3^4 A MAlJUAL OP ANATOMY 



The tendon appears about the centre of the forearm, and becomes 

 free from fleshy fibres in the lower third. It descends beneath the 

 tendon of the long radial extensor, except near its insertion, where 

 it lies internal to that tendon. It accompanies the long tendon 

 beneath the posterior annular ligament, and passes with it through 

 the outermost groove on the back of the lower end of the radius. 



Muscles of the Back of the Forearm. — ^These are arranged in two 

 layers — superficial and deep. 



Superficial Layer. — ^The muscles of this layer, which are serially 

 continuous with those of the outer side of the forearm, are four 

 in number, namely, the extensor communis digitorum, extensor 

 minimi digiti, extensor carpi ulnaris, and anconeus. The first three 

 have a common tendon of origin, which they share with the extensor 

 carpi radialis brevior. 



• I. Extensor Communis Digitorum — Origin. — (i) The external epi- 

 condyle of the humerus by the common tendon ; (2) the deep fascia ; 

 and (3) the intermuscular septa between it and adjacent muscles. 



Insertion. — ^The muscle ends in four tendons, which pass beneath 

 the posterior annular ligament to the dorsum of the hand. Here 

 they diverge and pass to the four inner fingers, that for the index 

 finger being accompanied internally by the tendon of the extensor 

 indicis. Having crossed the metacarpo-phalangeal joints, where 

 they give fibres to their lateral ligaments, the tendons form 

 broad expansions covering the backs of the first phalanges, which 

 give insertion to the lumbricales and interossei. Towards the 

 distal end of the first phalanx the expansion divides into three 

 parts — central and two lateral. The central part is inserted into 

 the back of the base of the second phalanx. The two lateral parts 

 unite on the back of the second phalanx, and are inserted into the 

 back of the base of the distal phalanx. The index and middle 

 finger tendons are connected by a weak band of fibres. The ring- 

 finger tendon is connected by a strong band of fibres with the 

 tendon on either side of it, which explains the very limited amount 

 of extension of which the ring-finger is capable, unless the middle 

 and little fingers are extended along with it. The little finger 

 tendon may divide into two parts, one joining the ring-finger tendon 

 and the other the tendon of the extensor minimi digiti. 



Nerve-supply. — ^The posterior interosseous nerve. 



Action. — (i) To extend the first phalanges of the four inner 

 fingers, and also the second and third phalanges slightly, these 

 being extended principally by the interossei and lumbricales ; 

 and (2) to assist in extension of the wrist-joint. 



The common extensor tendons, along with that of the extensor 

 indicis, in passing beneath the posterior annular ligament, occupy 

 the broad innermost groove on the back of the radius. 



2. Extensor Minimi Digiti — Origin. — (i) The external epicondyle 

 of the humerus by the common tendon; (2) the deep fascia; and 

 (3) the intermuscular septum on either side. 



Insertion. — Having passed beneath the posterior annular liga- 



