The clavicle, or collar bone, is a slender bone with a double curve
like an italic _f_, and extends from the outer angle of the shoulder-blade
to the top of the breastbone. It thus serves like the keystone of an arch
to hold the shoulder-blade firmly in its place, but its chief use is to
keep the shoulders wide apart, that the arm may enjoy a freer range of
motion. This bone is often broken by falls upon the shoulder or arm.

The humerus is the strongest bone of the upper extremity. As already
mentioned, its rounded head fits into the socket of the shoulder-blade,
forming a ball-and-socket joint, which permits great freedom of motion.
The shoulder joint resembles what mechanics call a universal joint, for
there is no part of the body which cannot be touched by the hand.

[Illustration: Fig. 18.--Left Scapula, or Shoulder-Blade.]

When the shoulder is dislocated the head of the humerus has been forced
out of its socket. The lower end of the bone is grooved to help form a
hinge joint at the elbow with the bones of the forearm (Fig. 27).

44. The Forearm. The forearm contains two long bones, the
ulna and the radius. The ulna, so called because it forms
the elbow, is the longer and larger bone of the forearm, and is on the
same side as the little finger. It is connected with the humerus by a
hinge joint at the elbow. It is prevented from moving too far back by a
hook-like projection called the _olecranon process_, which makes the sharp
point of the elbow.

The radius is the shorter of the two bones of the forearm, and is on
the same side as the thumb. Its slender, upper end articulates with the
ulna and humerus; its lower end is enlarged and gives attachment in part