In human anatomy we learned how a joint is held solely by muscular activity. It is tiring to stand for long, and even more so to remain with knees half bent. Quadrupeds, however, are in a position of rest with their knees half bent. Two other factors contribute to the tension of their joints: there are ligaments which hold them taut, and there is a slight hollow on one side of the joint surfaces, and a corresponding convexity on the other, so that the two parts can rest against each other.
The joints are held close by tendons, which are stretched only by muscular activity, causing the joint to move. This is why horses can stand all day without tiring, and why full-grown animals need never lie down and can even sleep standing up.
All animals, especially those with long necks, use the neck arid head as a counterweight when walking. A horse nods its head constantly while it runs, both up and down and slightly sideways. The movement occurs mainly between the skull and the highest neck vertebra, although the whole neck is brought into play to some extent.
The tail also is used for balancing, although it is mainly a sort of rudder against wind resistance and serves horses and cattle as a fly whisk. Animals also give unconscious expression' to their feelings with the tail. Dogs wag their tails when they are pleased, stick them up when they are aggressive, as do cats, and put them between their legs when they are afraid.
The pelvis of the elephant and giraffe falls steeply away from the horizontal backbone. It is flatter in the horse and flatter still in cattle. The drawings of skeletons show the differences of proportion and position of leg bones peculiar to each animal. In some animals the kneecap is grown together with the upper thighbone. Sometimes the shinbone has disappeared in part or completely. Both kneecap and shinbone are needed in humans for the twisting of the thighbone along its long axis. Where this is unnecessary in animals, these alterations, if we regard the human skeleton as the norm, take place.
In all toe-walkers the instep bones and heel bone have grown together or joined up, more completely in hoofed animals than in felines. The toes, too, have become one in hoofed animals, and a double hoof in cloven-footed animals. Carnivores still have all their toes. The double arch of the foot does not exist in any animals, not even in those which walk on their soles and whose feet are in other respects similar to human feet. The soles of bears' feet are rough and grip the ground well, while monkeys' feet, which are adapted to tree climbing, developed like hands. The carnivore has to grip its prey; it developed thick pads which allow a quiet tread and protect the sharp claws from wearing away.
The front foot developed in a similar way. Only where there is some reason for the forepaw to twist around do the humerus and ulna exist and rotate around each other, as, for instance, in cats, which can give short side pushes. They can turn around only one eighth as much as the human forearm.