THE LIMBS OF THE HOESE 31 



the triceps, with the lateral head of which it is very closely associated. 

 Its fibres arise from the distal part of the posterior surface of the 

 humerus, and its insertion is to the lateral surface of the olecranon. 

 The deeper fibres of the muscle are adherent to the capsule of the 

 elbow joint. 



N. EADiALis. — At a previous stage of the dissection, the radial nerve 

 was noted as passing down the proximal part of the arm in company 

 with the ulnar nerve which separated it from the axillary vein. About 

 the middle of the arm the radial nerve dipped between the medial and 

 long heads of the triceps muscle, giving off a branch to the tensor 

 fasciae antibrachii as it did so. More of the course of the nerve is now 

 open to examination. 



The nerve runs obliquely downwards and in a lateral direction along 

 with a branch of the deep humeral artery, in that spiral groove of the 

 humerus which is mainly occupied by the brachialis muscle. Laterally 

 the nerve is crossed by the lateral head of the triceps and the radial 

 extensor of the carpus. An oblique and somewhat spiral course brings 

 it to the flexor aspect of the elbow joint, where it will be encountered 

 at a later stage in the dissection. 



From that part of the radial nerve which is now visible arises 

 a cutaneous branch (n. cutaneus antibrachii dorsalis) that either 

 passes through the lateral head of the triceps or emerges from beneath 

 the distal border of the muscle, and is distributed over the lateral and 

 anterior part of the forearm. 



In his examination of the rest of the limb, the dissector should 

 constantly bear in mind that he is dealing with an animal in which 

 evolution has produced profound modifications. Inasmuch as the digits 

 have been reduced in number to one only, the horse is as far removed 

 as he conceivably can be from the typical pentadactyl mammal. In 

 association with the disappearance of all the digits except one, the 

 metacarpal bones have been modified. The first and fifth metacarpals 

 have disappeared entirely. The second and fourth are represented by 

 comparatively slender and tapering rods, which fail to stretch the full 

 length of the metacarpal region. At the distal end of each rudimen- 

 tary bone it seems probable that sometimes, at least, there may be 

 imperfect representatives of phalanges ; but these are very variable 

 in size, and always firmly blended with the metacarpal bone to which 

 they belong. The third metacarpal bone has been transformed in 

 the opposite direction. It has increased in length and strength, and, 

 like the phalanges it carries, has attained a considerable volume. 



As these bony changes were taking place, and as the manus as 



