A limb girdle is defined as the bones that connect a limb to the axial skeleton. The bones of the pectoral or shoulder girdle are the clavicle and scapula. Only one small joint connects the girdle to the rest of the skeleton: the sternoclavicular joint, and the two bones are joined to one another by an even smaller joint, the acromioclavicular. The remaining attachment to the axial skeleton is purely muscular, and this helps to account for the greater mobility of the shoulder girdle compared with its lower limb counterpart, where three bones (ilium, ischium and pubis) fuse to form the solid hip bone or pelvic girdle.
In terms of comparative anatomy, the human scapula represents two bones that have become fused together; the (dorsal) scapula proper and the (ventral) coracoid. The epiphyseal line across the glenoid cavity is the line of fusion. They are the counterparts of the ilium and ischium of the pelvic girdle. The counterpart of the pubis is a tiny piece of bone (precoracoid) that ossifies separately at the tip of the coracoid process; it takes no part in the formation of the shoulder joint. The clavicle, ossifying in membrane, is an added bone (it has no counterpart in the pelvic girdle) whose purpose is to act as a strut preventing medial movement of the scapula. The strong coracoclavicular ligament attaches the clavicle and scapula to each other, and the clavicle is anchored to the first costal cartilage by the costoclavicular ligament. Forces from the upper limb are transmitted by the clavicle to the axial skeleton through these ligaments, and neither end of the clavicle normally transmits much force.
Embryology of pectoral girdle:
Muscles developed in the upper limb are supplied by branches from the brachial plexus. During their development some of these muscles migrate for considerable distances to gain attachment to the trunk; but they continue to be innervated from the brachial plexus. Motor supply, once established in the embryo, never changes thereafter. An example is latissimus dorsi which, notwithstanding its very wide attachment to the trunk, still retains its supply from the posterior cord of the brachial plexus. Other muscles, not developed in the limb, migrate from the trunk and gain attachment to the girdle; e.g. sternocleidomastoid and trapezius, supplied by the accessory nerve, attach themselves to clavicle and scapula. The nerve supply tells the story of the development of any muscle.
Movements of the pectoral girdle:
The essential functional requirement of the pectoral girdle is mobility on the thorax to enhance the mobility of the shoulder joint. And it is a fact that all movement between humerus and glenoid cavity, except very slight movement, is accompanied by an appropriate movement of the scapula itself. Nor can the scapula move without making its supporting strut, the clavicle, move also. Generally speaking the shoulder joint, the acromioclavicular and sternoclavicular joints all move together in harmony, providing a kind of thoracohumeral articulation. It is important to appreciate this, for functional defects in any part of the ‘thoracohumeral articulation’ must impair the whole.
Muscles of the pectoral girdle:
The muscular attachments between pectoral girdle and trunk are direct and indirect. Direct attachment of the pectoral girdle to the trunk is provided by muscles that are inserted into the clavicle or scapula from the axial skeleton. These muscles are pectoralis minor, subclavius, trapezius, the rhomboids, levator scapulae and serratus anterior. Indirect attachment to the axial skeleton is secured by the great muscles of the axillary folds (pectoralis major and latissimus dorsi); these muscles, by way of the upper end of the humerus, move the pectoral girdle on the trunk. Their descriptions are included here.
The muscular attachments between upper limb and pectoral girdle include the deltoid and short scapular muscles, which are inserted about the upper end of the humerus, and the biceps and long head of triceps which, running over the humerus, are inserted beyond the elbow joint into the bones of the forearm. All these muscles are important factors in giving stability to the very mobile shoulder joint across which they lie, and are described with the shoulder region.