EXTENSOR DIVISION 265 



togethor with vostip;es of tlie proper extensors of the (lip;its. Usually at least 

 two heads may be distinguished. The principal or humeral heatl (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 tlu^ susj^ensory liga- 

 ment, and the tendon thus becomes much wider. Two synovial membranes 

 facilitate the play of the tendon. The jM-oximal one is a sheath which begins about 

 three inches (ca. 7 to 8 cm.) above the carijus, and terminates at the ])r()ximal 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 Pliillii)s)' arises from the outer tul>erosity 

 and outer border of the radius, and from the lateral ligam(>nt of the elbow joint. 

 The flat belly is succeeded l)y 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 jM-oximal extremity of the first i)halanx, or ends in the fascia here. 

 The smaller and deeper division (Caput ulnare, muscle of Thiernesse)-' is usually 

 somewhat diflficult to isolat(\ It arises from the ulna close to the interosseous 

 space. It has a small rounded l)elly 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 racHus and ulna, the extensor carpi 

 obliquus, and the anterior radial vessels and radial nerve; in front and internally, 

 the extensor carjii 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 i)halanges; extensor suffraginis). — This muscle is 

 much smaller than the preceding, l)ehind 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 tuberosity 

 of the distal end of the radius, then over the carpus, and, gradually inclining toward 

 the front, but not reaching the middle fine 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. 



