HOW ANIMALS MOVE. 
163 
legs, eacli of five parts: the coxa ; trochanter; femur • 
tibia , or shank; and tarsus. The last is subdivided usu¬ 
ally into five joints and a pair of claws. Such as can 
walk upside down, as the Fly, have, in addition, two or 
three pads between the claws. 84 These pads bear hairs 
which secrete a sticky fluid, by means of which the Fly 
adheres to the surface. While the leg-bones of Verte¬ 
brates are covered by the muscles which move them, the 
limbs of Insects are hollow, and the muscles inside. The 
fore legs are directed forward, and the two hinder pairs 
backward. In motion, the fore and hind feet on one side, 
and the middle one on the other, are moved simultane¬ 
ously, and then the remaining three. 
The four-legged animals have essentially the same appa¬ 
ratus and method of motion. The Crocodile has an awk¬ 
ward gait, owing to the fact that the limbs are short, and 
placed far apart, so that the muscles act at a mechanical dis¬ 
advantage. The Tortoise is proverbially slow, for a similar 
reason. Both swim better than they walk. Lizards are light 
and agile,but progression is aided by a wriggling of the body. 
The locomotive organs of the mammalian quadrupeds 
are much more highly organized. The bones are more 
compact; 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 most 
marked 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 
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 
