COATS OF THE EYE. 
417 
structure c, which is termed the Cornea. The manner in which 
this cornea is set upon the sclerotic coat, so as to serve as the 
continuation of it, may he compared to that in which a watch- 
glass is made to serve as the con¬ 
tinuation of the watch-case over 
the dial. The cornea is rather 
more convex than the rest of 
the sphere of the eye; so that 
the globe makes a slight addi¬ 
tional projection in that part. 
—When the sclerotic coat is 
removed, we come upon the 
second coat ch, which is termed 
the Choroid; this is much more 
delicate in its structure, con¬ 
sisting almost entirely of blood¬ 
vessels and nerves ; and it has 
a deep black hue, owing to its 
being lined with a thick layer 
of black -pigment , which consists 
of cells that have the power of 
secreting a black granular matter in their interior. 1 This coat 
also changes its character in the front of the eye; being there 
continuous with the Iris , or coloured portion, i, which forms 
a sort of curtain that hangs-down behind the cornea. The 
surface of the iris is flat, or nearly so; and there is con¬ 
sequently a space between it and the cornea, like that which 
intervenes between the dial-plate and the glass of a watch; this 
space is termed the anterior chamber of the eye. The iris is 
perforated in its middle by an aperture p, termed the Pupil. 
This aperture is always round in Man; but in animals whose 
range of vision is required to extend widely in a horizontal 
direction (such as the Buminants, and others which' feed 
upon herbage), it is an ellipse with the long diameter hori¬ 
zontal ; whilst in animals which rather seek their food above 
or below them (such as the Cat and other Carnivora which 
naturally live among trees and high places), the pupil is an 
ellipse whose long diameter is vertical. 
1 Similar pigment-cells, having great variety in their form, are to be 
found composing the black spots on the skin of the Frog, Water 
Newt, &c. 
ch s' s cr 
s' r v pc b 
Fig. 208.— Interior of the Eye. 
c, cornea; s, sclerotic; s', portion of 
the sclerotic turned back to show 
the parts beneath; ch, choroid; 
r, retina; n, optic nerve; ca, an¬ 
terior chamber; i, iris; p, pupil; 
cr, crystalline lens; pc, ciliary pro¬ 
cesses ; v, vitreous humor; bb, con¬ 
junctiva. 
E E 
