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.
[Illustration:
FIG. 190.—MUSCLES OF LEFT THORACIC LIMB OF HORSE, FROM ELBOW DOWNWARD,
ANTERIOR VIEW.
_a_, Extensor carpi radialis; _g′_, superficial pectoral muscle.
(After Ellenberger-Baum, Anat. für Künstler.)
]
_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. 443). The
larger of these (Caput radiale, muscle of Phillips)[59] 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)[60]
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.
[Illustration:
FIG. 191.—MUSCLES OF LEFT THORACIC LIMB OF HORSE FROM ELBOW DOWNWARD;
EXTERNAL VIEW.
_a_, Extensor carpi radialis; _g_, brachialis; _g′_, anterior
superficial pectoral; _c_, anterior or common digital extensor; _e_,
flexor carpi externus. (After Ellenberger-Baum, Anat. für Künstler.)
]
_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=.
[Illustration: