The stance of animals at rest and moving is best understood by taking two lines of gravity, one running down the foreand one down the hind leg-a system different from that of men and birds. Held for any length of time, it is unnatural if the two lines of gravity coincide.
A horse's levade or a dog's up
right stance is a matter of training, but a horse that bucks or rears or a cat that stands up gives a momentary push to increase the weight of recoil against an obstacle which is thereby overthrown and whose resistance should, therefore, be taken into consideration as part of the equilibrium of the maneuver.
The position and build of the skeleton is naturally affected by the four-legged gait. Most quadrupeds walk on their toes, in contrast to humans, who walk on the soles of their feet, thus requiring a larger ground surface for balancing. It is an advantage for quadrupeds to touch the ground with the smallest possible area, as this enables them to move off more quickly.
Bears and monkeys walk on the soles of their feet and are much slower. The instep, affected by this gait, has grown into a single bone. This, and the necessity to be always ready to jump in self-defense, has brought about the manifold bends, particularly noticeable in the hind legs, which exist even in a position of rest. A sudden, powerful movement forward can be provided only by stretching the leg joints. A runner who needs to make a quick start waits in a crouched position for the starting gun. A frightened deer can give a bound from a position of rest. The beast of prey crouches for its leap, as will any cornered animal.
The bends in the foreleg are less visible from outside. At rest they are present only between the shoulder blade, short upper arm, and forearm. The upper arm lies inside the skin of the trunk, and the bends of the joints can be understood only by a study of the skeleton. Drawings of animals often come to grief at this point.
Elasticity of movement is much helped by the curve of the upper arm, which is much more flexibly attached to the trunk than the human arm, which, being used for work, requires a stronger hold. The shoulder girdle in humans is still joined flexibly to the rib cage by the collar bone, but with very little mobility. Quadrupeds have no collar bone, which means that the shoulder blades are much more mobile. This is plainly visible in cats; at each step when they walk the shoulder blade rises as high as the withers. The collar bone is replaced by cords of sinewy muscle which are very supple and can act as brakes, like the springs of a shock absorber. Only animals which use their forelegs more for work than running have collar bones: monkeys, which climb, moles, which dig, and the kangaroo are among the few.
Where the forearm is more firm iy attached with a collar bone, the joint of the limb is always more mobile, especially so in man. Quadrupeds who use the foreleg mainly for running and jumping do not need the rotary action of hip and shoulder; it would only require extra muscles to strengthen the scissor movement while they were running. Their shoulder and hip muscles work like hinge joints, and the limbs cannot move much sideways. This alters the structure of the pelvis.
Herbivorous animals, which need to be able to run far, have a pelvis proportionately as wide as humans. Carnivores, which catch their prey by jumping, have a very narrow pelvis. The widest point is at the socket joints of the thigh bones. In horses and cattle the thigh sockets and hip bone knobs stand out equally. Beasts of prey also have much shorter necks than herbivores, which feed while walking and so are always bending their necks downwards.