294 



LAND MAMMALS IN THE WESTEEN HEMISPHERE 



Mtl^ 



'-m.nr. 



Fig. 148. —Left pes of 

 Horse. Cal., cal- 

 caneum. As., as- 

 tragalus. N., navic- 

 iilar. Cn. 3; third 

 cuneiform. Mt. Ill, 

 functional (third) 

 metatarsal. Mt. II 

 and Mt. IV, splints. 



primitive five-toed foot ; and, as this toe has 

 to carry the whole weight supported by its 

 leg, it is necessarily much larger than in ani- 

 mals which distribute the weight among sev- 

 eral digits. The horses are therefore said to 

 be monodactyl, or single-toed, but the term 

 is not strictly accurate, for on each side of 

 the functional digit is a rudimentary or ves- 

 tigial one, the 2d and 4th of the original five. 

 These rudimentary digits, which are not visi- 

 ble externally, have no phalanges and are 

 merely "splint-bones," metapodials (see p. 

 90) which have very slender shafts and end 

 below in blunt 

 points. The sin- 

 gle functional 

 metapodial has ^c.^ 

 encircling its 

 lower articular 

 end a prominent 

 ridge or keel, 

 which fits into 

 a corresponding 



groove on the up- 

 per end of the 

 first phalanx and 

 serves to prevent 

 lateral dislocation. In most mam- 

 mals this keel is merely a projec- 

 tion from the lower articular sur- 

 face and is confined to the pos- 

 terior side, so as not to be visible 

 from the front. The terminal or un- 

 gual phalanx is much enlarged to 

 carry the great weight which it sup- 



•jf&zr 



Fig. 149. — Left manus of Horse, 

 front side ; to the right, rear view 

 of the metacarpus. S. , scaphoid. 

 L., lunar. Z';/., pyramidal. Pis., 

 pisiform. Td., trapezoid. M , 

 magnum. C7., unciform. Mc.II, 

 Mc. IV, rudimentary second and 

 fourth metacarpals, or splints. 



