390 A MANUAL OF ANATOMY 



third, and reaching to near the lower end of the bone ; and (2) the 

 lower part of the interosseous membrane. 



Insertion. — The inner side of the common extensor tendon of the 

 index finger, which it joins at the metacarpo-phalangeal joint. 



The muscle is directed downwards and outwards, and its tendon 

 passes beneath the posterior annular ligament, where it occupies 

 the innermost groove on the back of the radius, in company with 

 the common extensor tendons, beneath which it lies. 



Nerve-supply. — The posterior interosseous nerve. 



Action. — (i) The muscle is the special extensor of the index finger, 

 as in pointing ; and (2) it acts as a feeble auxiliary extensor of the 

 wrist- joint. 



The muscle is covered by the extensor communis digitorum, 

 extensor minimi digiti, and extensor carpi ulnaris. 



Posterior Annular Ligament. — ^This is a thickened portion of the 

 deep fascia of the back of the forearm, and it lies obliquely on the 

 back of the wrist. It is attached externally to the lower part of the 

 anterior border of the radius, and internally to the inner and back 

 part of the cuneiform and pisiform bones, as well as to the hypo- 

 thenar portion of the palmar fascia. Its direction is inwards and 

 downwards, and it bridges over the various grooves in this region, 

 sending deep processes to be attached to the ridges which separate 

 them. In this manner these grooves are converted into fibro- 

 osseous canals for the passage of the extensor tendons. These 

 canals are six in number, four being on the radius, one between that 

 bone and the ulna, and one on the ulna. Proceeding from without 

 inwards, the first canal corresponds with the groove on the outer 

 '"''..--/""■/'/'-' :^ ■'"I oid process of the radius, and it contains the 



surface ol tn^* ^^'^-^^ ^'"^ metacarpi pollicis and extensor brevis 



tendons of the extensor 'bsfe^^'-^^ds with the outer broad groove 



pollicis. The second canal correspoi ^e tendons of the extensor 



on the -back of the radius, and it contains'lP^'^^i^lis brevior. The 

 carpi radialis longior and extensor carpi "1^'^^^^^"^ groove on 

 inird canal corresponds with the narrow deeo^^'^^^c^ °^^^^^"g^ 

 the back of the radius, internal to the radial tubTr'cre'^ P^^^^^^^- ^he 



L^f^rl ""J"^^'"' *^^ *""^°" «f the extensor longusT^^ °" ^^'' 



fourth canal corresponds with the broad innermosT^^^ extensor 



back of the radius, and it transmits ^h^ tendZ of^V^ the pos- 



communis digitorum and extensor indids tog^^^^^^^^ 



tenor interosseous nerve and posterior branch of the ant^^*^^^" 



osseous artery The fifth canal corresponds with the^^^^^^^^^^^ 



the radius and ulna, and it contains th^ Z.ii\: ^/^u ^ve on 



trltk'1^^;. ^1'^ ^1^*^ can^l c^es^t^s^wZlhf^^^^^^^ 

 the back of the ulna, between the styloid process and hfnH^^""- 

 bone and it transmits the tendon of the^eSensor carnf1,f ^*^' 



Deep Fascia of the Back of the Hand tko a x ■ ■ Cand 

 region is continuous above w^tVfl^e ^^^ttToI^LfS fc^ 



