B. Flexor Division

1. Flexor carpi internus (M. flexor carpi radialis s. radialis volaris; internal flexor of the metacarpus).—This muscle lies on the inner surface of the forearm, immediately behind the inner border of the radius.

Origin.—The flexor (internal) epicondyle of the humerus, below and behind the lateral ligament.

Insertion.—The proximal end of the internal (second) metacarpal bone.

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

Structure.—The muscle has a short tendon of origin, which is succeeded by a somewhat flattened, fusiform belly. The tendon of insertion begins near the distal fourth of the radius. It is provided with a synovial sheath which begins two or three inches (ca. 5 to 8 cm.) above the carpus and extends almost to the insertion of the tendon.

Relations.—Superficially, the skin and fascia, the posterior superficial pectoral and the tensor fasciæ antibrachii; deeply, the elbow joint, the radius, the deep flexor, the flexor carpi medius, the posterior radial vessels, and the median nerve. At the elbow the artery and nerve lie in front of the muscle, but below they dip beneath it.

Blood-supply.—Posterior radial artery.

Nerve-supply.—Median nerve.

On removing the deep fascia on the inner surface of the elbow the student may notice a small muscle lying along the lateral ligament. This is the pronator teres, which is usually not present or a mere vestige in the horse. It arises by a small, flat tendon from the flexor epicondyle of the humerus, and is inserted into the internal lateral ligament of the elbow. On account of its small size and the fact that the forearm is fixed in the position of pronation, the muscle can have no appreciable function. It is usually represented by the long portion of the internal lateral ligament.

2. Flexor carpi medius (M. flexor carpi ulnaris s. ulnaris medialis; oblique or middle flexor of the metacarpus).—This muscle lies on the inner and posterior aspect of the forearm partly under, partly behind, the preceding muscle. It arises by two heads—humeral and ulnar.

Origin.—(1) The flexor (internal) epicondyle of the humerus just behind the preceding muscle; (2) the inner surface and posterior border of the olecranon.

Insertion.—The upper edge of the accessory carpal (pisiform) bone.

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

Structure.—The humeral head is much the larger, constituting, in fact, the bulk of the muscle. It is flattened, curved, and tapers at both ends. The ulnar head, much smaller and very thin, is covered by an aponeurosis from which many of its fibers arise. It joins the large head a little above the middle of the forearm. The tendon of insertion is short and strong; it blends with the posterior annular ligament of the carpus.

Relations.—Superficially, the tensor fasciæ antibrachii, superficial pectoral, and flexor carpi internus, the skin and fascia, and cutaneous branches of the ulnar nerve; deeply, the superficial and deep flexors of the digit. In the distal half of the forearm the ulnar vessels and nerve lie between the outer edge of this muscle and the external flexor of the carpus.

Blood-supply.—Ulnar and posterior radial arteries.

Nerve-supply.—Ulnar and median nerves.

3. Flexor carpi externus (M. extensor carpi ulnaris s. ulnaris lateralis; external flexor of the metacarpus).—This muscle lies on the outer face of the forearm, behind the lateral extensor of the digit.

Origin.—The extensor (external) epicondyle of the humerus, behind and below the lateral ligament.

Insertion.—(1) The outer surface and upper edge of the accessory carpal bone; (2) the proximal extremity of the outer (fourth) metacarpal bone.

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

Structure.—The belly of the muscle is flattened and is intersected by a good deal of tendinous tissue. There are two tendons of insertion. The short tendon is inserted into the accessory carpal bone. The long tendon is detached just above the carpus; it is smaller and rounded; it passes downward and a little forward through a groove on the outer surface of the accessory carpal bone, enveloped by a synovial sheath, to reach its insertion on the outer metacarpal bone. A synovial pouch lies under the origin of the muscle at the elbow joint, with the cavity of which it communicates.

Relations.—Superficially, the skin, fascia, and cutaneous branches of the ulnar nerve; deeply, the elbow joint, the ulna, and the flexors of the digit; in front, the lateral extensor of the digit; behind, the middle flexor of the carpus, the ulnar head of the deep flexor, and the ulnar vessels and nerve.

Blood-supply.—Interosseous, ulnar, and median arteries.

Nerve-supply.—Radial nerve.

4. Superficial digital flexor or flexor perforatus (M. flexor digitalis sublimis; superficial flexor of the phalanges).—This muscle is situated in the middle of the flexor group, chiefly under cover of the middle flexor of the carpus.

Origin.—(1) The flexor epicondyle of the humerus, between the flexor carpi internus and the deep flexor of the digit; (2) a ridge on the posterior surface of the radius, below its middle and near the internal border.

Insertion.—(1) The eminences on the proximal extremity of the second phalanx behind the lateral ligaments; (2) the distal extremity of the first phalanx, also behind the lateral ligaments.

Action.—To flex the digit and carpus, and to extend the elbow.

Structure.—The fleshy portion of the muscle or humeral head takes origin from the humerus. The radial head (Caput tendineum) consists of a strong fibrous band, usually termed the radial or superior check ligament, which fuses with the tendon near the carpus. The belly of the muscle is intersected by tendinous strands, and fuses more or less with that of the deep flexor. Near the carpus it is succeeded by a strong, thick tendon which passes down through the carpal canal, formed by the posterior ligament of the carpus, the accessory carpal bone, and the posterior annular ligament (Ligamentum carpi transversum). Here it is enveloped by a synovial sheath, in common with the deep flexor. This carpal sheath (Vagina carpea) begins three or four inches (8 to 10 cm.) above the carpus, and extends downward nearly to the middle of the metacarpus. Below the carpus the tendon becomes flattened and broader. Near the fetlock it forms a ring through which the tendon of the deep flexor passes (Fig. ). Here the two tendons are bound down in the sesamoid groove by the posterior annular ligament, which fuses more or less with the superficial flexor tendon. At the distal end of the first phalanx the tendon divides into two branches which diverge to reach their points of insertion, and between these branches the tendon of the deep flexor emerges (Fig. ). A second synovial sheath, the digital or sesamoidean (Vagina digitalis), begins at the distal third of the metacarpus, three to four inches (8 to 10 cm.) above the fetlock, and extends to the middle of the second phalanx.

Relations.—The belly of the muscle is related superficially to the ulnar head of the deep flexor, the flexor carpi medius, and, at its origin, to the ulnar vessels and nerve; deeply to the humeral head of the deep flexor. The tendon is related superficially to the skin and fascia; deeply to the deep flexor tendon.

Blood-supply.—Posterior radial artery.

Nerve-supply.—Median nerve.

5. Deep digital flexor or flexor perforans (M. flexor digitalis profundus; deep flexor of the phalanges).—The fleshy part of this muscle lies on the posterior surface of the radius, under cover of the preceding muscles. It is the largest muscle of the flexor group.

Origin.—(1) The flexor (internal) epicondyle of the humerus; (2) the inner surface of the olecranon; (3) the middle of the posterior surface of the radius and a small adjacent area of the ulna.

Insertion.—The semilunar crest of the third phalanx, and the adjacent surface of the lateral cartilage.

Action.—To flex the digit and carpus, and to extend the elbow.

Structure.—This muscle consists of three principal heads. The humeral head (Caput humerale) constitutes the bulk of the muscle. It is marked by tendinous intersections, and is separable into three secondary heads. The ulnar head (Caput ulnare, ulnaris accessorius) is much smaller, and is at first superficially situated between the outer and middle flexors of the carpus, the radial head (Caput radiale, radialis accessorius) is the smallest, and is not always present; it is situated on the distal two-thirds of the posterior surface of the radius, under the humeral head. Each of these heads is provided with a tendon. The principal tendon—that of the humeral head—begins about three or four inches (8 to 10 cm.) above the carpus. It is soon joined by the small tendon of the ulnar head, which begins about the middle of the forearm. The tendon of the radial head fuses with the principal tendon close to the carpus. The conjoined tendon passes downward through the carpal canal, being included in the carpal synovial sheath with the superficial flexor tendon, as previously described. Continuing downward, it is joined about the middle of the metacarpus by a strong fibrous band, the so-called inferior or subcarpal check ligament (Caput tendineum). This is a direct continuation of the posterior ligament of the carpus. Below this the tendon passes through the ring formed by the perforatus, then in succession over the sesamoid groove, the inferior sesamoidean ligaments, and the tendon surface of the third sesamoid, to its insertion (Figs. and 151). Its terminal part is much widened. From the distal third of the metacarpus to the distal end of the second phalanx it is inclosed in the digital synovial sheath described in connection with the perforatus. The navicular bursa or bursa podotrochlearis is found between the tendon and the third sesamoid or navicular bone. The terminal part of the tendon is bound down by the fibrous sheet described with the fascia.

Relations.—The belly of the muscle is related posteriorly to the perforatus and the middle flexor of the carpus; internally, to the internal carpal flexor, the radial check ligament, and the posterior radial vessels and median nerve; externally, to the external flexor of the carpus; anteriorly, to the radius and ulna and branches of the posterior radial artery and median nerve. Below the carpus the tendon is accompanied by the vessels and nerves of the digit. It may also be noted that the muscle is not entirely covered by the other flexors; it comes in contact with the skin and fascia on the postero-external aspect of the proximal half of the forearm, and also on the outer aspect of the distal fourth.

Blood-supply.—Posterior radial and ulnar arteries.

Nerve-supply.—Median and ulnar nerves.