of the comparative weakness of the ligaments. 



In supination, a movement the reverse of what is described, 

 takes place ; the radius revolves outwardly, and is brought pa- 

 rallel with the ulna. If by any force it be carried beyond this 

 line, a dislocation may occur, in which the little head of the 

 ulna, abandoning the sigmoid cavity of the radius, will be thrown 

 in front of it. An accident, however, said to be very unusual. 



Bichat considers the cartilage between the ulna and the cu- 

 neiforme as a principal obstacle to these luxations; but when it 

 is insulated or separated from the cartilage of the radius, as 

 sometimes occurs, the joint is very much weakened thereby, 

 and more exposed to dislocations. 



SECT. X. OF THE MOTIONS OF THE HAND. 



The hand, as a whole, performs upon the forearm, flexion, 

 extension, lateral inclination, and circumduction. As it only 

 follows the motion of the radius in pronation and supination, 

 and does not contribute in the slightest degree to either, its ap- 

 propriate motions can all be performed independently of them. 



In flexion the convex head, formed by the first range of car- 

 pal bones, slides from before backwards in the concavity which 

 receives it. The posterior part of the capsufor ligament is 

 stretched, and the anterior thrown into folds, while the lateral 

 ligaments remain at their ease. In extension, with the excep- 

 tion of the lateral ligaments, the phenomena are reversed. This 

 extension, as is well known, not only brings the hand into the 

 same line with the bones of the fore arm, but carries it beyond 

 that line till it forms almost a right angle with it. The wrist 

 joint, in this respect, differs from the other ginglymous articu- 

 lations; but what it gains in extension it loses in flexion, as it 

 cannot be bent so much as either the elbow or knee. The ar- 

 rangement, however, gives great facility to the use of the hand. 



In the lateral inclinations of the hand, the capsule in front of 

 and behind the wrist, is but little affected, but the lateral liga- 

 ments are alternately relaxed and lightened. As the articular 

 surfaces are extensive in the line of these motions, dislocations 

 in the direction of either of them are very uncommon, and 

 when they do occur they are for the most part incomplete. 



