9 shows an enlarged, diagram of a small portion of this, the
percipient part of the eye. The optic nerve (o.n. in Figure 8) enters
the eye at a spot called the blind spot (B.S.), and the nerve fibres
spread thence over the inner retinal surface. From this layer of nerve
fibres (o.n. in Figure 9) threads run outward, through certain clear
and granular layers, to an outermost stratum of little rods (r.) and
fusiform bodies called cones (c.), lying side by side. The whole of
the retina consists of quite transparent matter, and it is this outermost
layer of rods and cones (r. and c.) that receives and records the visual
impression. This turning of the recipient ends away from the light is
hardly what one would at first expect-- it seems such a roundabout
arrangement-- but it obtains in all vertebrata, and it is a striking point
of comparison with the ordinary invertebrate eye.