HOW ANIMALS MOVE. 161 
are much more highly organized. The bones are more 
gompact; the vertebral column is arched, and yet elastic, 
between the shoulder and hip, and the limbs are placed 
vertically underneath the body. The bones of the fore 
limb are nearly in a line; but those of the hind limb, 
which is mainly used to project the body forward, are 
more or less inclined to one another, the angle being 
greatest in animals of great speed, as the Horse. Some 
walk on hoofs, as the Ox (Ungulate); some on the toes, 
as the Cat (Digitigrade); others on the sole, touching the 
ground with the heel, as the Bear (Plantigrade). In the 
Fig. 127.—Feet of Carnivores: A, Plantigrade (Bear); B, Pinnigrade (Seal); ©, 
Digitigrade (Lion). 
Pinnigrade Seal, half of the fore limb is buried under the 
skin, and the hind limbs are turned backward to form a 
fin with the tail. The normal number of toes is five; but 
some may be wanting, so that we have one-toed animals 
(as Horse), two-toed (as Ox), three-toed (as Rhinoceros), 
four-toed (as Hippopotamus), and five-toed (as the Ele- 
phant). The Horse steps on what corresponds to the nail 
of the middle finger; and its swiftness is mainly owing 
to the solidity of the extremities of the limbs. Horses 
of the greatest speed have the shoulder-joints directed at 
a considerable angle with the arm. 
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