EXTENSOR DIVISION 307 



capsule, the anterior radial artery, and the radial nerve; laterally, the common ex- 

 tensor; medially, at the elbow, the brachialis and biceps. 



Blood-supply. — Anterior radial artery. 



Nerve-supply. — Radial nerve. 



2. Common digital extensor (M. extensor digitalis communis). '— This muscle 

 -lies lateral to the foregoing, which it resembles in general form, although less bulky. 



Origin.~~{l) The front of the distal extremity of the humerus, in and lateral 

 to the coronoid fossa; (2) the lateral tuberosity on the proximal extremity of the 

 radius, the lateral ligament of the elbow, and the lateral bordex of the radius at the 

 junction of its proximal and middle thirds; (3) the lateral surface of the shaft of 

 the ulna; (4) the fascia of the forearm. 



Insertion. — (1) The extensor process of the third phalanx; (2) the dorsal sur- 

 face 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, although the division is always more or less artificial 

 so far as the muscular part is concerned. The htmieral head (Caput humerale), 

 which constitutes the bulk of the muscle, arises from the front of the lateral epi- 

 condyle of the humerus in common with the extensor carpi; the tendon of origin 

 is adherent to the capsule of the elbow joint. 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 gradually inclines medially, 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 synovial sheath which begins about three inches' (ca. 7-8 cm.) 

 above the carpus, and terminates at the proximal -end of the metacarpus. At the 

 fetlock a biorsa 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. 568) . - The larger of these is the radial head 

 (Caput radiale) f it arises from the lateral tuberosity and border of the radius, and 

 from the lateral ligament of the elbow joint. The flat belly is succeeded by a deli- 

 cate tendon, which accompanies the principal tendon over the carpus (included in 

 the same sheath), and then passes outward to fuse mth the tendon of the lateral 

 extensor, or it may continue downward between the common and lateral extensor 

 tendons to the fetlock. Usually a slip is detached which is inserted on the prox- 

 imal extremity of the first phalanx, or ends in the fascia here. The smaller and 

 deeper division is the ulnar head (Caput ulnare) f it 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 joint 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 medially, 



1 Termed also the anterior extensor of the phalanges or extensor pedis. .. : ■ 



2 This (formerly called the muscle of Phillips) is considered to represent the part of the 

 common extensor for the fourth and fifth digits. " . ' ■ " 



^ Martin considers that this muscle (formerly termed the muscle of Thiernesse) represents 

 the extensor indiois proprius and the part of the common extensor for the second digit. ■ - 



