THE LIMBS OF THE HOKSE 39 



crosses that of the radial extensor of the carpus to reach the base of the 

 second metacarpal bone to which it is inserted. The insertion affords 

 a good example of the shifting of a muscular attachment rendered 

 necessary by modification of the skeleton. In the dog the insertion 

 is into the first metacarpal bone. In consequence of the disappear- 

 ance of this bone in the horse, the insertion has been transferred to 

 the nearest available bony point. 



Dissection. — In order to expose the nerves and blood-vessels of the 



front of the forearm it is necessary to reflect the radial and ulnar 



extensors of the carpus. Cut across each muscle about the middle of its 



fleshy belly, bearing in mind that the collateral radial artery is closely 



- - applied to the deep surface of the radial extensor. 



It may be necessary to reflect the biceps also, but it is better to 

 avoid doing this if possible. 



N. RADiALis. — In the dissection of the arm the radial nerve was 

 observed to follow the bracliial muscle round the humerus, accompanied 

 by a branch of the deep artery of the arm. The continuation of the 

 nerve will now be found in front of the elbow joint between the 

 brachialis and the radial extensor of the carpus. Branches to the 

 following muscles should be demonstrated: brachialis, extensor carpi 

 radialis, extensor digitorum communis, extensor digiti quinti, and 

 extensor carpi ulnaris. A thin branch continues down the forearm 

 between the radius and the common extensor of the digits to end in 

 the m. abductor pollicis longus. 



A. PROFUNDA BEACHii. — The terminal division of the deep artery of 

 the arm was disclosed by reflection of the lateral head of the triceps. 

 All that now remains to be noted is that a twig from the artery enters 

 the radial extensor of the carpus, while another passes underneath , 

 the ulnar extensor to join the recurrent interosseous artery. An 

 anastomosis is also effected with the collateral radial artery. 



A. COLLATERALIS RADIALIS (distalis). — Immediately on its origin 

 from the brachial the collateral radial artery disappears between the 

 humerus and the biceps. The present dissection shows that the artery 

 next lies between the brachialis muscle and the humerus, and then 

 crosses the flexor aspect of the elbow joint, between the brachialis and 

 radial extensor, where it comes into contact with the radial nerve. 

 Branches from the artery supply the radial extensor, the common 

 extensor, and the long abductor of the pollex ; and an anastomosis is 

 established with the dorsal interosseous artery. The small terminal 

 branch of the vessel joins the dorsal rete of the carpus. 



A. INTEROSSEA DOESALis. — The dorsal interosseous artery arises from 

 the common interosseous at the back of the forearm and enters the 

 extensor region by traversing the opening between the radius and the 



