THE ORGAN OF SIGHT. 413 
Through the cornea can be seen the yellow curtain-like iris, 
with an opening in its centre, the pupil. 
Coats of the Eye (Fig. 167).—The outer coat of the eye- 
ball is formed by the sclerotic (or sclera) (4) and the cornea (c). 
The sclerotic is the whitish opaque portion covering the caudal 
three-fourths of the eye. It forms a tough sac serving as a 
protection for the structures within. The cornea (c) is the cir- 
cular transparent portion of the outer coat, covering the free 
surface of the eye; it is a direct continuation of the sclerotic. 
Just before passing into the cornea the sclerotic is much thick- 
ened, forming a broad white zone (d’) about the eyeball near 
its equator. To this zone, at its caudal margin, are attached 
the rectus muscles of the eyeball. The sclerotic has many 
pigment-cells on its inner surface, giving it a brownish appear- 
ance and forming the lamina fusca. 
The cornea (c) is transparent and more strongly curved 
than the sclerotic. It is thicker than 
most of the sclerotic coat, but not so 
thick as the white zone of the latter. 
Within the sclerotic is the second 
coat of the eye, formed by the choroid 
{d) and the iris (¢). The choroid (@) 
is a vascular layer, and contains much 
pigment, giving it a dark color. It 
lines the inner surface of the sclerotic, 
but does not cover the inner surface of 
the cornea. At the white zone of the ee ecicti ea es 
sclerotic it is thrown into a largenum- gq, optic nerve: 4, sclerotic 
ber (about seventy) of meridional folds, ee ae 
“7° . z ’ 4. , ’ irs; 
the plicz ciliares or ciliary folds, which /,"ciliary body; g, retina; 4, 
together constitute the so-called ciliary ae Ns i airiaget 
body (corpus ciliare) (/). This forms eye; 4, vitreous humor. 
a circular zone on the inner surface of the white zone of the 
sclerotic, and is continuous with the iris (¢). The choroid coat 
is furnished in certain parts of its area with a layer of cells 
containing crystals, which give it a brilliant color, forming the 
tapetum. The color of the tapetum is a metallic yellowish 
