CHAPTER III. 

 THE REGION OF THE SHOULDER. 



ANATOMY. 



ACTION OF MUSCLES. DISLOCATIONS. 



LANDMARKS. ATROPHY OF DELTOID. 



SYNOVITIS. OPERATIONS. 



The Shoulder Joint is an enarthrodial or ball-and- 

 socket joint, and is composed of a humeral head, nearly 

 hemispherical in form, pointing upwards, inwards and 

 backwards, and articulating with the shallow glenoid fos- 

 sa of the scapula. It is protected, from above, by an arch 

 composed of tlhe coracoid and the acromion processes with 

 the coraco-acromial ligament uniting them. The capsular 

 ligament which encircles the articulation is lax and does 

 not maintain the joint surfaces in apposition, so that, were 

 it not for the atmospheric pressure and the tonicity of the 

 muscles, that, controlling the joint, are attached to the 

 capsule, the humerus would be separated at least one inch 

 from the glenoid fossa. The area of the articular end of 

 the humerus is about three and a half times that of the 

 glenoid fossa, and this arrangement, whereby a larger 

 surface rests on a smaller one, permits great freedom of 

 movement at the joint. The articular surface of the hu- 

 merus is not exactly hemispherical, being longest in a di- 

 rection, downwards and inwards from the upper end of 

 the greater tuberosity, and consequently, elevation of the 

 arm is freest in the reverse direction, viz., upwards and 

 outwards, as evidenced by the ease with which the hand 

 may be placed behind the head. In an antero-posterior 

 direction the greatest movement is permitted when the 

 arm is raised to a right angle, since this position allows the 

 widest part of the head of the bone to be brought into play 

 on the glenoid surface. 



