THE SENSE ORGANS 



409 



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. The "centers of 

 vision," however, shift from the roof of the mid-brain to the occipital 

 lobes of the cerebral hemispheres, where they are located in mammals. 

 But the mid-brain of mammals persists as a reflex center. Having once 



LIBRAR 



MASS. 



FOVEA CENTRALIS 



Fig. 359. — A diagram of a median section of the eye. (Redrawn after Sobotta.) 



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

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

 therefore, serve for vertebrates generally. 



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 or body. A ray of light entering the eye passes 

 first through the cornea, then successively through aqueous humor, pupil, 



