The shoulder girdle bones are three in number. The scapula, the clavicle (collarbone), and the humerus. They meet at the superior, lateral corner of trunk to form two joints: the shoulder joint and the acromioclavicular joint; the former is the union of the head of humerus with the superior lateral corner of scapula; the latter is the union of collarbone with the acromion of scapula. All three bones are united at this angle by strong ligaments and cartilage.
The humerus and clavicle are longitudinal bones, but the scapula is rather flat and irregular in shape. It projects laterally and upwardly two longitudinal bony protrusions; the acromion and the coracoid process, which resembles a crow's beak. The short head of biceps brachii, the strongest muscle of arm, and the coracobrachialis muscle are attached to the coracoid process.
But the scapula also features on its superior lateral aspect a concave depression; it is the glenoid cavity, into which the head of humerus fits. This is called the shoulder joint. Meanwhile, the lateral end of the clavicle articulates with the acromion of scapula; this is called the acromioclavicular joint.
Below, a shoulder girdle radiograph shows the bones of this portion of the human body. 1- first rib; 2- medial border of scapula; 3- intercostal space; 4- lateral border of scapula; 5- scapula; 6- glenoid cavity of scapula; 7- humerus; 8- greater tuberosity of humerus; 9- anatomical neck; 10- acromion; 11- coracoid process of scapula; 12- clavicle; 13- spine of scapula; 14- superior border of scapula; 15- head of humerus; 16- surgical head.
