202 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
whole is richly vascular, and the chorioid, supplied by the ciliary 
arteries which enter at the side, is the chief source of nourishment 
for the rod and cone cells. To the vascular part certain other portions 
are added in some groups. Thus just outside the blood-vessels there 
may be a large lymph space, and outside of this, in most fishes and 
some amphibia and turtles, there is an argenteal layer containing 
calcic crystals which give the layer a whitish appearance. On the 
other hand, the side toward the retina frequently develops ‘a somewhat 
similar tapetum lucidum, with a metallic lustre, which reflects 
light strongly and is the cause of the apparent shining at night of the 
eyes of many selachians and some other fishes and carnivore mammals. 
In a few teleostomes (those with a pseudobranch) there is a so-called 
chorioid gland just oustide the vascular layer, near the entrance of 
the optic nerve. It partakes of the nature of a rete mirabile. 
The chorioid extends as far forward as does the retina, when its 
anterior edge is produced into a circular ciliary process, which is best 
developed in the amniotes, though appearing here and there in the 
ichthyopsida. This process is muscular (ciliary muscles) at its base and 
is connected at its margin with the delicate capsule surrounding the 
lens by a double fenestrated membrane, the zonula ciliaris (Zinnii). 
By the action of the muscles the lens is moved toward or away from the 
retina, while variations in tension may slightly alter its shape, thus 
changing {ts focal point (accommodation of the eye). 
Beyond the ciliary process the vascular tunic continues in front 
of the lens as the iris, a circular curtain with a central opening, the 
pupil. Pigment in the posterior layer (uvea) of the iris renders it 
opaque, while in many fishes the outer surface is silvery owing to the 
continuation of the argentea into this region. The rest of the iris is 
muscular, the muscles increasing in extent from the lower to the 
higher forms. They are arranged in two groups. The circular muscles 
(sphincter pupillz), by their contraction, diminish the size of the pupil; 
the radial (dilator pupillz) are antagonistic and effect an enlarge- 
ment of the opening in the iris. In the sauropsida these iridial muscles 
are cross banded, in amphibia and mammals of the smooth variety. 
Surrounding all of the structures of the eye so far described is the 
sense capsule, which differs from all other sense capsules (p. 62) in 
not being connected with the rest of the skull, as a result of its neces- 
sity for movement. In the capsule two parts are distinguished, the 
sclera which covers the proximal side of the eye, and the cornea, 
