Petrus Camper’s Metamorphosesfrog
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These facts of anatomy can be applied to art.

Except for one person, no one has ever given particular rules to draw all the animals with any kind of real precision. The skeletons — which in animals like in man furnish the real base of their exterior appearance — are so poorly represented and so erroneously treated that it is impossible for painters to get the least use from them.

The exception was Chrispyn van de Pas, who, in his Ouvrage, Part V, page 6, gave an easy method to sketch a horse or cow without ruler or compass:

De Pas Horse
De Pas Cow

According to him, one should draw a square, divide it into 9 equal parts and draw 3 circles: one for the shoulders and chest; a 2nd for the belly; and the 3rd circle for the rump. However, everything in van de Pas’s method is either ultimately based on conjecture or is actually physically incorrect. Nor is it possible to draw the animals in a different perspective.

Camper has therefore derived four general rules.

Plate V, Figure 8. Back to the Horse
horse again
  1. Trace ABC (the purple line) according to the direction it must have, i.e. according to the characteristic of the animal that must be represented, either in a horizontal angle or in a more diagonal one towards "C" such as would be required in the sheep or camel.
    Complete the sloping circle to "D" (dotted purple line).
    Draw FE (green line on the left) for the bone of the shoulder-plate and CH (green line on the right) for the bone of the hip which is equal to 2/3 of the head for the horse but equal to the head itself in the cow.
    Sketch the bone of the arm EG (dotted green line on the left) and the bone of the thigh IK (dotted green line on the right) so that the elbow and the knee in the horse, cow, etc. are of equal height and in the same alignment with the stomach.
    Complete the contour of the front and hind legs, i.e. trace out KL, MN, NO for the back and GR, RS, ST for the front (all blue lines).
    When "R" and "L" (blue circles) are of equal length, the "heel" ML (red line) is naturally raised.
    Outline the neck, according to the particular characteristic of the animal, and then the head.
    This fundamental outline — with Camper’s general rules from the first lesson — can be applied to all kinds of animals.
  2. To get the shapes of the front and back legs, add the muscles at "Q" (the brown circle) to the arm bone and the rest of the muscles (brown dotted lines) in the front and back.
  3. The first ribs are always straight and covered by the shoulder blade. Those in the back are always placed diagonally backward.
    In the horse they extend until the hip bone, but in the cow, as that part of the loins is longer, a triangular cavity or hollow is formed (blue shading in Figure 2).
  4. In all the animals with sole or clefts, the hand and foot are very long, like in RS and MN (dotted red lines).
    In the animals which leap, like lions, dogs, rabbits, the bone of the leg is very long but the foot very short.

This second lesson is illustrated first with two examples and then a final two.

Plate VI, Figure 9. Application of these Rules to Birds

Trace a new oval. Situate the arms in AB (purple line) which, if the bird does not fly, must be folded as in BC (dotted purple line). Take CD for the hand, DF for the thumb, and DE for the other fingers.
Then trace GH (green line) for the bones of the hip and the rump.
The red line (IK) gives you the thigh KL (red dotted line) the bone of the leg under it, the feet, and the toes.

Trace the neck observing its proportion to the height of the body and culminate the head. In some species the upper jawbone is very mobile, e.g. the eagle, parrot, and duck.
When the bird is of a species that flies, it must have a bony eminence in the form of a ridge on the breast bone in order to insert muscles. Birds that do not fly, like the ostrich, do not have this crest.
Add muscles and feathers to complete the animal.


Having proved that the front feet of all quadrupeds and the birds are analogous to our arms, it follows that it is absolutely absurd to give wings to the human figure, as the custom has held for angels and cupids.

Plate VI, Figure 11. Proof that Fish are analogous to Quadrupeds in terms of their Interior Structure

Begin again with the form of the trunk BACG (purple line).
Seeing that the fish does not need a neck at all and therefore hardly has one (except for those that breathe which have a slight neck), the head is attached directly to the spine of the back at point "A" (red circle).
In this state the fish cannot make a single movement, as he is in equilibrium with the water, so it is necessary to give it a motor force in the form of an oar like in a boat (green line on the left) with which the fish can be compared. But because this motor force must of necessity reside in the fish itself, the tail CH (brown line) and the transversal suspensions of the vertebrae, to insert its muscles, are indispensible. The longer the tail, the faster the fish can swim.


It has been established that a boat has the least mobility possible than the center of motion and of gravity meet at the same point. This is not possible in a boat, but always takes place in the fish; the latter therefore has the ability to swim in a straight line whereas boats can only advance by making movements from side to side with the prow.
But, as the fish needs to keep itself upright, it has fins at the chest (BF) and on the stomach at point "G" (green lines on the right). If one cuts the fins BF, as did Actedi, the fish cannot support itself and rolls upside down.


The center of motion must vary in ratio to the weight of the fish's head and from that also must depend the length of the tail.
From this, therefore, the forms of fish are the most susceptible of being diversified and they have more varieties than among the quadrupeds.
Linnaeus counted 212 species of quadrupeds to the 480 among the fish.

It is also obvious that it would be ridiculous to believe in the existence of mermaids.

Finally, seeing that fish advance by tail movements, explains why they do not need feet, thighs, leg bones, etc.

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