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Muscles of the Back

arm, muscle and fixed

MUSCLES OF THE BACK.

A great number of muscles massed together lies all along the back from the neck to the hip on each side of the middle line, filling up the hollow between the back - bone and the most projecting parts of the ribs. Their function is chiefly to straighten the back after it has been bent. Two muscles, placed above them, may be mentioned particularly. The latissimus dorsi (Pl. X.,fig. 1, 10), or broad muscle of the back, is attached to the spines of the lower half of the back-bone and to the crest of the hip-bone, and passes upwards over the lower corner of the shoulder-blade, towards the arm pit, which it covers in behind, to be inserted into a groove near the head of the upper arm bone. Its position is shown in the figure. The action of this muscle is readily understood. If the arm be elevated it will pull it downwards and backwards, and owing to the position of the groove of the arm-bone, in which it is fixed, it will also turn the arm so that the palm looks backwards, making the arm perform the move ment described in swimming. If, however, the arm be raised and fixed, the muscle will pull the body upwards towards the arm, as when the trunk is pulled up by the arms in climbing.

The trapezius (Pl. X., fig. 1, 6) covers the upper part of the back, and arises from a ridge on the occipital bone of the skull, and from the spines of some of the cervical and all the dorsal vertebrae. The‘fibres spread outwards so as to be inserted into the outer portion of the collar bone and the spine of the shoulder-blade. The muscles of the two sides are so disposed as to form a sort of "tippet," covering the upper part of the back, and stretching outwards to the shoulders. The trapezius elevates the shoulder if it acts from the head as the fixed point. If, however, the shoulder be counted the fixed point, the head will be pulled back towards one side if the muscle of that side only contract, and if the muscle of both sides contract, the head is thrown backwards, the chin being pro jected forwards by the movement. These are the two chief muscles that attach the trunk to the upper limbs behincL