This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The upper extremity being an organ of prehension and not of support, the shoulder-joint, which is the articulation which connects it with the trunk through the shoulder-girdle, is constructed with the idea in view of favoring and permitting motion, and not of supporting weight or resisting force. Hence we find it to be a ball-and-socket joint, the one which allows of the freest movements.
The glenoid cavity is a shallow excavation, not a deep cup, as in the hip. The articulating surface of the head of the humerus is extensive but not so large as it would have been had the scapula not been made to move on the thorax. The clavicle keeps the joint well out from the side of the body; hence the neck of the humerus is short. The movements of the arm are so extensive and free that we do not have the tuberosities of the humerus so large and set so far away from the articular surface as is the case with the femur and its trochanters.
If the upper portion of the femur was like the upper end of the humerus, the lower extremity would be continually rolling in or out, making walking or running at least difficult if not impossible. Thus we see that the shape of the bones is dependent on the character of their functions.
The ligaments of the joints are inelastic tissues; hence those that enter into the construction of a movable joint must be loose, and the more movable a joint is, the more does its security depend not on its ligaments, but on its muscles.
The shoulder-joint, like other joints, has a capsular ligament which is attached to the adjacent bones and serves to keep the lubricating synovial fluid applied to the articulating surfaces. In certain positions this ligament may also serve to a limited extent to keep the ends of the bones of the joint in contact.
Besides this capsular ligament there are fibrous bands which strengthen it at certain places as they pass from adjacent processes of bone on one side of the joint to those on the other. The muscles and their tendons pass across the capsule and sometimes blend with it, so that there is an intimate relation between the muscles and their tendons and the ligaments; finally, there is a third structure called the glenoid ligament, which is in reality a fibrocartilage that serves to deepen the glenoid cavity. The capsular ligament is attached on one side to the edge of the glenoid cavity, the anatomical neck of the scapula, and the rim of the glenoid ligament. On the other side it is attached above or externally to the anatomical neck of the humerus just at the edge of the articulating surface, but on the lower or inner surface it is attached some distance below the articular surface (approximately 1 cm.) and then turns upward toward the edge of the articular cartilage. Thus a fracture through the anatomical neck might pass outside of the joint above, and inside of it below. The positions assumed by the capsule in abduction and adduction are shown in Figs. 237 and 238.
Fig. 236. - Acromioclavicular and shoulder joints.
Fig. 237. - Section of shoulder-joint, arm adducted, showing the position assumed by the capsule and the points of its attachment.
Fig. 238. - Section of shoulder-joint, arm abducted, showing the attachment of the capsule and the position 10 assumes when the arm is in this position.
The capsular ligament, per se, has not much strength. There are two openings in it - one is for the long tendon of the biceps and the other is the opening of the bursa beneath the subscapularis muscle. Sometimes there is a synovial extension beneath the supraspinatus tendon and rarely, in old people, a communication with the subacromial bursa. It is evident that in case of suppuration within the joint the pus would tend to find vent first through these openings.
The laxity of the capsule is such that after the muscles are removed the head can be drawn a considerable distance away from the glenoid cavity. When the muscles are paralyzed the weight of the arm causes the head to fall away and a depression is seen beneath the acromion process. The capsule is strengthened by two definite and separate ligamentous bands called the coracohumeral and the gleno-humeral ligaments.
Fig. 239. - View of the left scapula and outer end of the clavicle from in front, showing the ligaments pass-g from the coracoid process to the clavicle and acromion process, the glenoid ligament, and attachments of the long heads of the biceps and triceps muscles.
The coracohumeral ligament passes from the root of the coracoid process to the anterior portion of the greater tuberosity. It is supposed by Sutton to be a regression of the tendon of the pectoralis minor muscle.
The glenohumeral ligament is a ribbon-like band seen lying alongside of the inner edge of the biceps tendon as it passes through the interior of the joint. It passes from the root of the coracoid process near the edge of the glenoid cavity to a dimple in the lesser tuberosity of the humerus. Sutton considers it a regression of the tendon of the subclavius muscle and homologous with the ligamentum teres in the hip. It is also called the superior glenohumeral ligament, in contradistinction to some bands on the lower anterior part of the capsule which are called by some anatomists the middle and inferior glenohumeral ligaments. Between the superior above and the middle and inferior below is the opening by which the bursa of the subscapularis muscle communicates with the joint.
The glenoid ligament is the wedge- or cup-shaped ring of fibrocartilage which deepens the glenoid fossa. It is attached around its edge to the rim of the fossa and at its upper end receives the long tendon of the biceps, which divides and blends with it on each side. At its lower part when it is attached to the bone it blends with the anterior edge of the long tendon of the triceps.