216 



ANIMAL BIOLOGY 



affords an example of similar responses of different organisms to 

 similar needs giving rise to analogous structures. (Figs. 50, 150.) 



The wall of the Vertebrate eye, or eyeball, forms a more or less 

 hollow sphere which can be rotated by several relatively large 

 muscles. The anterior exposed part of the eyeball consists of two 

 transparent layers: the delicate conjunctiva, continuous with the 

 inner lining of the eyelid, and the rigid cornea beneath. The sides 



Pigmented 

 epithelium 



i ^Q^UOj L ^24 1 ^ i ^ 



Visual 

 layer 



Nervous <( 

 layer 



Rods 



and 



cones 



Fibers to 

 optic nerve 



Fig. 151. — Diagram of a vertical section of the Vertebrate retina (human). 

 The pigmented epithelium is the retinal layer farthest from the vitreous 

 chamber, and in contact with the choroid coat. 



and posterior part are also composed of two layers, the outer 

 sclerotic coat and the inner choroid coat. Suspended within 

 the eyeball is the lens which divides the cavity into two chief 

 parts; the one posterior to the lens, known as the vitreous 

 chamber, being lined by the retina whose nerve cells supply the 

 nerve fibers forming the optic, or second cranial nerve. 



The Vertebrate eye is an optical apparatus that may be com- 

 pared roughly with a camera, but this conspicuous difference 



