452 MYOLOGY 



The Extensor carpi radialis longus (Extensor carpi radialis longior) is placed partly 

 beneath the Brachioradialis. It arises from the lower third of the lateral supracon- 

 dylar ridge of the humerus, from the lateral intermuscular septum, and by a few 

 fibers from the common tendon of origin of the Extensor muscles of the forearm. 

 The fibers end at the upper third of the forearm in a flat tendon, which runs along 

 the lateral border of the radius, beneath the Abductor pollicis longus and Extensor 

 pollicis brevis ; it then passes beneath the dorsal carpal ligament, where it lies in a 

 groove on the back of the radius common to it and the Extensor carpi radialis brevis, 

 immediately behind the styloid process. It is inserted into the dorsal surface of 

 the base of the second metacarpal bone, on its radial side. 



The Extensor carpi radialis brevis (Extensor carpi radialis brevior) is shorter and 

 thicker than the preceding muscle, beneath which it is placed. It arises from the 

 lateral epicondyle of the humerus, by a tendon common to it and the three following 

 muscles; from the radial collateral ligament of the elbow-joint; from a strong 

 aponeurosis which covers its surface; and from the intermuscular septa between it 

 and the adjacent muscles. The fibers end about the middle of the forearm in a 

 flat tendon, which is closely connected with that of the preceding muscle, and 

 accompanies it to the wrist; it passes beneath the Abductor pollicis longus and 

 Extensor pollicis brevis, then beneath the dorsal carpal ligament, and is inserted 

 into the dorsal surface of the base of the third metacarpal bone on its radial side. 

 Under the dorsal carpal ligament the tendon lies on the back of the radius in 

 a shallow groove, to the ulnar side of that which lodges the tendon of the Extensor 

 carpi radialis, longus, and separated from it by a faint ridge. 



The tendons of the two preceding muscles pass through the same compartment 

 of the dorsal carpal ligament in a single mucous sheath. 



Variations. Either muscle may split into two or three tendons of insertion to the second and 

 third or even the fourth metacarpal. The two muscles may unite into a single belly with two 

 tendons. Cross slips between the two muscles may occur. The Extensor carpi radialis inter- 

 medius rarely arises as a distinct muscle from the humerus, but is not uncommon as an accessory 

 slip from one or both muscles to the second or third or both metacarpals. The Extensor carpi 

 radialis accessorius is occasionally found arising from the humerus with or below the Extensor carpi 

 radialis longus and inserted into the first metacarpal, the Abductor pollicis brevis, the First dorsal 

 interosseous, or elsewhere. 



The Extensor digitorum communis arises from the lateral epicondyle of the 

 humerus, by the common tendon; from the intermuscular septa between it and the 

 adjacent muscles, and from the antibrachial fascia. It divides below into four 

 tendons, which pass, together with that of the Extensor indicis proprius, through 

 a separate compartment of the dorsal carpal ligament, within a mucous sheath. 

 The tendons then diverge on the back of the hand, and are inserted into the second 

 and third phalanges of the fingers in the following manner. Opposite the meta- 

 carpophalangeal articulation each tendon is bound by fasciculi to the collateral 

 ligaments and serves as the dorsal ligament of this joint; after having crossed the 

 joint, it spreads out into a broad aponeurosis, which covers the dorsal surface of 

 the first phalanx and is reinforced, in this situation, by the tendons of the Inter- 

 ossei and Lumbricalis. Opposite the first interphalangeal joint this aponeurosis 

 divides into three slips; an intermediate and two collateral: the former is inserted 

 into the base of the second^ phalanx; and the two collateral, which are continued 

 onward along the sides of the second phalanx, unite by their contiguous margins, 

 and are inserted into the dorsal surface of the last phalanx. As the tendons cross 

 the interphalangeal joints, they furnish them \vith dorsal ligaments. The tendon 

 to the index finger is accompanied by the Extensor indicis proprius, which lies 

 on its ulnar side. On the back of the hand, the tendons to the middle, ring, and 

 little fingers are connected by two obliquely placed bands, one from the third 

 tendon passing downward and lateralward to the second tendon, and the other 



