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 —(1) 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 border 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 humeral 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 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. 568). The larger of these is the radial head 
(Caput radiale) ;? 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 with 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);? 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, 
1Termed also the anterior extensor of the phalanges or extensor pedis. 
* This (formerly called the muscle of Phillips) is considered to represent the part of the 
mmon extensor for the fourth and fifth digits. 
3 Martin considers that this muscle (formerly termed the muscle of Thiernesse) represents 
‘he extensor indicis proprius and the part of the common extensor for the second digit. 
