Because the source of the sensations is in the heart, therefore this is the part first formed in the whole animal, and because of the heat of this organ the cold forms the brain, where the blood-vessels terminate above, corresponding to the heat of the heart.Hence the parts about the head begin to form next in order after the heart, and surpass the other parts in size, for the brain is from the first large and fluid.
There is a difficulty about what happens with the eyes of animals.
Though from the beginning they appear very large in all creatures, whether they walk or swim or fly, yet they are the last of the parts to be formed completely, for in the intervening time they collapse.
The reason is this.The sense-organ of the eyes is set upon certain passages, as are the other sense-organs.Whereas those of touch and taste are simply the body itself or some part of the body of animals, those of smell and hearing are passages connecting with the external air and full themselves of innate spiritus; these passages end at the small blood-vessels about the brain which run thither from the heart.But the eye is the only sense-organ that has a bodily constitution peculiar to itself.It is fluid and cold, and does not exist from the first in the place which it occupies later in the same way as the other parts do, for they exist potentially to begin with and actually come into being later, but the eye is the purest part of the liquidity about the brain drained off through the passages which are visible running from them to the membrane round the brain.Aproof of this is that, apart from the brain, there is no other part in the head that is cold and fluid except the eye.Of necessity therefore this region is large at first but falls in later.For the same thing happens with the brain; at first it is liquid and large, but in course of evaporation and concoction it becomes more solid and falls in; this applies both to the brain and the eyes.The head is very large at first, on account of the brain, and the eyes appear large because of the liquid in them.They are the last organs to reach completion because the brain is formed with difficulty; for it is at a late period that it gets rid of its coldness and fluidity; this applies to all animals possessing a brain, but especially to man.For this reason the 'bregma' is the last of the bones to be formed; even after birth this bone is still soft in children.The cause of this being so with men more than with other animals is the fact that their brain is the most fluid and largest.This again is because the heat in man's heart is purest.His intellect shows how well he is tempered, for man is the wisest of animals.And children for a long time have no control over their heads on account of the heaviness of the brain; and the same applies to the parts which it is necessary to move, for it is late that the principle of motion gets control over the upper parts, and last of all over those whose motion is not connected directly with it, as that of the legs is not.Now the eyelid is such a part.But since Nature makes nothing superfluous nor in vain, it is clear also that she makes nothing too late or too soon, for if she did the result would be either in vain or superfluous.
Hence it is necessary that the eyelids should be separated at the same time as the heart is able to move them.So then the eyes of animals are perfected late because of the amount of concoction required by the brain, and last of all the parts because the motion must be very strong before it can affect parts so far from the first principle of motion and so cold.And it is plain that such is the nature of the eyelids, for if the head is affected by never so little heaviness through sleepiness or drunkenness or anything else of the kind, we cannot raise the eyelids though their own weight is so small.So much for the question how the eyes come into being, and why and for what cause they are the last to be fully developed.
Each of the other parts is formed out of the nutriment, those most honourable and participating in the sovereign principle from the nutriment which is first and purest and fully concocted, those which are only necessary for the sake of the former parts from the inferior nutriment and the residues left over from the other.For Nature, like a good householder, is not in the habit of throwing away anything from which it is possible to make anything useful.Now in a household the best part of the food that comes in is set apart for the free men, the inferior and the residue of the best for the slaves, and the worst is given to the animals that live with them.