32 



TOPOGRAPHICAL ANATOMY OF 



a whole was increasing in length, its relation to the ground became 

 gradually altered and more and more restricted. Not only has the 

 single remaining digit developed in such a way as to be now capable 

 of sustaining the whole weight of the body ; it does so by contact 

 with the ground by its extreme end only. That is, the modern horse 

 is no longer digitigrade : he is unguligrade. 



The bones of the forearm have also undergone a striking change. 

 While the radius has increased in size and strength, the ulna has been 

 reduced to a proximal extremity and a tapering shaft that does not 

 reach much beyond the middle of the radius. The disappearance of 



Carpus. 



Carpus. 



Fig. 19. — Evolution from Pentadactyl to Monodactyl Condition in the Manus of the Horse. 

 The lower part of the figure shows the human hand for comparison. 



A. = Pentadactyl, hypothetical ancestral stage. B. = Tetradactyl, Protohippus stage. 

 C. = Tetradactyl-tridactyl, Epihippus stage (three digits touching the ground, one 

 slightly raised). D. = Tridactyl, Mesohippus stage. E. = Monodactyl, Equus stage ; 

 modern horse. (After H. F. Osborn.) 



part of the shaft has caused the detachment of the distal end of the 

 ulna, which has fused with the distal end of the radius. These modifi- 

 cations in the bones of the forearm have been accompanied by their 

 intimate bony union with each other, and the consequent loss of any 

 possibility of movement between them. The limb, therefore, has been 

 fixed permanently in the position of pronation. 



The above alterations in the bony skeleton have, naturally, been 

 accompanied by great modification in the muscular apparatus. Some 

 muscles, for example the supinators, have lost their function and have 

 disappeared. Some, of which the abductor of the pollex is an illustra- 

 tion, have been compelled to change their attachments. Other muscles 

 have assumed a different structure and a different function, as is 

 exemplified in a remarkable manner by the interosseous muscle of 

 the metacarpus. Naturally, also, the blood-vessels and nerves of the 

 manus have been simplified. In short, it must always be remembered 



