THE SHOULDER JOINT 73 



the degree of convexity of the latter. It is surrounded by a well-defined 

 rim which is not complete, since it presents antero-internally a well- 

 marked notch. The rim affords the superior attachment to the capsular 

 ligament. 



The joint is of the enarthrodial variety, and permits of abduction, 

 adduction, flexion, extension, circumduction, and rotation. 



A peculiarity in connection with this joint is that it possesses only 

 one ligament, namely, the capsular, which is in the form of a double- 

 mouthed sac attached superiorly and inferiorly as described above. The 

 sac supports by its inner surface the synovial membrane of the joint. 

 Posteriorly, the small scapulo-humeralis gracilis muscle lies on the liga- 

 ment, and protects the ligament from injury by being folded in between 

 the sharp posterior edge of the glenoid cavity and the head of the 

 humerus during extreme flexion of the joint. But the dearth of liga- 

 mentous union is compensated by the number of powerful tendons which 

 play over the joint and help to maintain the articular surfaces in apposi- 

 tion. Thus we have the outer and inner divisions of the tendon of the 

 supraspinatus muscle playing over the front of the joint and obtaining 

 insertion into the anterior division of the internal tuberosity and the 

 summit of the external tuberosity of the humerus respectively. Even 

 when this muscle is in a non-contractible state these tendons are quite 

 tense, so that they play an important part in maintaining the humeral 

 head in its position in the glenoid cavity. On the outer side of the joint 

 we have the tendons of the infraspinatus and deltoid muscles. That of 

 the former muscle splits into two portions, the inner of which is inserted 

 into the convexity or posterior division of the external tuberosity, and 

 the outer passes to its insertion into the upper extremity of the ridge 

 which descends from the summit of the external tuberosity to the deltoid 

 tubercle. To the tubercle just mentioned the tendon of the deltoid 

 muscle is attached. 



Again, we have the tendons of the biceps and coraco-humeralis 

 muscles arising from the coracoid process and the special tubercle on its 



