Mathematical Mirror

A schematic anatomical diagram illustrates the optic chiasm. A large circle representing the cranium is labeled with letters 'a', 'b', and 'c'. Inside the cranium, two areas represent the anterior brain cavities: 'd' on the right and 'e' on the left. Lines representing the optic nerves emerge from these areas, crossing at a point labeled "Contactus" contact before extending to two smaller circles at the top which represent the eyes, labeled 'f' (left eye) and 'g' (right eye). To the left of the cranium are the labels "Sinistra" left and "Opposita" opposite.

side of the anterior brain were to go to the right eye, then an angle would be formed in the common nerve where they meet, and the nerve would become curved and not straightly extended to the eye. But this would impede vision, because vision always chooses straight lines as much as it can. And then, when the bone of the eye becomes concave, having a hole toward the head, the nerve enters the foramen of the eye and expands itself in the concavity of the bone in the manner of the instrument by which wine is put into casks. Let a b c therefore be the cranium, and let d be the right part of the anterior concavity of the brain, and e the left; these two parts are wrapped in the pia mater, from the base of which two nerves emerge from the right and from the left, meeting in the foramen of the skull, and afterward dividing themselves, so that the nerve which comes from the right goes to the left eye, which is f, and the one from the left goes to the right, which is g. And they enter the foramina of the concave bones so that they may expand in that concavity, as is evident in the figure. But it must be understood that just as two nerves are made from the pia mater, so are they from the dura mater hard mother/outer membrane of the brain, and similarly from the skin of the skull in which it is wrapped externally. And these three nerves are concave and meet in the foramen, and one nerve is made having three nervous tunics. And this nerve, composed in this way, goes to each eye, and each eye has naturally a similar position with respect to their concurrence in the foramen, and an equal distance from it, so that vision is completed more quickly.