OSTEOLOGY AND ARTHROLOGY 69 



and is shorter. It corresponds to the region which, situated 

 between the pastern and the hoof, is known as the cornet. 



The third phalanx, situated entirely witliin the hoof, 

 has the same direction as the first and second. It is 

 large and broad, and presents three surfaces separated by 

 well-marked angular borders (see Fig. 96). The anterior 

 surface is oblique downwards and forwards ; it is convex 

 transversely. The inferior surface is slightly hollowed, 

 and is in relation with the sole, or plantar surface of the 

 hoof. 



The superior surface, which is articular, is divided by a 

 median ridge into two lateral cavities, which correspond to 

 the trochlea on the inferior surface of the lower extremity 

 of the second phalanx. The inferior border corresponds 

 in shape with the hoof. The superior border presents 

 in its median part a projection, the Pyramidal eminence, 

 which prolongs at this level the anterior surface of the bone. 

 Finally, the posterior border, which is concave, is in contact 

 with a sesamoid bone, the lesser sesamoid, which increases 

 the superior articular surface behind, and is also in contact 

 with the second phalanx. 



As we have just seen, the horse possesses but one digit. 

 In the ancestors of the animal — that is, in the prehistoric 

 species which are now extinct {orohippus, miohippus, proto- 

 hippus, or hipparion) — the number of digits was larger ; this 

 fact conclusively proves that the rudimentary metacarpals of 

 the existing horse are vestiges of digits which have dis- 

 appeared through want of use. In the first of those ancestors 

 — orohippus — there were four digits ; all save the first, the 

 thumb, being then developed. In the others of the series 

 there existed but three digits. It must, however, be noted 

 that in those animals it is always the digit which corresponds 

 to the middle finger of the pentedactyl hand that is longest. 

 In other less ancient species the lateral fingers are reduced 

 to the condition of mere splints of bone. It follows from 

 what has been said that the digit which persists in the equine 

 species should be considered as the third finger, and that the 

 rudimentary metacarpals represent lateral digits considerably 

 atrophied. 



