The shoulder-joint is an enarthrodial or ball-and-socket joint.
The bones entering into its formation are the hemispherical head of the humerus and the shallow glenoid cavity of the scapula, an arrangement which permits of very considerable movement, while the joint itself is protected against displacement by the tendons which surround it.
The ligaments do not maintain the joint surfaces in apposition, because when they alone remain the humerus can be separated to a considerable extent from the glenoid cavity; their use, therefore, is to limit the amount of movement.
The joint is protected above by an arch, formed by the coracoid process, the acromion, and the coracoacromial ligament.
The articular cartilage on the head of the humerus is thicker at the center than at the circumference, the reverse being the case with the articular cartilage of the glenoid cavity.
The ligaments of the shoulder are:
The articular capsule completely encircles the joint, being attached, above, to the circumference of the glenoid cavity beyond the glenoidal labrum; below, to the anatomical neck of the humerus, approaching nearer to the articular cartilage above than in the rest of its extent. It is thicker above and below than elsewhere, and is so remarkably loose and lax, that it has no action in keeping the bones in contact, but allows them to be separated from each other more than 2.5 cm., an evident provision for that extreme freedom of movement which is peculiar to this articulation. It is strengthened, above, by the Supraspinatus; below, by the long head of the Triceps brachii; behind, by the tendons of the Infraspinatus and Teres minor; and in front, by the tendon of the Subscapularis. There are usually three openings in the capsule. One anteriorly, below the coracoid process, establishes a communication between the joint and a bursa beneath the tendon of the Subscapularis. The second, which is not constant, is at the posterior part, where an opening sometimes exists between the joint and a bursal sac under the tendon of the Infraspinatus. The third is between the tubercles of the humerus, for the passage of the long tendon of the Biceps brachii.
Synovial Membrane.—The synovial membrane is reflected from the margin of the glenoid cavity over the labrum; it is then reflected over the inner surface of the capsule, and covers the lower part and sides of the anatomical neck of the humerus as far as the articular cartilage on the head of the bone. The tendon of the long head of the Biceps brachii passes through the capsule and is enclosed in a tubular sheath of synovial membrane, which is reflected upon it from the summit of the glenoid cavity and is continued around the tendon into the intertubercular groove as far as the surgical neck of the humerus. The tendon thus traverses the articulation, but it is not contained within the synovial cavity.
Bursæ.—The bursæ in the neighborhood of the shoulder-joint are the following: (1) A constant bursa is situated between the tendon of the Subscapularis muscle and the capsule; it communicates with the synovial cavity through an opening in the front of the capsule; (2) a bursa which occasionally communicates with the joint is sometimes found between the tendon of the Infraspinatus and the capsule; (3) a large bursa exists between the under surface of the Deltoideus and the capsule, but does not communicate with the joint; this bursa is prolonged under the acromion and coraco-acromial ligament, and intervenes between these structures and the capsule; (4) a large bursa is situated on the summit of the acromion; (5) a bursa is frequently found between the coracoid process and the capsule; (6) a bursa exists beneath the Coracobrachialis; (7) one lies between the Teres major and the long head of the Triceps brachii; (8) one is placed in front of, and another behind, the tendon of the Latissimus dorsi.
The muscles in relation with the joint are, above, the Supraspinatus; below, the long head of the Triceps brachii; in front, the Subscapularis; behind, the Infraspinatus and Teres minor; within, the tendon of the long head of the Biceps brachii. The Deltoideus covers the articulation in front, behind, and laterally.
The arteries supplying the joint are articular branches of the anterior and posterior humeral circumflex, and transverse scapular.
Movements.—The shoulder-joint is capable of every variety of movement, flexion, extension, abduction, adduction, circumduction, and rotation. The humerus is flexed (drawn forward) by the Pectoralis major, anterior fibers of the Deltoideus, Coracobrachialis, and when the forearm is flexed, by the Biceps brachii; extended (drawn backward) by the Latissimus dorsi, Teres major, posterior fibers of the Deltoideus, and, when the forearm is extended, by the Triceps brachii; it is abducted by the Deltoideus and Supraspinatus; it isadducted by the Subscapularis, Pectoralis major, Latissimus dorsi, and Teres major, and by the weight of the limb; it is rotated outward by the Infraspinatus and Teres minor; and it isrotated inward by the Subscapularis,