324 THE MUSCLES 



intermuscular septa which separate it from the origins of the adjacent muscles; and 

 (3) the external lateral ligament of the elbow-joint. 



Insertion. — The back of the bases of the second and third metacarpal bones 

 near the Inwvr part of their line of contact. 



Structure. — This nmscle is thicker than the preceding, and its fibres have a 

 more decitledly penniform arrangement, as they ])ass from their long fibrous origin 

 ol)liiiuelv downwards and forwards to the tendon which appears first ui)on the inner 

 border of the anterior aspect of the muscle about the middle, and is free at the 

 junction of the lower and middle thirds of the forearm. It passes through the 

 second compartment of the posterior annular ligament with the tendon of the pre- 

 ceding muscle. The tendons of this and the preceding muscle are ' invested in a 

 synovial sheath as they pass through the posterior annular ligament, and a small 

 bursa lies Ix^neath each of them close to its insertion. 



Nerve-supply. — From the posterior cord of the lirachial plexus (through the 

 sixth and seventh cervical nerves) by filaments from the posterior interosseous 

 branch of the musculo-spiral nerve which enter the upper part of the anterior 

 surface of the muscle. 



Action. — (1) To extend the wrist; and (2) to feebly extend the elbow-joint. 



Relations. — It is covered by the extensor carpi radialis longiorand the tendons 

 of the three thumb extensors. It lies upon the supinator brevis, the pronator radii 

 teres, the outer surface of the radius and the wrist- joint. 



Variations. — Its muscular portion may be blended witli that of the preceding muscle, or slijis 

 may pass between the muscles. The tendon is often divided, and may send separate insertions 

 to the second and third metacarpal bones. 



MUSCLES OF THE BACK OF THE FOREARM 



The muscles upon the back of the forearm form two lavers: the superficial, 

 consisting of those arising from the back of *the external condyle of the humerus; 

 and the deep layer, of those which, with one exception, arise from the bones of the 

 forearm only. 



The superficial layer continues upon the back of the forearm the series of the 

 radial extensors. It consists of four muscles: the extensor communis digitorum; 

 the extensor minimi digiti; the extensor carpi ulnaris; and the anconeus. 



Superficial Layer 



1. EXTENSOR COMMUNIS DIGITORUM 



The extensor communis digitorum — named from its conmion action upon 

 the four lingers — is fusiform and somewhat flattened, and divides below into four 

 tendons. 



Origin. — (l) The common tendon from the lower part of the external condyle; 

 (2) the deep fascia of the forearm; and (3) the intermuscular se})ta which separate 

 it from the extensor carpi radialis brevior, the extensor minimi digiti, and the 

 supinator brevis. 



Insertion. — (1) The lateral ligaments of the metacarpo-phalangeal joints of 

 the four fingers; the back of the bases of (2) the second, and (3) the third 

 phalanges of the four fingers. 



Structure. — Arising from the interior of tlie aponeurotic case formed liy the 

 deep fascia and the internuiscular septa, the fleshy fibres soon break \\\) into se]ia- 

 rate masses, and converge upon the four tendons which, beginning al)Out the middle 

 of the forearm, l)ecome free a short distance above the wrist, and lie side by side in 

 the fourth compartment of the posterior annular ligament; those for the ring and 

 little fingers being more closely connected than the others. After their emergence 

 at the lower border of the ligament, the four tendons, which still retain their flat- 



