151. The principal muscle that carries the arm downwards is the latissimus dorsi (fig. LXXII. 2), the broadest muscle of the body, which, after having covered all the lower part of the back and loins, terminates in a thin but strong tendon which stretches to the arm, and is implanted into the humerus (fig. LXXII. 2), near the tendon of a muscle immediately to be described,—the great pectoral. When the arm is raised by the deltoid and its assistant muscles, the latissimus dorsi carries it downwards with force, and its powerful action is increased by that of other muscles which arise from the scapula and are inserted into the arm. 152. The principal muscle that carries the arm forwards towards the chest, is the great pectoral (fig. LXXI. 1), which, arising partly from the clavicle (fig. LXXI. 1), partly from the sternum (fig. LXXI. 1), and partly from the cartilages of the second, third, fourth, fifth, and sixth ribs (fig. LXXI. 1), covers the greater part of the breast (fig. LXXI. 1). Its fibres, converging, terminate in a strong tendon, which is inserted near the tendon of the longissimus dorsi into the humerus, about four inches below its head (fig. LXXI. 1). These two muscles form the axilla or armpit, the anterior border of the axilla consisting of the pectoral muscle. Though each of these muscles has its own peculiar office, yet they often act in concert, thereby greatly increasing their power, and the result of their combined action is to carry the arm either directly downwards or to the side of the chest. [Illustration: Fig. LXXII. View of the muscles seated on the back part of the trunk that act upon the shoulder and arm. 1. The muscle called the trapezius; 2. the latissimus dorsi; 3. the deltoid.] 153. Some of the muscles that elevate the arm carry it inwards, and others outwards; the muscles that carry it forwards likewise carry it inwards; while of the muscles that pull it downwards, some direct it forwards and inwards, and others backwards and outwards (151 and 152). 154. It has been already stated that the shoulder-joint is completely surrounded by the muscular fibres or the tendinous expansions of several of these powerful muscles, which have a far greater effect in maintaining the head of the humerus in its socket than the fibrous capsule of the joint; the latter being necessarily loose, in order to allow of the extended and varied motions of the arm, whereas the muscles that encompass the joint adhere closely and firmly to it. Moreover, by virtue of their vital power these muscles act with an efficiency which a mere ligamentous band is incapable of exerting; for they apportion the strength of resistance to the separating force, and react with an energy proportioned to the violence applied.