Whether skin and web [tela] are necessary to an animal.

Further one asks whether skin and web [tela] are necessary to an animal.

1. It seems not. Whatever impedes sensation is unnecessary for an animal. But the skin impedes sensation, for by interposing itself it keeps the object at a distance from the organ of sense, and therefore, etc.

2. Besides, the skin is at an even greater distance from the principal parts than is the flesh. But flesh is not necessary because it ebbs and flows, and in some parts there is no flesh at all; therefore, etc. interchangeable with "membrane" at this juncture of the QDA, although tela is not as commonly used as membranum.

3. The same seems to hold for a web, for "nature does nothing in vain." But the interior members are adequately preserved by the exterior members, and therefore the web is not needed.

To the contrary: these are necessary because these both exist in animals, which would not be so if they were unnecessary.

One must reply that skin and web are necessary. And the reason is that every moisture not contained in something solid flows and is not bounded. Because there are fluids [humiditates] arising throughout an animal's body from the nutriment, it is necessary for the animal's body to be wrapped with something solid, so that the fluids do not flow out of it. That which contains them is the skin. In addition, the soft members of an animal are prone to injury, and therefore they need some things that are solid to protect them from injuries. The skin is such a solid, and therefore, etc.

One can be persuaded by the same argument for the web, because the principal members, like the heart and the brain, are enclosed by hard partsthe heart by the bones of the chest and the brain by the skull. Therefore, these members need some parts thinner than bone so that neither the principal members nor, as a result, all the others, will be harmed by their hardness, because once a principal member suffers something, all the rest suffer as well. The webs, however, are parts just like this, and this is why they are surrounded by a web, that is, by the tunics of webs or pannicular-membranes.

1. On to the arguments. To the first, one must reply that something can be a medium between a sense and its sensible in two ways. Either it is there in such a way that it will be susceptible to the sensible species by means of a medium in the sense. Such a medium does not impede sensation but is rather necessarily required by it, because "a sensible placed upon a sense is not perceived," according to the Philosopher's way of think-ing. Thus a medium is necessarily required so that, if something were in the heaven and the medium were a vacuum, it would not be seen. And the Philosopher proves this. [Or] something else can be a medium which is not susceptible to the species but conveys it to the sense, like rock or wood, and such a thing impedes [sensation]. Skin is a medium in the first way, keeping the object at a distance from the sense, and this is why it does not impede sensation but strengthens it.

2. To the second argument one must reply that something is at a distance from a principal part in two ways: either according to its location or according to its nature. According to location, the skin is more distant from the principal parts than is the flesh, but not according to nature, because skin is made from parts that are more spermatic than is flesh, and this is why flesh that is cut away grows again, but skin does not, except when it is closely joined to fleshy parts, because facial skin and the skin of the prepuce or the labia does not grow back when it has been cut away.

3. To the third argument one must reply that an impediment can arise from the principal parts in two ways: either from outsideand the skin and the bones are opposed to such an impedimentor an impediment can arise from the hardness of the adjacent members, and the webs and the pannicular membranes are opposed to this impediment. Again, the webs exist for the sake of something else, namely, so that the interior fluids will not flow out too much, that is, due to either some intrinsic or extrinsic cause.