584 



COMPARATIVE ANATOMY 



carried into the anterior brain vesicle, so that the retina became inverted 

 and each eye grew in the form of a hollow ball towards the skin and the 

 source of light. The photoreceptors, however, come to lie on the side of 

 the retina farthest from the source of light, and hence are "inverted" in 

 position. Possibly a lateral line organ was converted into a lens, which by 

 enlarging and pushing into the optic vesicle, converted this into a two- 

 layered cup with an inner retinal, and an outer pigment layer. Finally, 

 the surrounding mesenchyma was converted into a sclerotic layer and the 

 eye-ball thus formed became connected with the eye muscles. 



The paired eyes of all vertebrates are essentially similar. Except in 

 size, the eye of petromyzon is like that of mammals. Having once 

 invented a camera-like eye in cyclostomes, "nature" has pursued the 

 policy of letting well enough alone. A description of the human eye will, 

 therefore, serve for vertebrates generally. 



CCONJUNCTIVA. 



FOVEA CENTHALIS 



Fig. 481. — A diagram of a median section of the eye. (Redrawn after Sobotta.) 



The Eye of Man 



Comparison of the eye with a camera holds for many details. Both are 

 mechanisms by which an image is focused upon a sensitive layer, the film 

 or the retina, the opaque sclerotic corresponding with the box of the 

 camera. Each has a lens to form the image, and an iris diaphragm to 

 regulate the amount of light. The eye, however, is filled with liquid, 

 the humors of the eye. That between the translucent cornea and the 

 lens is the aqueous humor ; the semisolid material between lens and retina 

 is the vitreous humor. A ray of light entering the eye passes first through 

 the cornea, then successively through aqueous humor, pupil, lens, vitreous 



