The lower extremity consists of an articular and a non-articular portion. The articular has a small head externally for the radius, and a pulley internally for the movements of the ulna in flexion and extension of the limb. The non-articular part has pro jections on its inner and outer aspects, the internal and external condyles; each is surmounted by a supracondylar ridge, and the internal condyle and ridge attach the muscles passing to the flexor surface of the forearm, while the external are for those passing to the extensor surface.
Before describing the two bones of the forearm, the range of movement which can take place between them should be noticed. In one position, supine, they lie parallel to each other, the radius being the more external bone, and the palm of the hand being directed forward ; in the other or prone position the radius crosses obliquely in front of the ulna, and the palm of the hand is directed backward. Not only the bones of the forearm, but those of the hand are supposed to be in the supine position when they are described.
Radius.—The radius (fig. 14) is the outer bone of the forearm, and possesses a shaft and two extremities. The upper extremity or head has a shallow, smooth cup for articulation with the humerus ; the outer margin of the cup is also smooth, for articula tion with the ulna and orbicular ligament; below the cup is a constricted neck, and immediately below the neck a tuberosity for the insertion of the biceps. The shaft of the bone possesses three surfaces for the attachment of muscles and a sharp inner border for the interosseous membrane. The lower end of the bone is much broader than the upper, and is marked posteriorly by grooves for the lodgment of tendons passing to the back of the hand: from its outer border a pointed styloid process projects downward ; its inner border has a smooth shallow fossa for articu lation with the ulna, and its broad lower surface is smooth and concave, for articulation with the scaphoid and semilunar bones of the wrist.
Ulna.—The ulna (fig. 14) is also a long bone. Its upper end is subdivided into two strong processes by a deep fossa which pos sesses a smooth surface for articulation with the humerus. The anterior process is rough in front for the insertion of the brachialis anticus, whilst the posterior or olecranon process gives insertion to the large triceps muscle of the upper arm. Im
mediately below the outer border of the great fossa is the small cavity for articulation with the side of the head of the radius. The shaft of the bone has three surfaces for the attach ment of muscles, and a sharp outer border for the interosseous membrane. The lower end, much smaller than the upper, has a pointed styloid process and a smooth articular surface, the outer portion of which is for the lower end of the radius, the lower part for moving on a cartilage of the wrist joint called the triangular fibro-cartilage.
Hand.—The hand consists of the carpus or wrist, the meta carpus or palm and the free digits, the thumb and four fingers. Anatomists describe it with the palm turned to the front, and with its axis in line with the axis of the forearm.
The carpal or wrist bones (fig. 14) are eight in number arranged in two rows, a proximal, consisting of the scaphoid, semilunar, cuneiform and pisiform, and a distal, consisting of a trapezium, trapezoid, os magnum and unciform ; the bones in each row being named in the order they are met with, from the radial or outer to the ulnar or inner side of the wrist.
The metacarpal bones are five in number (fig. 14). They are miniature long bones, and each possesses a shaft and two ex tremities. The metacarpal of the thumb is the shortest, and diverges outward from the rest ; its carpal extremity is saddle shaped, for articulation with the trapezium ; its shaft is some what compressed, and its phalangeal end is smooth and rounded, for the first phalanx of the thumb. The four other metacarpal bones belong to the four fingers. Their carpal ends articulate with the trapezoid, os magnum and unciform : their phalangeal ends articulate with the proximal phalanges of the fingers.
The number of digits in the hand is five. They are distinguished by the names of pollex or thumb, index, medius, annularis and minimus. Their skeleton consists of fourteen bones named phalanges, of which the thumb has two, and each of the four fingers three. Each is a miniature long bone, with two articular extremities and an intermediate shaft, except the terminal phalanges in which the distal end is rounded for the nail.