FLEXOR DIVISION 313 
the forearm the ulnar vessels and nerve lie between the lateral edge of this muscle 
and the lateral flexor of the carpus. 
Blood-supply.—UInar and median arteries. 
Nerve-supply—Ulnar and median nerves. 
3. Ulnaris lateralis (or lateral flexor of the carpus)..—This muscle lies on the 
lateral face of the forearm, behind the lateral extensor of the digit. 
Origin.—The lateral epicondyle of the humerus, behind and below the lateral 
ligament. 
Insertion.—(1) The lateral surface and proximal border of the accessory carpal 
bone; (2) the proximal extremity of the lateral (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 lateral surface of the accessory carpal bone, enveloped by 
a synovial sheath, to reach its insertion on the lateral 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? (M. flexor digitalis superficialis)—This muscle is 
situated in the middle of the flexor group, chiefly under cover of the middle flexor 
of the carpus. 
Origin.—(1) The medial epicondyle of the humerus; (2) a ridge on the pos- 
terior surface of the radius, below its middle and near the medial border. 
Insertion —(1) The eminences on the proximal extremity of the second phalanx, 
behind the collateral ligaments; (2) the distal extremity of the first phalanx, also 
behind the collateral ligaments. 
Action.—To flex the digit and carpus and to extend the elbow. 
Structure-—The fleshy part of the muscle is the humeral head (Caput humerale); 
it 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, from which 
it is therefore somewhat difficult to separate. Near the carpus it is succeeded by a 
| strong, thick tendon which passes down through the carpal canal and is enveloped 
by a synovial sheath, in common with the deep flexor. This, the carpal sheath 
(Vagina carpea), begins three or four inches (8-10 em.) above the carpus, and ex- 
tends downward nearly to the middle of the metacarpus. Below the carpus the 
tendon becomes flattened and broader and at the fetlock it widens greatly. Near 
the fetlock it forms a ring through which the tendon of the deep flexor passes (Fig. 
286). Here the two tendons are bound down in the sesamoid groove by the volar 
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 di- 
verge to reach their points of insertion, and between these branches the tendon of 
_ 1 Also known as the flexor carpi (s. metacarpi) externus or extensor carpi ulnaris. Morpho- 
logically it belongs to the extensor group. 
? Also commonly known as the flexor perforatus or superficial flexor of the phalanges. 
