EXTENSOR COMMUNI9 DIGITORUM. 



231 



The fourth tendon lies close to the third, and divides below into two parts, the 

 outer, which is usually the larger, being destined for the ring ringer, and uniting 

 wifch the foregoing tendon over the head of the metacarpal bone, while the inner 

 joins the outer division of the extensor minimi digiti tendon. 



On the fingers the tendons have the following arrangement. Opposite the 

 metacarpo-phalangeal articulation the tendon is bound down by bands of transverse 

 fibres, which pass forwards on each side to become continuous with the anterior 

 ligament of the joint. It then expands, and is joined by tendinous slips from the 

 interosseous muscles, and on the radial side also by the tendinous insertion of the 

 lumbrical muscle of the finger, the whole forming an aponeurosis which covers the 



Fig. 239. SUPERFICIAL MUSCLES AND TEN- 

 DONS ON THE BACK OP THE WRIST AND 



HAND. (Modified from Bourgery. ) i 



The posterior annular ligament of the 

 wrist is represented. 1, extensor ossis meta- 

 carpi pollicis ; 1', its insertion ; 2, extensor 

 brevis pollicis ; 2', its insertion ; 3, tendon 

 of extensor longus pollicis ; 4, extensor com- 

 munis digitorum ; 4', tendon to the middle 

 finger, receiving the insertion of the second 

 and third dorsal interosseous muscles ; 4", 

 division of the tendon into three portions, 

 of which the median is inserted into the 

 second phalanx, the two lateral passing on 

 to be inserted at 4'", into the terminal 

 phalanx ; 5, extensoi- minimi digiti ; 5', its 

 junction with the slip of the common ex- 

 tensor ; 6, placed on the lower end of the 

 ulna, points to the extensor carpi ulnaris ; 

 6', insertion of this muscle into the base of 

 the fifth metacarpal bone ; 7, part of the 

 fiexor carpi ulnaris ; 8, placed on the os 

 magnum, points to the insertion of the ex- 

 tensor carpi radialis brevior ; 8', placed on 

 the base of the second metacarpal bone, 

 points to the insertion of the extensor carpi 

 radialis longior ; 9, tendon of extensor 

 indicis ; 10, small part of the adductor pol- 

 licis ; 11, first dorsal interosseous or abduc- 

 tor indicis : in the other three interosseous 

 spaces are seen in succession, from the 

 radial side inwards, the insertion of the 

 first palmar, second dorsal, third dorsal, 

 second palmar, fourth dorsal, and third palmar interosseous muscles ; 12, abductor minimi digiti. 



back of the first and second phalanges and ends upon the third phalanx. Over the 

 lower part of the first phalanx the aponeurosis divides into three slips ; the central 

 one is much thinner than the others and is inserted into the base of the second 

 phalanx ; the two lateral parts, continuing onwards, join together below, and are 

 inserted into the base of the last phalanx. A thin layer of transverse and oblique 

 fibres bridges over the interval between the three parts of the aponeurosis and binds 

 the lateral slips down to the sides of the first interphalangeal articulation. 



The index finger portion of the muscle is the most distinct ; its fleshy belly is confined to 

 the middle two-fourths of the forearm. The middle finger portion is the strongest ; its tendon 

 becomes free from muscular fibres at the middle of the forearm. The fourth tendon is the 

 smallest, and receives fleshy fibres as far as the wrist. 



Varieties. The varieties of the extensor communis digitorum resolve themselves chiefly 

 into the following, viz., 1st, the occasional deficiency of one or more of the tendons of 

 insertion, and 2nd, more frequently an increase in their number. This last goes in some 

 instances to the extent of doubling the tendon to each of the fingers, and even of tripling it 



