BACK OF THE FOREARM AND HAND. 14t 



The EXTENSOR COMMUNIS DIGITORUM occupies the cen- 

 tral portion of the posterior region of the forearm ; it 

 arises from the external condyle by the tendon common to 

 the extensor muscles, and from the intermuscular septa 

 between it and the contiguous muscles ; at the lower part 

 of the arm it divides into three tendons, which pass 

 through the annular ligament in a compartment with the 

 extensor indicis ; escaping from the ligament, the most 

 internal tendon divides into two, and the four tendons pass 

 along the dorsum of the hand, forming a flattened sheath 

 for the back of each finger. Opposite the first phalangeal 

 articulation this expanded tendon divides into three slips ; 

 the central one is inserted into the base of the second 

 phalanx, the two lateral continue onward and are inserted 

 into the dorsal surface of the last phalanx. Oblique ten- 

 dinous bands connect the tendons with each other on the 

 back of the hand, and upon the fingers they are reinforced 

 )y tendinous slips from the lumbricales and interossei mus- 



The EXTENSOR MINIMI DIGITI is generally a part of the 



:tensor communis ; occasionally it is separable from it. 

 [ts origin is the same, and it passes through a separate ring 

 )f the annular ligament ; its tendon, which is split into two 

 directly afterward, terminates in an expansion on the back 

 of the little finger. 



The EXTENSOR CARPI ULNARIS arises from the external 

 condyle, and the common tendon of the extensors, and 

 from the upper part of the ulna ; its tendon passes through 

 a separate sheath of the annular ligament, just over the car- 

 pal end of the ulna, to be inserted into the base of the 

 metacarpal bone of the little finger. 



The ANCONEUS MUSCLE is a small triangular muscle 

 placed upon the posterior part of the elbow-joint, which it 

 partly covers, and is sometimes considered as a part of the 

 triceps extensor cubiti ; it arises from the outer condyle 

 by a distinct tendon posterior to the common tendon of 

 the extensor muscles, and is inserted into the radial side of 

 the olecranon and the adjacent surface of the ulna. 



The extensor muscles of the arm, which have been described, are 

 now to be divided in the middle, and their two ends reflected ; how- 

 ever careful he may have been, the student must expect the deep 

 layer of muscles to present a ragged appearance. 



The posterior interosseous artery perforates the interos- 



