RADIAL REGION. 



311 



the thumb. By its deep 

 surface, with the elbow-joint, 

 the Extensor carpi radialis 

 brevior, and back part of the 

 wrist. 



The Extensor Carpi Ea- 

 dialis Brevior is shorter, as 

 its name implies, and thicker 

 than the preceding muscle, 

 beneath which it is placed. 

 It arises from the external 

 condyle of the humerus by a 

 tendon common to it and the 

 three muscles next to be de- 

 scribed ; from the external late- 

 ral ligament of the elbow-joint; 

 from a strong aponeurosis 

 which covers its surface; and 

 from the intermuscular septa 

 between it and the adjacent 

 muscles. The fibres terminate 

 about the middle of the forearm 

 in a flat tendon, which is closely 

 connected with that of the 

 preceding muscle, accompanies 

 it to the wrist, lying in the 

 same groove on the posterior 

 surface of the radius; passes 

 beneath the annular ligament, 

 and, diverging somewhat from 

 its fellow, is inserted into the 

 base of the metacarpal bone 

 of the middle finger, its radial 

 side. 



The tendons of the two pre- 

 ceding muscles pass through 

 the same compartment of the 

 annular ligament, are lubri- 

 cated by a single synovial 

 membrane, but separated from 

 each other by a small vertical 

 ridge of bone, as they lie in 

 the groove at the back of the 

 radius. 



Relations. By its super- 

 ficial surface, with the Ex- 

 tensor carpi radialis longior, 

 and crossed by the Extensor 

 muscles of the thumb. By 

 its deep surface, with the 

 Supinator brevis, tendon of 

 the Pronator radii teres, 

 radius, and wrist-joint. By 

 its ulnar border, with the 

 Extensor cornmunis digito- 

 rurn. 



Fig. 174. Posterior Surface of Forearm. Superficial Muscles. 



