THE MUSCLES OF THE UPPER EXTREMITY. 125 



the, principal extensor of the hand upon the forearna. 

 It arises by two strong tendinous heads : the outer one 

 from the tubercle of the external condyle of the humerus, 

 just above the origin of the tendon of the tensor patagii 

 brevis ; and the inner and stronger one from a tubercle 

 found above the oblique trochlear facet of the distal end 

 of the same bone, for the radius. The median nerve 

 passes between the two heads, after which they unite to 

 form a large fusiform muscle, the highest of the group, 

 seen on lateral aspect of the arm, which, running 

 parallel with the ulna, becomes converted into a flat, 

 broad, and strong . tendon at about the middle of the 

 forearm. This passes directly on to become inserted 

 into the apex of the anchylosed first metacarpal of the 

 carpo-metacarpus. 



The attachment that the tensor pataijii hrevis makes 

 with the present muscle has already been described 

 above. The distal extremity of the radius is distinctly 

 grooved in a lonoitudinal direction to admit of the 

 lodgment of the tendon of the extensor metacariyi 

 radialis longior, before it makes its final attachment. 

 Owen says that this muscle " raises the hand, draws it 

 forward toward the radial margin of the forearm, and 

 retains it in the same plane. In the Penguin this 

 muscle is extremely feeble, and the tendon is lost in 

 that of the tensor plicce alaris." 



74. The extensor digitorum communis ^ is a smaller 

 muscle than the one just described, and arises from the 



Le long supinateur. Gervais et Alix, p. 26. 

 „ „ „ Alix, p. 408. 



Extensor carpi radialis. Selenka, p. 129, No. 58; Carlsson, p. 22. 

 Extensor carpi radialis Irevis. Selenka, p. 130, No. 59a." 



^ This is 92. M. extensor digitorum communis of Gadow (Bronn's 

 Klassen des Thier-Eeichs, vi. Band, p. 282) and of Selenka (loc. cit., 

 p. 131, No. 61). 



