How does the skin form?

When a body that is not liquid is produced from the joining together of the small parts of some fluid, like those I have mentioned, some of its parts will necessarily be external to others, and these exteriors must be arranged in a different way from the interiors because they touch a body of a different nature (that is to say, one whose small parts have another shape, or are arranged or move in a different way) than those of which they are composed, for if this were not the case, they would combine with one another, and there would be no surface distinguishing the two bodies.

Thus as soon as the seed begins to gather and acquire a structure, those of its parts that touch the womb, as well as some others that are very close, are forced by this contact to turn, to arrange themselves, and to join with one another in a different way from the manner in which those further away turn, arrange themselves, and join together.

In this way these parts of the seed that are closer to the womb begin to form a skin which has to enclose the whole offspring.

But it only achieves this some time afterwards when all the internal parts of the seed have already been driven towards the heart through the arteries and through the veins which take their place, and finally these arteries and these veins also go towards the exteriors, which flow through the veins towards the heart, to the extent that the arteries advance and produce many small filaments from whose tissue this skin is composed.

As for those surfaces which are formed from a body dividing into two, they cannot have any cause other than this division.

In general all divisions are caused by this alone: one part of the body that is dividing is carried by its movement towards one side while the other part that is joined to it remains where it is, or is carried by its movement towards another side; for this is the only way in which they can be separated.

Thus the parts of the seed making up the heart at the beginning are joined to those that make up the pericardium and the sides, so that, all together, they form only one body.

But the expansion of the blood in the cavities of the heart moved the matter that surrounded this cavity, in a way other than that which elongated it slightly, and at the same time the animal spirits descending from the brain through the spinal column towards the sides of the body also moved the matter near the sides in a different way.

In this way, the matter between the two, not being able to follow the two different motions together, began gradually to come apart from the sides and from the heart and then began to form the pericardium.

Next, to the extent to which the parts of the seed which made it up flowed towards the heart, the arteries of the different places through which they passed sent tiny filaments in their place, which, joining one to the other, formed the skin of which it is made.

What then made this skin hard enough is that, on the one hand, many parts of the blood which dilated in the heart entered into the whole of its flesh, and collected there between it and the pericardium, no longer being able to proceed any further, because, on the other hand, many vapours from the blood contained in the lungs also left, to the extent to which they began to grow, and these were collected between the same pericardium and the sides.

And so these vapours, compressing it from one side and another, made the fibres very hard and caused there to be always some space between it and the heart, which is filled only with these vapours, one part of which is condensed in the form of water, the other remaining in the form of air.