306 FASCIA 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 aimular 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 aimular 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 fiexor 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 j oint. 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. — Superflcially, 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 metacarpi magnus. 



