208 SKELETON. 



cranon, having left its cavity, is placed below the condyles. 

 In this state the capsular ligament is stretched at its posterior 

 part, while the anterior is thrown into folds, and is relaxed 

 along with the lateral ligaments. In the demi-flexion of the 

 arm, there is a more equal degree of tension of the several li- 

 gaments. When the os humeri is reposing in its most easy at- 

 titude, at the side of the body, if the fore arm be flexed, its line 

 of motion directs the hand towards the mouth; a circumstance 

 which is accounted for by the peculiar obliquity of the troch- 

 lea, on the lower part of the os humeri, upon which the ulna 

 revolves, and is independent of any special act of volition. It 

 is said that man, above all other animals, has the mechanism 

 of the upper extremity most particularly addressed to the latter 

 motion, to the perfection of which the clavicle is indispensable. 

 It is in consequence of this application of the clavicle, that if it 

 be broken, man, like animals which are entirely deprived of it, 

 will, in the flexions of the fore arm, more easily carry the hand 

 to the opposite shoulder than to the mouth. 



In. the full extension of the fore arm, the olecranon process 

 being received into its cavity, is much above the condyles of 

 the os humeri. The lateral ligaments, as well as that part of 

 the capsule on the front of the joint, are in a state of tension. 

 When the extremity is in this position, a fall upon the hand 

 may produce a dislocation backwards. In this case the fore 

 arm being fixed, the coronoid process affords the surface upon 

 which the principal momentum of the fall is felt. If the liga- 

 ments on the front of the joint be not strong enough to with- 

 stand the force, they are lacerated, and the articular surfaces, 

 passing each other, the upper parts of the ulna and radius are 

 driven behind the os humeri. Bichat asserts, that nothing is 

 more easy than to produce such a luxation on the dead body by 

 a similar proceeding, and that he has repeatedly done it that 

 it is about as easy to produce this dislocation, as it is difficult to 

 effect one at the scapulo-humeral articulation. In a moderate 

 extension of the fore arm, produced by a small weight suspend- 

 ed on the hand at arm's length, there is a well marked pressure 

 of the inferior extremity of the os humeri against the ligaments 

 in front of the articulation, which is augmented by a tendency 

 of the ulna to describe the arc of a circle, from above down- 

 wards, and to separate itself from the os humeri. In this case 



