Mathematical Mirror

They are ordered according to prior and posterior. These, however, are positioned according to right and left, as Constantinus Constantine the African says. For the whole cell of the brain can be divided into an anterior and posterior part, as has been said above. And nonetheless according to right and left. And its anterior, namely the seat of the sensus communis common sense, has a right and a left, where there are two ventricles in a certain way distinct. And from the pia mater soft mother/delicate membrane covering the brain, which covers each ventricle, a double nerve exits, one from the right ventricle, and the other from the left; and they are the optic nerves, that is to say, concave according to the said authors. And the concavity does not begin from the middle of the anterior part of the brain, because there is the instrument of odoratus the sense of smell, which is one nerve having on its sides two caruncles similar to the tips of nipples, according to what Avicenna teaches in book 10 of On Animals. But according to Avicenna, the author of the Perspectiva, and Constantinus, these visual nerves of which we speak exit from the base of the ventricles of the anterior part, as Constantinus explains, teaching one to inspect the heads of large animals when they are slaughtered, not in summer nor in heat, and one will find a small hole in the skull through which the nerve passes. And afterward, one should carefully examine the membrane of the pia mater so that it is not broken, and one will find the nerve exiting from the base of the anterior part of the brain. But the two nerves, as has been said, from the two parts—namely the right and the left—according to everyone, concur and become one nerve, and after the junction, they are divided again. It would have been better, however, that they should concur in the foramen than before or after. For whether it is this way or that, they would make two holes in the bone of the head, but it is better to be made through one than through two. And the bone is stronger the less it is perforated. Therefore, since nature does things in the best way it can, the concurrence will be in the foramen of the skull, from which place they are again divided. And the nerve that comes from the right side goes to the left eye, and the one from the left goes to the right, so that the extension of the nerves from their origin to the eyes is straight. For if the one that comes from the right