24 THE ANATOMY OP THE HOESB. 



■which, after being reinforced by a fibrous band from the back of the 

 radius, passes through the carpal sheath behind the carpus, and is 

 ultimately inserted by a bifid tendon into the second phalanx. The 

 examination of this and the succeeding muscle, from the carpus down 

 wards, must be postponed till the dissection of the metacarpus and 

 digit is undertaken. 



Acticm. — The muscle flexes successively the pastern, fetlock, and 

 carpal joints. 



The Deep Flexor of the Digit (flexor pedis perforans) (Plate 6). 

 This muscle is situated in contact with the posterior surface of the 

 radius, and consists of three divisions, which may be distinguished as 

 the humeral, the radial, and the ulnar portions. The humeral or main 

 division arises, in common with the preceding muscle, from the lower 

 extremity of the ridge bounding the olecranon fossa on the inside. The 

 radial portion, or radialis accessorius, is deeply placed, and arises from 

 the back of the radius. The ulnar division, or ulif.aris accessorius, is 

 placed beneath the deep fascia of the fore-arm, where it lies between the 

 external and oblique flexors of the metacai-pus, and is accompanied by 

 the ulnar nerve and vessels. It arises from the summit and posterior 

 border of the olecranon. These three divisions unite above the carpus, 

 and have a common tendon which passes through the carpal sheath, 

 and is ultimately inserted into the os pedis. 



Action. — It flexes successively from below upwards the inter-phalan- 

 geal joints, the fetlock, and the carpus. 



Directions. — The front of the fore-arm must now be dissected ; and 

 here it will be convenient to turn attention in the first place to muscles ; 

 but while these are being isolated, care is to be taken of the interosseous 

 vessels, which descend along the lateral extensor at the outer side of the 

 region, and of the tendon of the oblique extensor where it crosses over 

 the tendon of the extensor metacarpi magnus above the carpus. 



The Extensor Metacarpi Magnus (Plates 7 and 8) corresponds to the 

 long and short radial extensors of the wrist in the human subject. It is 

 a powerful muscle, having at its upper end a massive muscular belly, 

 which tapers downwards, and terminates a few inches above the carpus 

 in a tendon. It arises from the anterior and upper part of the outer 

 ridge of the olecranon fossa (the outer condyloid ridge), where this rido-e 

 bounds the musculo-spiral groove ; and by a second tendon, in common 

 with the extensor pedis, from a depression which is placed external to 

 the coronoid fossa. Its inferior tendon lies in the largest and most 

 internal of the vertical grooves at the lower end of the radius ; and after 

 gliding over the front of the carpus in a synovial sheath, it is inserted 

 into a special tubercle on the upper end of the large metacarpal bone at 

 its inner side. 



Action. — It extends the manus on the fore-arm. 



