De praecipius visus symptomatis eorumque causis Physica & Medica contemplatio

XV.
This reason was noted by Galen, by a certain divine intuition of genius, when he attributed some symptoms of dimming vision to the thickness and obscurity of the spirits, which, although they might be suitable for guiding motion and touch elsewhere, provided no harm was detected in these, could not, however, maintain the sharpness of sight equally.

XVI.
For the optic nerve does not provide the sense of tactile qualities, of which colors are devoid, although it has not been pleasing to anatomists to inquire into this more diligently until now. Yet those cases can be an argument, where the optic nerves are completely obstructed by a suffusion a cataract or similar opacity or any other kind of disease, such that the faculty of seeing is destroyed by the lack of the flowing spirit, while the sense and motion of the eye remain nonetheless intact. Moreover, the crystalline lens, surrounded on all sides by the optic nerves, lacks that sense.

XVII.
But it is consistent that the proper and genuine species of each spirit is, in part, consigned in the brain, as the principle of all faculties, like some character of a future duty; and in part, it is confirmed and perfected by the innate temperament of each sensorium, especially adapted by nature for this use—just as the temperament of the skin serves touch, that of the crystalline humor serves vision, and that of the tongue serves taste, each with a certain power and gift in which they individually excel.

XVIII.
Therefore, just as faculties and the senses themselves differ from one another in species, so in a way will the spirits, as the proximate ton aistheseon ergodiontai workers of the senses, differ, equipped with peculiar powers; nor will the condition and nature of all be equal.

XIX.
Furthermore, just as elsewhere there is no sudden and immediate permutation of species, so neither in the senses of different spirits will an autapadosis reciprocal exchange consist, unless a new alteration intervenes. Nor will the akousikos auditory spirit be derived straight from the ear into the eye, nor will the optic spirit, conversely, flow back from the eye into the ear, without an accepted mark or certain episema sign/seal of another faculty consistent with the sensorium subject.