chap, xi.j THE SHOULDER-JOINT. 189 



The shoulder-joint. From one surgical point 

 of view, joints may be divided into (1) those that 

 depend for their strength mainly upon ligaments ; 

 (2) those that are mechanically strong, and that 

 derive their stability to a great extent from the 

 arrangement of their component bones ; and (3) those 

 that rely for their support principally upon muscles. 

 As an example of the first kind may be cited the 

 sterno-clavicular joint, of the second form the elbow- 

 joint, and of the third the shoulder-joint. The articu- 

 lation the least prone to dislocation is the one that 

 derives its strength from tough unyielding ligaments, 

 while the one most often luxated belongs to the third 

 variety, its strength being greatly dependent upon 

 muscles that may be taken by surprise, and that may 

 themselves, from disordered action, prove sources of 

 weakness. These are, of course, not the only features 

 in the etiology of dislocation. A great deal depends 

 upon the amount of movement permitted in a given 

 joint, and the degree of leverage that can be brought 

 to bear upon its parts. 



The arch formed by the coracoid and acromion 

 processes and the ligament between them forms an 

 essential support to the head of the humerus, and is 

 an important constituent of the articulation. With 

 this arch the humeral head is in immediate relation, 

 although not in actual contact (Fig. 18). In paralysis 

 of the deltoid the head may be separated by some 

 distance from the coracoid process, and Nannoni 

 records the case of a child with old standing paralysis 

 of the deltoid, between whose humeral head and acro- 

 mial vault four fingers could be lodged. It is well to 

 note that at least two-thirds of the head of the bone 

 are not in contact with the glenoid cavity when the 

 arm hangs by the side, and Anger points out that in 

 this position three-fourths of the circumference of the 

 humeral head are in front of a vertical line drawn 



