A. Extensor Division

1. Extensor carpi radialis (M. extensor carpi radialis s. radialis dorsalis; extensor metacarpi magnus; anterior extensor of the metacarpus).—This is the largest muscle of the extensor division, and lies on the anterior (dorsal) surface of the radius.

Origin.—(1) The external condyloid crest of the humerus; (2) the coronoid fossa.

Insertion.—The tuberosity on the anterior (dorsal) surface of the proximal extremity of the large (third) metacarpal bone.

Action.—To extend and fix the carpal joint and to flex the elbow joint.

Structure.—The belly of the muscle is rounded, and runs out to a point at the distal third of the forearm. The tendon, which runs nearly the whole length of the fleshy portion, appears on the surface of the latter about its middle; here the muscle shows a distinctly pennate arrangement. The tendon passes through the middle groove at the distal extremity of the radius and over the capsule of the carpal joint, bound down by the anterior annular ligament and invested with a synovial sheath. The latter begins three to four inches (ca. 8 to 10 cm.) above the carpus and extends almost to the insertion of the tendon. In the lower half of the forearm the deep fascia blends with the tendon, and here the latter is joined by the long tendon of the biceps.

Relations.—Superficially, the skin, fascia, and the oblique extensor; deeply, the capsule of the elbow joint, the biceps tendon, the radius, the carpal joint capsule, the anterior radial artery, and the radial nerve; externally, the anterior or common extensor; internally, at the elbow, the brachialis and biceps.

Blood-supply.—Anterior radial artery.

Nerve-supply.—Radial nerve.

2. Anterior or common digital extensor (M. extensor digitalis communis; anterior extensor of the phalanges; extensor pedis).—This muscle lies external to the foregoing, which it resembles in general form, although less bulky.

Origin.—(1) The front of the distal extremity of the humerus, just external to the coronoid fossa; (2) the external tuberosity on the proximal extremity of the radius, the external lateral ligament of the elbow, and the external border of the radius at the junction of its proximal and middle thirds; (3) the external surface of the shaft of the ulna; (4) the fascia of the forearm.

Insertion.—(1) The extensor (or pyramidal) process of the third phalanx; (2) the anterior surface of the proximal extremities of the first and second phalanges.

Action.—To extend the digital and carpal joints, and to flex the elbow joint.

Structure.—The muscle is a compound one, representing the common extensor, together with vestiges of the proper extensors of the digits. Usually at least two heads may be distinguished. The principal or humeral head (Caput humerale) arises from the front of the extensor epicondyle of the humerus in common with the extensor carpi. Its belly is fusiform, and terminates in a point near the distal third of the radius. The tendon appears on the surface of the muscle about the middle of the belly, the arrangement being pennate. The tendon passes downward through the outer of the two large grooves on the front of the distal end of the radius, and over the capsule of the carpal joint. Passing down over the front of the metacarpus, it inclines gradually inward, reaching the middle line of the limb near the fetlock. A little below the middle of the first phalanx it is joined by the branches of the suspensory ligament, and the tendon thus becomes much wider. Two synovial membranes facilitate the play of the tendon. The proximal one is a sheath which begins about three inches (ca. 7 to 8 cm.) above the carpus, and terminates at the proximal end of the metacarpus. At the fetlock a bursa occurs between the tendon and the joint capsule, but otherwise the two are adherent. The smaller head, arising chiefly from the radius and ulna, is often divisible into two parts (Fig. ). The larger of these (Caput radiale, muscle of Phillips) arises from the outer tuberosity and outer border of the radius, and from the lateral ligament of the elbow joint. The flat belly is succeeded by a delicate tendon, which accompanies the principal tendon over the carpus (included in the same sheath), and then passes outward to fuse with the tendon of the lateral extensor. Usually a slip is detached which is inserted on the proximal extremity of the first phalanx, or ends in the fascia here. The smaller and deeper division (Caput ulnare, muscle of Thiernesse) is usually somewhat difficult to isolate. It arises from the ulna close to the interosseous space. It has a small rounded belly and is provided with a delicate tendon which may fuse with the principal tendon or may be inserted into the capsule and the fascia in front of the fetlock joint.

Relations.—The chief relations of the belly of the muscle are: superficially, the skin and fascia; deeply, the elbow joint, the radius and ulna, the extensor carpi obliquus, and the anterior radial vessels and radial nerve; in front and internally, the extensor carpi radialis; behind, the lateral extensor and the interosseous vessels.

Blood-supply.—Radial and interosseous arteries.

Nerve-supply.—Radial nerve.

3. Lateral digital extensor (M. extensor digitalis lateralis s. digiti quinti proprius; lateral extensor of the phalanges; extensor suffraginis).—This muscle is much smaller than the preceding, behind which it is situated.

Origin.—The external tuberosity of the radius and the lateral ligament of the elbow joint, the shaft of the ulna, the outer border of the radius, and the intermuscular septum.

Insertion.—An eminence on the front of the proximal extremity of the first phalanx.

Action.—To extend the digit and carpus.

Structure.—The muscle is pennate, and is inclosed in a sheath formed by the deep fascia, from which many fibers arise. The belly is thin and fusiform and terminates at the lower third of the forearm. From here the tendon (at first small and round) passes downward through the groove on the outer tuberosity of the distal end of the radius, then over the carpus, and, gradually inclining toward the front, but not reaching the middle line of the limb, it passes over the metacarpus and fetlock to its insertion. Two synovial membranes occur in connection with the tendon. A sheath envelops the tendon, beginning about three inches (ca. 6 to 8 cm.) above the carpus, and reaching to the proximal end of the metacarpus. At the fetlock a small bursa lies between the tendon and the joint capsule, but otherwise the tendon is adherent to the capsule. The tendon becomes flat and much larger below the carpus, having received the tendon of the radial head of the anterior extensor and a strong band from the accessory carpal bone.

Relations.—Superficially, the skin and fascia; deeply, the outer face of the radius and ulna; in front, the common extensor, the oblique extensor, and the interosseous artery; behind, the external flexor of the carpus and the deep flexor of the digit.

Blood-supply.—Interosseous artery.

Nerve-supply.—Radial nerve.

4. Extensor carpi obliquus (M. abductor pollicis longus et extensor pollicis brevis; oblique extensor of the metacarpus; extensor metacarpi obliquus).—This is a small muscle which curves obliquely over the distal half of the radius and the carpus.

Origin.—The external border and adjacent part of the anterior surface of the radius (the attachment area beginning at a point above the middle of the bone and extending down to its lower fourth).

Insertion.—The head of the inner (second) metacarpal bone.

Action.—To extend the carpal joint.

Structure.—The muscle is pennate and has a flat belly which curves downward, forward, and inward over the distal part of the radius. The tendon continues the direction of the muscle, crossing obliquely over the tendon of the extensor carpi radialis; it then occupies the oblique groove at the distal end of the radius, and crosses the inner face of the carpus. It is provided with a synovial sheath.

Relations.—Superficially, the skin and fascia, the lateral extensor, and the common extensor; deeply, the radius, the extensor carpi radialis, the carpal capsule, and the internal lateral ligament.

Blood-supply.—Interosseous and anterior radial arteries.

Nerve-supply.—Radial nerve.