CHAPTER II.

Concerning the interior virtues of the sensitive soul, which are the imagination and the common sense.

Organs of sensation and perception.

Since the optic nerves—that is, the concave ones that create vision—arise from the brain, and since authors of perspective ascribe to the discriminative faculty, by means of sight, the judgments to be made concerning the twenty species of visible things (which will be touched upon later), and since it is not known whether that discriminative faculty exists among the virtues of the soul whose organs are distinct in the brain, and since many other things to be treated below presuppose the certification of the virtues of the sensitive soul, it is therefore necessary to begin with the parts of the brain and the virtues of the soul so that we may find those things necessary for vision. And the authors of perspective give us the way to this, showing how the visual nerves descend from the membranes of the brain and the skin of the cranium; but no one explains all the necessary things in this part. I say, therefore, as all naturalists, physicians, and perspectivists agree, that the brain is wrapped in a double membrane: one is called the pia mater tender mother, which immediately contains the brain; and the other is called the dura mater hard mother, which adheres to the concavity of the bone of the head, which is called the cranium. For this one is harder, so that it may resist the bone, and the other is softer and more delicate because of the softness of the brain, whose substance is medullary and unctuous, in which phlegm dominates. And it has three distinctions, which are called thalami chambers, and cells, and parts. And there are two virtues in the divisions of the first cell: there is one, the common sense, existing in its anterior part, as Avicenna says in the first book De Anima On the Soul, which is like a fountain with respect to the particular senses, and like a center with respect to lines going out from the same point to the circumference, according to Aristotle in the second book De Anima, who judges concerning individual particular sensibles. For the judgment concerning a visible thing is not completed before the species comes to the common sense, and so it is with the audible and the others, as is evident from the end of De Sensu et Sensato.