82 Anatomy of Skeleton 



glenoid rim on the scapula and to the anatomical neck of the humerus, extend- 

 ing to the surgical neck below : it is very loose below and in front and tehind, 

 but fairly tense above when the arm is by the side for no further adduction is 

 possible. 



It has, however, one strong accessory band in the shape of the coraco-humeral 

 ligament, which passes on one side beyond the glenoid margin on to the coracoid 

 process, and on the other extends beyond the anatomical neck to the tops of both 

 tuberosities, on either side of the commencement of the bicipital groove, so that its 

 front part stands out as a fibrous edge between the Subscapular arid Supraspinatus 

 tendons, while it becomes continuous behind, under the latter tendon, with the plane 

 of the capsule (see Figs. 57 and 62). The long head of Biceps passes over the head 

 of the bone, in the articular cavity and surrounded by synovial membrane, to reach 

 the groove, and is regarded as an additional factor helping to keep the head in the 

 cavity : as it emerges from under the coraco-humeral ligament this is strengthened 

 over the top of the groove by transverse fibres. The coraco-humeral band would tend 

 to check extreme outward rotation. Three accessory gleno-humeral ligaments, of no 

 apparent mechanical importance, are present inside the joint, making prominent folds 

 of the synovial membrane on its front wall. They are directed down and out from the 

 upper and front glenoid rim, and the upper band is attached at the fovea humeri, an 

 impression on the anatomical neck just above the small tuberosity. The opening 

 for the subscapular bursa is between the upper and middle bands, and here, 

 therefore, the subscapular tendon comes into relation with the articular surface. 

 The head of the humerus is almost a third of a sphere, set at an angle of 

 140 degrees with the axis of the shaft, and covered by hyaline cartilage that 

 is thicker in the centre, thus contrasting with the opposing cartilage that lines 

 the glenoid cavity. The area of its articular surface is more than twice that of 

 the glenoid surface, so that a large part must always lie in contact, not with 

 the glenoid cavity, but with the surrounding capsule : when the arm is by the 

 side this capsular contact is mainly above and behind, but is below when the arm 

 is raised. 



The want of security evident in such a shallow articulation is a consequence of 

 the necessity for having free movement in all directions, and the two bones cannot be 

 firmly held together by ligaments, for this would restrict the range of motion. So the 

 security of the joint must depend on the surrounding muscles, and the closer these are 

 attached to the articular surfaces, and especially to the more movable one, the more 

 efficiently can they perform the function of bands which are always tense in any 

 position or action of the joint. This is probably the reason why we find strong 

 tendons grouped closely round three surfaces of the capsule and inserted into the 

 humerus immediately outside the anatomical neck to which the capsule is attached 

 (see Fig. 62). 



They are the Subscapularis, Supraspinatus, Infraspinatus, and Teres minor : the 

 two spinatus muscles are inserted into their corresponding facets, but the other two 

 extend beyond their evident markings on the bone, as usually seen. 



The Teres minor (Fig. 63) is inserted by tendon on its facet, but in addition is 

 attached by muscle fibre to the shaft for inch below this, thus not causing a 

 secondary marking here. The two modes of insertion probably depend on the 

 fact that the portion on the tuberosity, being raised, is under pressure from the 

 Deltoid, whereas that below the prominence escapes such pressure : occasionally 

 the lower portion is also tendinous, and so there is occasionally a marking for it 

 on the bone. 



