306 FASCL® AND MUSCLES OF THE HORSE 
the grooves and binding down the extensor tendons and their synovial sheaths. Be~ 
hind it is greatly thickened and forms the volar annular or transverse ligament of 
the carpus (Lig. carpi transversum). This stretches across from the accessory car- 
pal bone to the medial collateral ligament and the proximal extremity of the medial 
metacarpal bone. It thus completes the carpal canal (Canalis carpi), in which lie 
the flexor tendons, the carpal synovial sheath, the common digital artery, and 
the medial volar nerve. 
The superficial fascia of the metacarpus and digit presents no special features, 
but the deep fascia (Fascia metacarpea et digitalis) is complicated by the existence 
of several annular ligaments. In the metacarpus it is hardly distinguishable from 
the periosteum in front. On the proximal part of the volar surface it forms a strong 
and close sheath for the flexor tendons, and is attached to the metacarpal bone 
on each side. Lower down and between the annular ligaments it is thin. On the 
flexion surface of the fetlock joint it is much thickened by fibers passing transversely 
from one sesamoid bone to the other, forming the volar annular ligament of the fet- 
lock, which binds down the flexor tendons in the sesamoid groove and converts the 
latter into a canal. Distal to this is a second thick quadrilateral sheet, the prox- 
imal digital annular ligament (Lig. vaginale), which covers and is adherent to the 
tendon of the superficial flexor. It is attached on either side by two bands to the 
ends of the borders of the first phalanx, thus firmly binding down the flexor tendons. 
A little further down a crescentic fibro-elastic sheet, the distal digital annular liga- 
ment, covers the terminal expansion of the deep flexor tendon. It is attached 
on either side by a strong band to the side of the first phalanx about its middle; its 
superficial face is largely covered by the digital cushion and its deep surface is in 
great part adherent to the deep flexor tendon. It is also connected with the so- 
called tendon or ligament of the ergot (Fig. 572). This is a thin and narrow 
fibrous band, which begins in the fibrous basis of the ergot, as the mass of horn at the 
fetlock is called. It descends to the side of the pastern joint, crossing over the 
digital artery and nerve; here it widens out and blends with the fibro-elastic sheet 
just described. 
A. EXTENSOR DIVISION 
1. Extensor carpi radialis (M. radialis dorsalis).'—This is the largest muscle 
of the extensor division, and lies on the dorsal surface of the radius. 
Origin—(1) The lateral condyloid crest of the humerus; (2) the coronoid 
fossa; (3) the deep fascia of the arm and forearm and the intermuscular septum 
between this muscle and the common extensor. 
Insertion.—The metacarpal tuberosity. 
Action.—To extend and fix the carpal joint and to flex the elbow joint. 
Structure—The tendon of origin blends with that of the common extensor 
and is adherent to the capsule of the elbow joint. 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 dorsal annular ligament and | 
invested with a synovial sheath. The latter begins three to four inches (ca. 8- 
10 cm.) above the carpus and extends to the middle of the carpus. Distal to this 
the tendon is attached to the joint capsule, but there is usually a small bursa at the 
level of the third carpal bone. In the distal 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 short biceps tendon, the radius, the carpal joint | 
1 Also commonly termed the extensor metacarpl magnus. 
