Now, so far as we can make out, the histological structure of our auditory centre is the same as that of our visual centre.

Both are a collection of cells diverse in form, multipolar, and maintained by a conjunctive pellicule (_stroma_).

The structure of the fibres and cells varies slightly in the motor and sensory regions, but no means have yet been discovered of perceiving a settled difference between the nerve-cells of the optic centre and those of the auditory centre.
There should be a difference, as our mind demands it; but our eye fails to note it.
Let us suppose, however, that to-morrow, or several centuries hence, an improved _technique_ should show us a material difference between the visual and the auditory neurone.

There is no absurdity in this supposition; it is a possible discovery, since it is of the order of material facts.

Such a discovery, however, would lead us very far, for what terribly complicates this problem is that we cannot directly know the structure of our nervous system.