Opus Majus of Roger Bacon, Vol. II (Burke English trans.)

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CHAPTER II

SINCE the optic nerves—that is, the concave nerves causing vision—have their origin in the brain, and writers on optics ascribe to a distinct function through the medium of vision the formation of judgments concerning twenty species of visible things (which will be considered later), and since it is not known whether that distinct function is among the functions of the soul—the organs of which are distinct in the brain—and since many other things to be treated of later require a definition of the functions of the sensitive soul, we must begin with the parts of the brain and the functions of the soul to discover those things necessary for vision.

Writers on optics give us a means to this end by showing how the visual nerves descend from the membranes of the brain and from the lining of the cranium, but no one explains everything necessary in this matter. I say, then, as all writers on natural subjects, all physicians, and authorities on optics agree, that the brain is enfolded by a double membrane. One is called the pia mater (gentle mother), which enfolds the brain by direct contact; the other is the dura mater (hard mother), which adheres to the concave side of the bone of the head called the cranium. This latter membrane is harder so that it may resist the bone, while the other is softer and more tender due to the softness of the brain. The brain's substance is like marrow and ointment, with phlegm as the chief constituent, and it possesses three distinctions, which are called chambers, cells, parts, or divisions.

In the first cell, there are two faculties: the one in the anterior part is the common sense, as Avicenna states in the first book on the Soul. This is like a fountain with respect to the particular senses, and like the center with respect to the lines extending from that same point to the circumference, according to Aristotle in the second book on the Soul. This common sense judges concerning each particular sensation. The judgment is not completed regarding what is seen before the form comes to the common sense, and the same is true regarding what is heard and the other senses, as is clear from