ly
pigmented layer, the choroid, upon which the percipient nervous layer,
the retina (r.) rests. The chief chamber of the eye is filled with
a transparent jelly, the vitreous humour (v.h.). In front of the eye, the
white sclerotic passes into the transparent cornea (c.). The
epidermis is continued over the outer face of this as a thin,
transparent epithelium. The choroid coat is continued in front by a
ring-shaped muscle, the iris (ir.) the coloured portion of the eyes. This
iris enlarges or contracts its central aperture (the black pupil)
by reflex action, as the amount of light diminishes or increases.
Immediately behind this curtain is the crystalline lens (l.), the
curvature of the anterior face or which is controlled by the ciliary
muscle (c.m.). In front of the lens is the aqueous humour (a.h.). The
description of the action of this apparatus involves the explanation of
several of the elementary principles of optics, and will be found by
the student in any text-book of that subject. Here it would have no
very instructive bearing, either on general physiological considerations
or upon anatomical fact.

Section 112. The structure of the retina demands fuller notice. Figure
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.

Section 113. We may pause to call the student's attention to a little
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 abs