When thou examinest the first ventricle thou wilt see three things before thou comest to the second ventricle.
[a] The first is itself double, and is formed of the very substance of the brain, so that it forms the base of the anterior ventricle both right and left [= corpora striata].
[b] To the side of this is another thing like a subterranean worm, red as blood, yet tethered by certain ligaments and nervelets [= choroid plexus and taenia semicircularis]. And this worm when it lengthens itself closes these passages, and thus blocks the path between the first ventricle and the second. Nature has wrought it thus, so that when a man wills he may cease from cogitation and thought; and similarly when, on the other hand, he would think and contemplate, this worm contracts itself again and opens these passages and thus frees the way between one ventricle and another.
[c] The third structure is a little lower and is a lacuna or rounded concavity [= infundibulum]. In the middle of this is a hole which passes down towards the palate, and this lacuna provides also a direct passage which descends from the middle ventricle to its colature [= sieve-like structure, i.e. certain parts of the sphenoid bone]. And this lacuna has around it certain large round eminences which support the veins and arteries that ascend to the ventricle. This passage is wide above and narrow below, and by it the first and second ventricles purge themselves of their superfluities, but the anterior part [of the first ventricle] purges itself more by the colature of the nose [= cribriform plate]. Thus nature has made two passages to cleanse the superfluities of the brain.
When thou hast seen these three structures there will appear the second or middle ventricle which is as a passage and transit from the anterior to the posterior ventricle. Here are two faculties. One, the estimative, deduces [Italian elicere] the insensible from the sensible. The other, called the cognitive, comprehends both things sensible and things insensible, synthesizing and analysing them (componendo e dividendo). These [two] faculties in the middle ventricle minister to the intellect. Now all the other faculties described, and even the power of memory, are found in brute animals, but this [intellectual power] is encountered in man alone.
Now will appear the third ventricle in the posterior part; and it is hard, for it gives rise to the greater part of the motive nerves which are of a strong and firm nature. This ventricle is pyramidal in shape, and culminates in an apex directed upwards where images of visible things (spetie) are conserved, for these are better stored in a strait than in an ample space; but the part below is wide to receive these images, which are better received in an ample than in a strait place. This ventricle has two functions: it gives rise to the spinal cord [nucha, an Arabian term] and motor nerves; and it is also the storehouse of the memorative faculties.
From what [has been said] it will be apparent that when the back of the head is injured, the memory immediately suffers; when the middle part is injured, the estimative and cognitive faculties suffer; and when the anterior part is injured, the faculties of common sensation and of imagination (fantasia) suffer. And thus it is that the doctors have become aware of the location of these powers.
This being disposed of, thou wilt next raise the brain carefully so as not to break the nerves. Commencing now with the part in front, there will first appear two small fleshy protuberances like two nipples, of like substance to the brain in which they originate, and covered by a thin membrane, the pia mater. These are the olfactory organs, wherein is the sense of smell.
From the brain arise seven pairs of nerves. Proceed therefore farther with the anterior part, and thou wilt see the first pair of these nerves, which are large, and called the nervi optici. These have their origin in the front ventricle of the brain and proceed towards the eyes. But before they pass through the pia mater, they join together, and at their place of union there is a perforated spot. Galen maintains that these nerves only join or rather unite, but do not intersect, so that the nerve that comes from the right after union returns again towards the right, and similarly with the nerve coming from the left, which after the union returns towards the left eye.[185] But Rhazes maintains the contrary,[186] although the opinion of Galen is the more common. These nerves are subservient to sight, and they are united so that the images of the things received by the two eyes and conveyed by the two nerves should return in unity; so that one thing should not appear as two.