SECOND DISTINCTION

CHAPTER I

CLEARLY, therefore, so that there may be no doubt in what follows, the structure of the eye must be studied, because without this, nothing can be known regarding the method of vision. Some writers say less, some more, and in certain matters, they disagree. Authors on perspective commonly pass over these details, assuming a prior knowledge of natural philosophy and medicine, as if everyone who reads the science of perspective has already consulted medical and natural science texts. Therefore, their own discussion is obscure, and we cannot understand it unless we look to the more comprehensive treatment of the subject by medical and natural philosophers.

For this reason, I must say more on this topic than is found in the texts on perspective. Although it is difficult to verify these matters and explain them, I hope these facts can be clarified through certain authors. So that I do not rely too heavily on the opinions of individuals, I will confine my description of the eye's structure mainly to three authorities: Alhazen in his first book on Perspective, Constantinus in his book On the Eye, and Avicenna in his books. These writers are sufficient, and they treat the matters of our interest most definitively. I cannot, however, provide the exact words of each, as they sometimes differ due to faulty translations, but I shall form a single, truthful statement from them all. All, then, agree that there are two parts in the anterior cavity of the brain, which are called ventricles, concavities, or cells. These ventricles cannot be the organs of "common sense" and "imagination," which I mentioned previously. Those faculties are arranged anteriorly and posteriorly, whereas these ventricles are placed on the right and on the left, as Constantinus states. For the entire brain cell can be divided into an