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.
[Illustration: