THE SHOULDER AND ARM
The shoulder girdles attach the arms to the axial skele-ton. Each consists of a scapula (shoulder blade) and clavicle (collarbone). Thescapula is a large, flat bone with several projections (the spine of the scapula, the coracoid process) that anchor some of the muscles that move the upper arm and the forearm. A shallow depression called the glenoid fossa forms a ball-and-socket joint with the humerus, the bone of the upper arm (Fig. 6–12).
Each clavicle articulates laterally with a scapula and medially with the manubrium of the sternum. In this position the clavicles act as braces for the scapulae and prevent the shoulders from coming too far for-ward. Although the shoulder joint is capable of a wide range of movement, the shoulder itself must be rela-tively stable if these movements are to be effective.
The humerus is the long bone of the upper arm. In Fig. 6–12, notice the deltoid tubercle (or tuberosity); the triangular deltoid muscle that caps the shoulder joint is anchored here. Proximally, the humerus forms a ball-and-socket joint with the scapula. Distally, the humerus forms a hinge joint with the ulna of the forearm. This hinge joint, the elbow, permits move-ment in one plane, that is, back and forth with no lat-eral movement.
The forearm bones are the ulna on the little finger side and the radius on the thumb side. The semilunar notch of the ulna is part of the hinge joint of the elbow; it articulates with the trochlea of the humerus. The radius and ulna articulate proximally to form a pivot joint, which permits turning the hand palm up to palm down. You can demonstrate this yourself by holding your arm palm up in front of you, and noting that the radius and ulna are parallel to each other. Then turn your hand palm down, and notice that your upper arm does not move. The radius crosses over the ulna, which permits the hand to perform a great vari-ety of movements without moving the entire arm.
The carpals are eight small bones in the wrist; gliding joints between them permit a sliding move-ment. The carpals also articulate with the distal ends of the ulna and radius, and with the proximal ends of the metacarpals, the five bones of the palm of the hand. All of the joints formed by the carpals and metacarpals make the hand very flexible at the wrist (try this yourself: flexion to extension should be almost 180 degrees), but the thumb is more movable than the fingers because of its carpometacarpal joint. This is a saddle joint, which enables the thumb to cross over the palm, and permits gripping.
The phalanges are the bones of the fingers. There are two phalanges in each thumb and three in each of the fingers. Between phalanges are hinge joints, which permit movement in one plane. Important parts of the shoulder and arm bones are described in Table 6–3
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