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. 



IV 1 ? 



FIG. 95. FORE-FOOT OF ANCESTRAL FORMS OF THE HORSE. 1. OROHIPPUS 

 ' (Eocene). 2. MESOIIIPPUS (Upper Eocene). 3. MIOHIPPUS (Miocene). 4. PROTO- 

 HIPPUS (Upper Pliocene). 5. PLIOHIPPUS (Uppermost Pliocene). 6. EQUUS--. 



3\i 



D F F 



FKJ. 96. SKELETON OF THE LEFT FORE-LIMB OF A, PIG ; B, HYOMOSCHUS ; C, 

 TKAGULUS ; D, ROEHUCK ; E, SUEEP ; 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 arid Birds. 



