MUSCLES OF THE CHEST.
Some of the chest muscles will be more ap propriately considered in the section on the respiratory system, since they are specially concerned in the act of breathing, and are therefore classified together under muscles of respiration. Under muscles of respiration is included the diaphragm, a muscle which forms an internal partition between the cavities of the chest and belly. Its action is also con sidered in the above-named section. Of the other muscles of the chest the chief are the pectoralis major, or larger chest muscle (Pl. X., fig. 1, 15), and the pectoralis minor, or smaller chest muscle, and the serratus magnus. The first rises partly from the an terior portion of the collar-bone, partly from the breast-bone, and partly from the cartilages of the ribs, and, passing in front of the arm-pit, is inserted into the side of the groove in the upper arm-bone. It is the opponent of the broad muscle of the back to this extent, that it draws the arm forwards, while it co-operates with the muscle of the back in pulling the limb down from the elevated position. If the arm
be fixed in the elevated position, then it co operates with the broad muscle in pulling the trunk up towards it. The pectoralis minor lies beneath the upper part of the major, and is inserted into the coracoid process of the shoulder-blade, which it depresses, thus oppos ing the trapezius. The serratus magnus muscle (Pl. X., fig. 1, yr) is in contact with the ribs in the upper and side part of the chest, rising by nine fleshy slips from eight upper ribs, and it stretches across to the shoulder-blade to which it is attached. If the ribs be fixed it pulls the shoulder-blade from the spinal column, and allows the arm to be farther outstretched ; if the shoulder-blade be fixed, the serratus inagims, pulling from it, will raise the ribs. These are the principal muscles that attach the trunk to the upper limbs in front.