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.
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.
155. The bones of the fore-arm are two, the ulna and the radius (figs. LXIX. and LXXIII.). The ulna is essentially the bone of the elbow (figs. LXIX. 5, and LXXIII. 3); the radius that of the hand (fig. LXXV.). Supposing the arm to hang by the side of the body, and the palm of the hand to be turned forwards, the ulna, in apposition with the little finger, occupies the inner; and the radius, in apposition with the thumb, occupies the outer part of the fore-arm (fig. XXXIV. 3).
1. The internal condyle of the humerus; 2. the external
condyle of the humerus; 3. the olecranon process of the
ulna; 4. the head of the radius.