418 CONTRACTION AND DILATATION OF THE PUPIL :—RETINA. 
534. By the contraction and relaxation of certain fibres in 
the Iris, the size of the Pupil is changed according to the 
degree of light to which the eye is exposed; the aperture being 
made to diminish in a strong light, in such a manner as to 
exclude the rays that would be superfluous, and to prevent 
too many from falling upon the expansion of the optic nerve; 
whilst it dilates in a faint light, so as to admit as many rays 
as possible. If we notice the pupil of a Man who is looking 
towards the mid-day sun, we shall see that it is contracted to 
a small round speck; but the pupil of a Sheep would be con¬ 
tracted, in similar circumstances, into a horizontal slit; and 
the pupil of a Cat into a vertical one. The alteration in the 
size of the pupil in accordance with the degree of light, may 
be easily observed by stationing oneself at a window pro¬ 
vided with shutters, and holding a looking-glass in the hand : 
if the light be at first strong, the pupil will be seen in a con¬ 
tracted state; but if the shutters be gradually closed, so as to 
diminish the amount of light that falls upon the eye, the 
pupil will be seen to enlarge; and it will diminish again 
when the shutters are re-opened. The blackness which the 
pupil always presents, in the healthy state of the eye, is due 
to our seeing the black lining of the back of the eye through 
it. In many Quadrupeds, the black pigment is replaced, in 
one portion of the eye, by a layer of a blueish colour, having 
an almost metallic lustre; and from this we see the light 
brilliantly reflected, when we look at their eyes in certain 
directions. 
535. On turning back the choroid coat, we come to the third, 
r, of the layers of which the wall of the eye is composed : this 
is an extremely delicate film, chiefly consisting of nerve-cells 
and nerve-fibres that spread-out from the optic-nerve, n ; and 
it is called the Retina (or net). It advances nearly as far as 
the iris ; but it is deficient in the front of the eye. The part 
of the retina which lines the globe at the point exactly 
opposite to the centre of the pupil, is distinguished from the 
rest by a peculiar yellow opacity, which causes it to be 
designated “ the yellow spot/ - ’ In this spot, visual sensibility 
is more acute than elsewhere. On the other hand, the part 
of the retina that covers the entrance of the optic-nerve 
(which is below “ the yellow spot,” and nearer to the nose) is 
so much less sensitive to light than the rest, that under 
