[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 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 long bones of the back of the hand.  They are attached to the wrist and to the finger bones, and may be easily felt by pressing the fingers of one hand over the back of the other.  The metacarpal bones of the fingers have little freedom of movement, while the thumb, unlike the others, is freely movable.  We are thus enabled to bring the thumb in opposition to each of the fingers, a matter of the highest importance in manipulation.  For this reason the loss of the thumb disables the hand far more than the loss of either of the fingers.  This very significant opposition of the thumb to the fingers, furnishing the complete grasp by the hand, is characteristic of the human race, and is wanting in the hand of the ape, chimpanzee, and ourang-outang.