CHAPTER XII.
THE UPPER EXTREMITIES.
The upper extremities include the shoulders, arms,
forearms, wrists, and hands and contain each thirty-two
bones. The bones of the two shoulders taken together
are called the shoulder girdle and consist of the
two clavicles or collar bones and the two scapulæ or shoulder
blades, which together make an almost complete
girdle of the shoulders.
The clavicle is a long slender bone extending almost horizontally from the sternum to the scapula and can be felt for its whole length in the living. For the inner two-thirds it is convex anteriorly, for the outer third concave. In woman it is generally less curved, smoother, and more slender than in man, and as bone is rough when the muscles attached are powerful, the right clavicle, being used more, is generally rougher and thicker than the left. Among the muscles attached are the large neck muscle, the sterno-cleido-mastoid, whose tendons form the presternal notch, the trapezius, the pectoralis major, and the deltoid.
Being slender and superficial the clavicle is most frequently broken of any bone in the body, generally by indirect violence, as by falling with the hand out, though old people in such a case are apt to get Colles' fracture at the wrist. The bone generally gives way at the juncture of the outer and middle thirds, with displacement of the parts inward, so that the fracture is seldom compound. Since, however, the main vessels of the upper arm, with their nerves, lie beneath the clavicle, there is danger of their being punctured. Such serious injury is guarded against by the presence of
