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 its 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 pupillae), by their contraction, diminish the size of the pupil; 

 the radial (dilator pupillae) 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, 



