EXTENSORS OF THE THUMB. 



233 



and posterior oblique lines. It is pierced by the posterior interosseous nerve, which, 

 effects a more or less complete division of the muscle into two layers. 



Varieties. The anterior fibres of the supinator brevis not unf requently form a detached 

 slip inserted into the orbicular lig-ament. An accessory fasciculus from the external condyle 

 has been observed, also fibres inserted into the tendon of the biceps, the bursa under that 

 tendon, and the tuberosity of the radius. 



Fig. 240. DEEP POSTERIOR MUSCLES OF THE FOREARM. 

 (Allen Thomson.) 



, humerus ; J, olecranon ; c, radius ; d, lower part of the 

 ulna, grooved for the tendon of the extensor carpi ulnaris, which 

 is cut short ; 1, anconeus ; 2, flexor profundus digitorum, ex- 

 posed by the removal 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 carpi radialis 

 longior ; e, their insertions into the second and third metacarpal 

 bones ; 6, extensor ossis metacarpi pollicis ; 6', its insertion into 

 the base of the first metacarpal bone ; 7, extensor brevis pollicis ; 

 7', its insertion into the base of the first phalanx ; 8, extensor 

 longus pollicis ; 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 intermetacarpal spaces the 

 four dorsal interosseous muscles are exposed, the tendons of the 

 common extensor having been removed ; and at 10, the insertions 

 of the second and third dorsal interosseous muscles, 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. 



The extensor ossis metacarpi pollicis muscle 

 (abductor longus pollicis) arises from a narrow ob- 

 lique impression occupying the upper part of the 

 outer division of the posterior surface of the ulna 

 (fig. 95) below the origin of the supinator brevis, 

 from the middle third of the posterior surface of the 

 radius below the insertion of the same muscle, and 

 from the interosseous membrane between. Thence 

 descending obliquely outwards, it ends in a tendon 

 which passes, in company with the extensor brevis 

 pollicis, through the groove on the outer side of the 

 lower extremity of the radius, and is inserted into 

 the base of the metacarpal bone of the thumb, on its 

 radial side. The tendon is commonly split near its 

 insertion, and one part is often attached to the tra- 

 pezium. 



Relations. The upper part of this muscle is concealed 

 by the common extensor, but it becomes superficial below, 



and together with the next muscle crosses the tendons of the radial extensors of the carpus, 

 conceals the insertion of the supinator longus, and, below the extremity of the radius, 

 crosses the radial artery. A bursa separates the two extensors of the thumb from the tendons 

 of the radial extensors. 



The extensor brevis pollicis muscle (ext. primi internodii poll.) lies close to 

 the lower border of the extensor ossis metacarpi, and is much smaller than that 

 muscle ; it arises from the interosseous ligament and a small part of the radius 

 below the middle of the forearm ; its tendon accompanies that of the extensor ossis 

 metacarpi through the same compartment of the annular ligament, and passes on to 

 be inserted into the proximal end of the first phalanx. 



