Pectoral 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 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 five joints that can be divided into two groups. Three of these joints are true anatomical joints while two 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.:20 In the first group, the scapulohumeral or glenohumeral joint is the anatomical joint mechanically linked to the physiological subdeltoid or suprahumeral joint (the 'second shoulder joint') so that movements in the suprahumeral joint results in movements in the glenohumeral joint. In the second group, the scapulocostal or scapulothoracic joint is the important physiological joint that can not function without the two anatomical joints in the group, the acromioclavicular and sternoclavicular joints, i.e. they join both ends of the clavicle.:20 The glenohumeral joint is the articulation between the head of the humerus and the glenoid cavity of the scapula. It is a ball and socket type of synovial joint with three rotatory and three translatory degree of freedom. The glenohumeral joint allows for adduction, abduction, medial and lateral rotation, flexion and extension of the arm. The acromioclavicular joint is the articulation between the acromion process of the scapula and the lateral end of the clavicle. It is a plane type of synovial joint. The acromion of the scapula rotates on the acromial end of the clavicle. The sternoclavicular joint is the articulation of the manubrium of the sternum and the first costal cartilage with the medial end of the clavicle. It is a saddle type of synovial joint but functions as a plane joint. The sternoclavicular joint accommodates a wide range of scapula movements and can be raised to a 60° angle.