488 THE MUSCLES AND FASCIAE. 



The Extensor brevis pollicis (Extensor primi internodii pollicis), the smallest 

 muscle of this group, lies on the inner side of the preceding. It arises from the pos- 

 terior surface of the shaft of the radius, below the Extensor ossis metacarpi pollicis, 

 and from the interosseous membrane. Its direction is similar to thai of the 

 Extensor ossis metacarpi pollicis, its tendon passing through the same groove on 

 the outer side of the styloid process, to be inserted into the base of the first phalanx 

 of the thumb. 



Relations. The same as those of the Extensor ossis metacarpi pollicis. 



The Extensor longus pollicis (Extensor secundi internodii pollicis) is much larger 

 than the preceding muscle, the origin of which it partly covers in. It arises from 

 the posterior surface of the shaft of the ulna, below the origin of the Extensor 

 ossis metacarpi pollicis, and from the interosseous membrane. It terminates in a 

 tendon which passes through a separate compartment in the annular ligament, 

 lying in a narrow, oblique groove at the back part of the lower end of the radius. 

 It then crosses obliquely the tendons of the Extensor carpi radialis longior and 

 brevior, being separated from the other extensor tendons of the thumb by a 

 triangular interval, in which the radial artery is found, and is finally inserted into 

 the base of the last phalanx of the thumb. 



Relations. By its superficial surface, with the same parts as the Extensor ossis 

 metacarpi pollicis ; by its deep surface, Avith the ulna, interosseous membrane, the 

 posterior interosseous nerve, radius, the wrist, the radial vessels, and metacarpal 

 bone of the thumb. 



The Extensor indicis is a narrow, elongated muscle placed on the inner side of, 

 and parallel with, the preceding. It arises from the posterior surface of the shaft 

 of the ulna, below the origin of the Extensor longus pollicis and from the inter- 

 osseous membrane. Its tendon passes with the Extensor communis digitorum 

 through the same canal in the annular ligament, and subsequently joins the tendon 

 of the Extensor communis which belongs to the index finger, opposite the lower 

 end of the corresponding metacarpal bone, lying to the ulnar side of the tendon 

 from the .common Extensor. 



Relations. The relations are similar to those of the preceding muscles. 



Nerves. The Supinator longus, Extensor carpi radialis longior, and Anconeus 

 are supplied by branches from the musculo-spiral nerve ; the remaining muscles 

 of the radial and posterior brachial regions, by the posterior interosseous nerve. 



Actions. The muscles of the radial and posterior brachial regions, which 

 comprise all the extensor and supinator muscles, act upon the forearm, wrist, and 

 hand ; they are the direct antagonists of the pronator and flexor muscles. The 

 Anconeus assists the Triceps in extending the forearm. The chief action of the 

 Supinator longus is that of a flexor of the elbow-joint, but in addition to this it 

 may act both as a supinator or a pronator ; that is to say, if the forearm is forcibly 

 pronated it will act as a supinator, and bring the bones into a position midway 

 between supination and pronation ; and, vice versd, if the arm is forcibly supinated, 

 it will act as a pronator, and bring the bones into the same position, midway 

 between supination and pronation. The action of the muscle is therefore to throw 

 the forearm and hand into the position they naturally occupy when placed across 

 the chest. The Supinator brevis is a supinator ; that is to say, when the radius 

 has been carried across the ulna in pronation and the back of the hand is directed 

 forward, this muscle carries the radius back again to its normal position on the 

 outer side of the ulna, and the palm of the hand is again directed forward. The 

 Extensor carpi radialis longior extends the wrist and abducts the hand. It may 

 also assist in bending the elbow-joint; at all events, it serves to fix or steady this 

 articulation. The Extensor carpi radialis brevior assists the Extensor carpi radi- 

 alis longior in extending the wrist, and may also act slightly as an abductor of the 

 hand. The Extensor carpi ulnaris helps to extend the hand, but when acting 

 alone inclines it toward the ulnar side ; by its continued action it extends the 

 ' elbow-joint. The Extensor communis digitorum extends the phalanges, then the 

 wrist, and finally the elbow. It acts principally on the proximal phalanges, the 



