414 SENSE-ORGANS AND INTEGUMENT. 
blue or green; it is this layer which causes the cat’s eye to 
“‘shine ’’ in the dark. 
The iris (¢) is a continuation of the choroid. It projects 
inward from the white zone of the sclerotic, forming a circular 
curtain lying some distance behind the cornea, and perforated 
by an opening, the pupil. The iris is usually yellow in color 
on its outer surface, darkly pigmented within. The pupil 
varies normally in size and form according to the amount of 
light to which the eye is subjected. In a cat killed with 
chloroform the pupil is very large and circular; in the living 
animal it is elliptical with the long axis dorsoventral, or when 
much contracted it is a mere dorsoventral slit. 
The inner coat of the eye is formed by the retina (zg). 
This is the part of the eye which is sensitive to light; it is 
formed by an expansion of the optic nerve (2). In a preserved 
eye it is usually soft and opaque, and may be seen to line the 
caudal half of the inner surface, extending apparently to the 
ciliary body. Here it seems to end as a free margin, the ora 
serrata; it really becomes thin and passes onto the surface of 
the ciliary body, forming the ciliary portion of the retina, 
and onto the inner surface of the iris, where it forms the uvea. 
Near the centre of the retina the entrance of the optic nerve is 
marked by a small round spot, the blind spot. 
The Crystalline Lens (/).—The lens is a biconvex trans- 
parent body situated within the eye a little in front of the ciliary 
body and just behind the iris. It is more strongly convex in 
front than behind. The lens is surrounded by a thin trans- 
parent sac, the capsule of the lens (capsula lentis). The 
capsule of the lens is attached all around the equator of the 
lens by fibrous bands to the ciliary body, this attachment 
forming the zonula ciliaris (or zonula Zinni) (7), by which 
the lens is suspended. 
Chambers of the Eye.—The lens (/) and zonula ciliaris (¢ } 
form a partition dividing the eye into two parts. The cavity 
in front of the lens contains a fluid, the aqueous humor. 
This cavity is partly subdivided by the iris into the azterzur 
and posterior chambers of the eye. 
The part of the eye lying behind the lens is much larger 
