THE POSTERIOR RADIOULNAR REGION 481 



the intermuscular septa between it and the adjacent muscles. The fibres termi- 

 nate about the middle of the forearm in a fiat 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. There is often a bursa between a portion of the base 

 of the bone and the tendon. 



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 teres, radius, and wrist-joint; by its ulnar border, with the Extensor 

 communis digitorum. 



10. The Posterior Radioulnar Region (Fig. 359). 



The muscles in this region are divided for purposes of description into two 

 groups or layers, superficial and deep. 



The Superficial Layer. 



Extensor communis digitorum. Extensor carpi ulnaris. 



Extensor minimi digiti. Anconeus. 



The Extensor communis digitorum (m. extensor digitorum communis) is situ- 

 ated 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. It divides below into four tendons 

 which pass, together with that of the Extensor indicis, through a separate com- 

 partment of the annular ligament, lubricated by a synovial membrane. The 

 tendons then diverge, pass across the back of the hand, and are inserted into the 

 second and third phalanges of the fingers in the following manner: The outermost 

 tendon, accompanied by the Extensor indicis, goes to the index finger (Figs. 

 359 and 362); the second tendon is sometimes connected to the first by a thin 

 transverse band, and receives a slip from the third tendon (Fig. 359); it goes to 

 the middle finger; the third tendon gives off the slip to the second (Fig. 359), 

 and receives a very considerable part of the fourth tendon; the fourth, or innermost 

 tendon, divides into two parts; one goes to join the third tendon, the other, rein- 

 forced by the Extensor minimi digiti, goes to the little finger. Each tendon 

 opposite the metacarpophalangeal articulation becomes narrow and thickened, 

 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 phalangeal joint this aponeu- 

 rosis 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 accessory 



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