THE POSTERIOR BRACHIAL REGION. 485 



minate 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, lying in the 

 same groove on the posterior surface of the radius ; it passes beneath the extensor 

 tendons of the thumb, then -beneath the annular ligament, and, diverging some- 

 what from its fellow, is inserted into the base of the metacarpal bone of the middle 

 finger, on its radial side. 



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

 of the annular ligament, and are lubricated by a single synovial membrane, but are 

 separated from each other by a small vertical ridge of bone as they lie in the 

 groove at the back of the radius. 



Relations. By its superficial surface, with the Extensor carpi radialis longior, 

 and with the Extensor muscles of the thumb which cross it ; by its deep surface, 

 with the Supinator brevis. tendon of the Pronator radii teres, radius, and wrist- 

 joint ; by its ulnar border, with the Extensor communis digitorum. 



Posterior Radio-Ulnar Region (Fig. 308). 



Superficial Layer. 



Extensor communis digitorum. Extensor carpi ulnaris. 



Extensor minimi digiti. Anconeus. 



The Extensor communis digitorum is situated at the back part of the forearm. 

 It arises from the external condyle of the humerus by the common tendon, from 

 the deep fascia, and the intermuscular septa between it and the adjacent muscles. 

 Just below the middle of the forearm it divides into three tendons, which pass, 

 together with the Extensor indicis, through a separate compartment of the annular 

 ligament, lubricated by a synovial membrane. The tendons then diverge, the 

 innermost one dividing into two ; and all, after passing across the back of the 

 hand, are inserted into the second and third phalanges of the fingers in the 

 following manner : Each tendon becomes narrow and thickened opposite the meta- 

 carpo-phalaugeal articulation, and gives off a thin fasciculus upon each side of the 

 joint, which blends with the lateral ligaments and serves as the posterior ligament ; 

 after having passed the joint it spreads out into a broad aponeurosis, which covers 

 the whole of the dorsal surface of the first phalanx, being reinforced, in this 

 situation, by the tendons of the Interossei and Lumbricales. Opposite the first 

 phalanges! joint this aponeurosis divides into three slips, a middle and two lateral : 

 the former is inserted into the base of the second phalanx ; and the two lateral, 

 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 phalangeal joints they furnish them with 

 posterior ligaments. The tendons of the middle, ring, and little fingers are 

 connected together, as they cross the hand, by small, oblique, tendinous slips, or 

 ' fa : those on each side of the ring finger are strong, and bind the tendon of 

 this finger closely to those of the middle and little finger, so that it cannot, in 

 general, be freely extended without moving the other two. Sometimes there is 

 also a thin slip between the tendons of the index and middle fingers. The tendons 

 of the index and little fingers also receive, before their division, the special extensor 

 tendons belonging to them. 



Relations. By its */ '-rticial surface, with the fascia of the forearm and hand, 

 the posterior annular ligament, and integument; by its deep surface, with the 

 Supinator T)revis, the Extensor muscles of the thumb and index finger, the 

 posterior interosseous vessels and nerve, the wrist-joint, carpus, metacarpus, and 

 phalanges ; by its radial border, with the Extensor carpi radialis brevior ; by its 

 ulnar border, with the Extensor minimi digiti and Extensor carpi ulnaris. 



The Extensor minimi digiti is a slender muscle placed on the inner side of 

 the Extensor communis, with which it is generally connected. It arises from the 

 common tendon by a thin, tendinous slip, and from the intermuscular septa 

 between it and the adjacent muscles. Its tendon runs through a separate 



