Shoulder girdle

The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. In humans it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists of the clavicle, scapula, and coracoid. Some mammalian species (such as the dog and the horse) have only the scapula. The pectoral girdles are to the upper limbs as the pelvic girdle is to the lower limbs; the girdles are the parts of the appendicular skeleton that anchor the appendages to the axial skeleton. In humans, the only true anatomical joints between the shoulder girdle and the axial skeleton are the sternoclavicular joints on each side. No anatomical joint exists between each scapula and the rib cage; instead the muscular connection or physiological joint between the two permits great mobility of the shoulder girdle compared to the compact pelvic girdle; because the upper limb is not usually involved in weight bearing, its stability has been sacrificed in exchange for greater mobility. In those species having only the scapula, no joint exists between the forelimb and the thorax, the only attachment being muscular. The shoulder girdle is the anatomical mechanism that allows for all upper arm and shoulder movement in humans. The shoulder girdle consists of five muscles that attach to the clavicle and scapula and allow for the motion of the sternoclavicular joint (connection between sternum and clavicle) and acromioclavicular joint (connection between clavicle and scapula). The five muscles that comprise the function of the shoulder girdle are the trapezius muscle (upper, middle, and lower), levator scapulae muscle, rhomboid muscles (major and minor), serratus anterior muscle, and pectoralis minor muscle. The shoulder girdle is a complex of 5 joints that can be divided into two groups. 3 of these joints are true anatomical joints, while 2 are physiological ("false") joints. Within each group, the joints are mechanically linked so that both groups simultaneously contribute to the different movements of the shoulder to variable degrees.