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
to the bones of the wrist. This bone radiates or turns on the ulna,
carrying the hand with it.

Experiment 10. Rest the forearm on a table, with the palm up (an
attitude called supination). The radius is on the outer side and
parallel with the ulna If now, without moving the elbow, we turn the
hand (pronation), as if to pick up something from the table, the radius
may be seen and felt crossing over the ulna, while the latter has not
moved.

[Illustration: Fig. 19.--Left Clavicle, or Collar Bone. (Anterior
surface.)]

45. The Hand. The hand is the executive or essential part of the
upper limb. Without it the arm would be almost useless. It consists of 27
separate bones, and is divided into three parts, the wrist, the
palm, and the fingers.

[Illustration: Fig. 20.--Left Humerus.]

[Illustration: Fig. 21.--Left Radius and Ulna.]

The carpus, or wrist, includes 8 short bones, arranged in two rows of
four each, so as to form a broad support for the hand. These bones are
closely packed, and tightly bound with ligaments which admit of ample
flexibility. Thus the wrist is much less liable to be broken than if it
were to consist of a single bone, while the elasticity from having the
eight bones movable on each other, neutralizes, to a great extent, a
shock caused by falling on the hands. Although each of the wrist bones has
a very limited mobility in relation to its neighbors, their combination
gives the hand that freedom of action upon the wrist, which is manifest in
countless examples of the most accurate and delicate manipulation.

The metacarpal bones are the five l