414 SENSE-ORGy4NS AND INTEGUMENT. 



blue or green; it is this layer which causes the cat's eye to 

 " shine " in the dark. 



The iris {e) 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 {g). 

 This is the part of the eye which is sensitive to light; it is 

 formed by an expansion of the optic nerve {a). 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) (/), by which 

 the lens is suspended. 



Chambers of the Eye. — The lens {h) 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 anterior 

 and posterior cJi ambers of the eye. 



The part of the eye lying behind the lens is much larger 



