EXTENSOR DIVISION 265 



togpther with vestiges of the proper extensors of the digits. Usually at least 

 two heads maj' 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 

 jioint 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 liga- 

 ment, 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 l^etween the tendon and the 

 joint capsule, but otherwise the two are adherent. The smaller head, arising 

 chiefly from the radius and ulna, is often divisil)le into two parts (Fig. 443). The 

 larger of these (Caput radiale, muscle of Phillips)^ arises from the outer tuberosity 

 and outer border of the radius, and from the lateral ligament of the ell)ow 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)" 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 pro- 

 prius; 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 inter- 

 muscular 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 tuberosit}^ 

 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 meta- 

 carpus and fetlock to its insertion. Two synovial membranes occur in connection 



1 This is considered to represent the part of the common extensor for the fourth and fifth 

 digits. 



^ Martin considers that this muscle represents the extensor indicis proprius and the part 

 of the common extensor for the second digit. 



