Animal anatomy is best understood by a comparison with human anatomy, working from the somewhat suspect idea that animals have adapted the structure of the human body to their own needs of existence. For artists, the idea that man is the measure of all things is always correct.
The point of view of exact science would enforce him to be continuously rethinking instead of reacting primarily to his faculty of vision. The understanding of visible form is basically a matter of comparison, and for the human being the most accessible object of comparison is the human body, if he is trying to understand the structure of other living bodies. many would consider this a falwed idea, but it suits our purposed for now.
We shall concern ourselves here only with vertebrates, and of these only those most conducive to pictorial representation: quadrupeds and birds. Their physical structure is very close to the human. It is composed of the spine, skull, thorax and shoulder girdle, pelvis, and four limbs.
The shape of the spine in quadrupeds is the first deviation from human anatomy because of the difference of posture. Its shape resembles a slightly curved arch, between the pelvis and shoulder girdle, and is not, like the human spine, S-shaped to balance the erect gait. Only the S-curve of the neck, composed with very few exceptions of seven vertebrae, is repeated in quadrupeds.
The sometimes considerable length of neck, in the giraffe, for instance, is due solely to the length of each vertabra. The vertebrae are never plastically noticeable in the neck, unlike those in the tail, which are a continuation of the spine beyond the pelvis. In human anatomy five vertebrae have grown together to form the os sacrum, but in most quadrupeds the more numerous vertebrae (many reptiles have up to 40) have remained mobile. The number of vertebrae in the back is greater in a number of animals; therefore the trunk is proportionately longer in animals than in humans.
It is important to study from the skeleton the line of the spine along the back in different animals in order to understand the arrangement and function of the pelvic and shoulder girdles. Unlike those of the human, their chest and loin vertebrae have very high projections. Thus, the outline of the back of an animal appears slightly S-shaped as it lifts over these projections at either end of the spinal arch.
In felines this outline has a downward curve, like a span roof, which turns where the projections change in slope. The turn of this curve is again marked on the skeleton by a very short vertical projection. The peculiar alternation of slope in these projections prevents a reverse bend of the spine and gives a powerful spring mechanism to the back for jumping, worked by strong, straight muscles.
The fore and hind legs of quadrupeds have different functions. The hind legs give the impetus and power to move forward; the forelegs primarily provide the main support for the greatest weight of the body, two thirds of which in the horse, for instance, rests on the forelegs. The mechanism of forward and backward movement in quadrupeds can be likened to a motor car with rear-wheel drive. Every quadruped begins a forward movement with a hind leg and immediately afterwards, almost simultaneously when running, moves the opposite foreleg forward to offset the change in gravity. A few animals move differently: the camel, for example goes forward with both legs, first those of one side, then the other. This gait can be taught to horses in dressage, but is not natural to them.