point in the physiology of nerves, very happily illustrated here. The
function of a nerve fibre is the conduction of impressions pure and
simple; the light radiates through the fibrous layer of the retina without
producing the slightest impression, and at the blind spot, where the
rods and cones are absent, and the nerve fibres are gathered
together, no visual impressions are recorded. If there is any doubt as
to the existence of a blind spot in the retinal picture, the proof is easy.
Let the reader shut his left eye, and regard these two asterisks, fixing
his gaze intently upon the left-hand one of them.
* *
At a distance of three or four inches from the paper, both spots will be
focussed on his retina, the left one in the centre of vision, and the
right one at some spot internal to this, and he will see them both
distinctly. Now, if he withdraws his head slowly, the right spot will of
course appear to approach the left, and at a distance of ten or twelve
inches it will, in its approach, pass over the blind spot and vanish, to
reappear as he continues to move his head away from the paper.
The function of nerve fibres is simply conduction, and the nature of the
impressions they convey is entirely determined by the nature of their
distal and proximal terminations.