EXTEXSOK COMMUNIS DIGITORUM. 



217 



onwards, are joined together towards the middle or fore part of the second 

 phalanx, and, having passed beyond this, are inserted into the last pha- 

 lanx. On the index and little fingers the tendons are joined before their 

 division, by the special extensor tendons of those digits. 



Fig. 177. — Deep PosTcraoR Muscles of the I^'ig- !'<'• 



Forearm. (A. T.) i 



(t, posterior surface of the humerus ; i, ole- 

 cranon ; c, radius ; d, lower part of the ulnar, 

 grooved for the tendon of the extensor carpi idnaris ; 

 €, liase of the second luetacariml hone ; 1, anconeus ; 

 2, riexor digitorum profundus, exposed by the re- 

 anoval of the aponeurotic tendon of 3, the flexor 

 carpi ulnaris ; + , supinator brevis ; 4, extensor 

 carpi radialis brevior, and 5, the cut tendon of the 

 extensor cai'pi radialis longior ; c, their insertion 

 into the second and third metacarpal bones ; 6, 

 extensor ossis metacarpi pollicis ; C, its insertion 

 into the first metacai-pal bone ; 7, extensor jirimi 

 internodii ijollicis ; 7', its insertion into the base of 

 the first phalanx ; 8, extensor secundi internodii 

 jwllicis ; 8', its insertion into the base of the last 

 phalanx ; 9, extensor indicis ; 9', its junction with 

 the tendon of the common extensor, which is cut 

 short : in the metacarpal spaces the foiu- dorsal 

 interossei muscles are represented, the tendons of 

 the common extensor ha\'iug been removed ; and at 

 10, the insertion of the second and third dorsal 

 interossei, by a triangular expansion, into the tendon 

 of the extensor communis, as well as the mode of 

 insertion of that tendon into the middle and last 

 phalanges, are shown. 



On the back of the hand the tendon of 

 the ring-finger gives otf two processes, 

 one on each side, the lateral vincula, 

 which pass obliquely downwards to join 

 the tendons of the middle and little 

 lingers : in consequence of this arrange- 

 ment it follows that, if those two fingers 

 be flexed, the tendon of the ring-finger 

 is held downwards, and it becomes impos- 

 sible to extend that finger by the action 

 of the muscle. The tendon of the index- 

 finger is only united to that of the 

 middle finger by a loose transverse, semi- 

 transparent baud of fibres, and is free in 

 its action. A synovial membrane invests 

 the tendons of this muscle as they pass 

 mider the annular ligament. 



VarUtics.— The varieties of the extensor communis digitomm resolve them- 

 selves chiefly into the following, viz., 1st, tlie 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 fingere, and even to tripling it to one or two of them. More 

 frequently, however, the increase of number of the tendons is limited to the 

 index or little finger alone. The muscular substance is also liable to be divided 

 more deeply tiian usual into distinct portions for a part or the whole of its length. 



