26 THE ANATOMY OF THE HORSE. 



tendon passes first through a vertical groove on the external tuberosity at 

 the lower end of the radius, then through a synovial sheath at the outer 

 side of the carpus, and it will subsequently be followed to its insertion 

 into the first phalanx. 



Action. — It is an extensor of the fetlock and of the carpus. 



Directions. — The nerves and blood-vessels on the front of the fore-arm 

 must next be sought, and, in order to fully expose them, some of the 

 foregoing- muscles must be cut. The biceps is to be cut about its middle 

 in order to follow the anterior radial artery ; and by dissecting deeply 

 down in front of the elbow, between the brachialis anticus and the 

 extensor metacarpi magnus, the artery will be found to meet the 

 musculo-spiral nerve. The extensor metacarpi magnus is to be cut 

 about its middle and carefully reflected in order to follow the artery, 

 which lies in relation to the deep face of the muscle ; and the extensor 

 pedis is to be similarly reflected to trace the termination of the mus- 

 culo-spiral nerve. 



The Anterior Radial Artery (Plate 8) is the smaller terminal 

 branch of the brachial. It separates at an acute angle from the pos- 

 terior radial, and passes forward beneath the biceps and then beneath 

 the brachialis anticus. It meets the musculo-spiral nerve in the inter- 

 space between the brachialis anticus and the extensor metacarpi magnus, 

 and afterwards descends on the anterior surface of the radius, where it 

 is covered by the last-mentioned muscle. It terminates at the carpus 

 by anastomosing inwardly with branches from th,e posterior radial, and 

 outwardly with the interosseous artery of the fore-arm. It supplies 

 articular branches to the elbow, and muscular branches to the muscles 

 on the front of the fore-arm. 



The Interosseous Artery of the fore-arm (Plate 7) is a branch 

 given off" by the median at the back of the fore-arm. It comes out- 

 wards through the radio-ulnar arch, and descends along the extensor 

 suffraginis, terminating in slender branches in front of the carpus. 

 It supplies articular branches to the elbow ; the nutrient artery of the 

 radius ; and muscular twigs to the extensor suffraginis, extensor pedis, 

 and extensor metacarpi obliquus. 



The anterior radial and interosseous arteries are generally compara- 

 tively slender vessels, but they are liable to some variation in size and 

 distribution, and the one may partly supplant the other. 



A bins. Satellite veins of the same names run in company with the 

 foregoing arteries. 



The Musculo-spiral Nerve in the fore-arm (Plate 8). In the dissec- 

 tion of the axilla and arm this nerve has already been seen as a large 

 trunk descending from the brachial plexus, and taking a spiral course 

 behind the humerus. It reaches the front of the elbow, where it meets 

 the radial artery in the interspace between the brachialis anticus inwardly, 



