110 



COMPARATIVE ANATOMY. 



" cannon-bone," while the others are gradually reduced. A similar reduction 

 takes place in the hind-foot, and is here as a rule more rapid. 



As far back as the Eocene period Ungulates were separated into Perisso- 

 and Artiodactyles ; a long series of ancestors is hereby indicated. 



FIG. 95. FoBE-FooT OF ANCESTRAL FORMS OF THE HORSE. 1. OROHIPPITS 

 (Eocene). 2. MESOHIPPUS (Upper Eocene). 3. MIOHIPPUS (Miocene). 4. PHOTO- 

 HIPPUS (Upper Pliocene). 6. PLIOHIPPUS (Uppermost Pliocene). 6. EQUUS. 



Fiu. D6. SKELETON OF THE LEFT FoRE-LiMu of A, PIG ; B, HYOMOSCHUS ; C, 

 TEAGULUS ; D, ROEBUCK ; E> SHEEP ; F, CAMEL. (From Bell, after Garrod.) 



The tibia is the most important bone of the shank, just as the 

 radius is of the fore-arm, and the thigh is usually shorter than the 

 shank. A sesamoid bone developed in the distal tendons of the 

 great extensor muscles of the shank is known as the knee-cap or 

 patella. This is already present in Lizards and Birds. 



