278' HUMAN ANATOMY. 



(Unless the arm is raised somewhat forward, it is stopped still sooner by the 

 acromion.) Forward movement is about equal to abduction, and is checked 

 in the same way. When the arm is at a right angle with the body, the range of 

 motion in a horizontal plane is about 90°. The degree of rotation in the shoulder 

 is very variable. It is greatest when the arm is partially abducted, when in a dis- 

 sected joint it may approximate 135°. When raised to a right angle it is about 

 90°, and in the hanging arm, if not closely adducted, nearly the same. Circum- 

 duction is free. 



Probably none of the important joints is so dependent on others as that of the 

 shoulder. The scapula takes part in practically all the movements, not waiting till 

 the range of movement at the shoulder is exhausted, but sharing in it from the start. 

 The acromion and coraco-acromial ligament make an extra socket under certain cir- 

 cumstances, as when the body is supported by the arms, the subacromial bursa act- 

 ing as a synovial membrane. The long head of the biceps is a great assistance to 

 the stability of the joint, the muscle pulling the bones firmly together and making 

 them rigid under circumstances of strain. It has the further advantage over a liga- 

 ment that its tension can vary without change of position. ^ 



PRACTICAL CONSIDERATIONS. 



The extremely wide range of motion of the humerus upon the scapula in the 

 human species is associated, for mechanical reasons, with many anatomical conditions 

 of interest to the surgeon. The most important of these conditions in relation to 

 displacement are: (i) The shallowness of the glenoid cavity. (2) The relatively 

 large size of the humeral head, only one-third of which is in contact with the glenoid 

 surface when the arm is by the side of the body. (3) The thinness and the great 

 laxity of the capsule, which, if fully distended, would accommodate a bulk twice as 

 large as the head of the humerus. This laxity (to permit of free elevation of the 

 arm) is greater at the inferior portion of the joint. The primary office of the cap- 

 sular ligament in this joint is not to maintain apposition, but to limit movement. 

 (4) The maintenance of the contact between the articular surfaces by muscular 

 action, aided by atmospheric pressure, and not by the ligamentous or capsular at- 

 tachments. (5) The length of the humerus, affording a very long leverage ; and 

 the exposed position of the shoulder. 



All these circumstances favor dislocation, and render this joint more frequently 

 the subject of that accident than any other joint in the body. The usual position in 

 which luxation occurs is that of abduction and advancement of the arm, as in falls 

 on the hand. 



It is to be noted that this attitude brings the most prominent part of the lower 

 edge of the head of the bone against the thinnest and weakest part of the capsule. 

 Moreover, the greatest diameter of the head of the humerus is involved, adding to 

 the pressure against the capsule (Fig. 293). As such accidents happen suddenly, 

 the muscles are usually taken off their guard, and hence that source of protection to 

 the joint is lacking. 



As opposed to these factors, and as tending to prevent displacement, should be 

 mentioned : (i) The exceptional relation of the biceps tendon to the joint, strength- 

 ening the capsule at its upper portion, preventing the humerus from being too 

 strongly pressed against the acromion by the powerful deltoid and the other 

 elevators of the arm ; steadying the head of the bone through its connection in the 

 bicipital groove, in which way ' ' it serves the purpose of a ligament, with the advan- 

 tage of being available in all positions of the joint, and without restricting the range 

 of movement in any direction" (Humphry). (2) The arrangement of the glenoid 

 cup, the inner and lower edges of which are more prominent than the outer and 

 upper, resisting somewhat the tendency of the powerful axillary muscles, and of 

 blows on the shoulder, to displace the humerus inward, and of falls with the arm in 

 abduction, to displace it downward. (3) The glenoid ligament deepening the 

 articular cavJty, and aided in this by the insertions of the long head of the biceps 

 above and the scapular head of the triceps below. (4) The resistance offered by 

 the strong coraco-acromial ligament to any upward displacement. If it were not for 



