232 



ELEMENTARY BIOLOGY 



273. The human eye. We may think of our eye as a small 

 camera with sensitive nerve endings in the place where the 



film or plate would be (see Fig. 97). 

 The space between the lens and 

 the retina is filled with a trans- 

 parent, jellylike mass, and in front 

 of the lens the space under the 

 protective coat contains a watery 

 fluid. Finally, the eye is moved 

 about in its setting by muscles 

 attached to the bony framework, 

 and is further protected by the 

 movable lids. 



274. Other vertebrate eyes. While 

 our eye is in general very much like 

 the eyes of other backboned animals, 

 there are important differences, cor- 

 responding to the habits and the habi- 

 tats of the different groups. Animals 

 living in the water, for example, have 

 a different kind of lens ; animals that 

 prowl about at night have a different 

 kind of pupil. The fishes (except the 

 sharks) lack eyelids. Snakes have 

 their eyelids permanently closed, but 

 transparent. Among the birds and 

 in many reptiles there is a single 

 eyelid that passes over the eyeball 

 from the inner corner, under the 

 pair of eyelids (Fig. 98). 



A B 



FIG. 96. Compound eye 



In the Arthropoda, or jointed-legged 

 animals, there are compound eyes as 

 well as simple ones. /4, head of a 

 locust, showing the compound eye 

 with its many facets, each repre- 

 senting the exposed surface of an 

 ommatidium, or single eye. B, an om- 

 matidium, seen in section cut length- 

 wise, a, corneal lens ; , lens-growing 

 cells ; c, cone ; d, iris cells ; <?, retinal 

 cells, receiving light impressions; 

 /, retinal pigment ; g, perforated sup- 

 porting membrane 



275. The seeing eye. Although 

 sensitiveness to light is found 

 among all branches of the plant 



world and among all branches of the animal world, there are 

 only three main groups of animal that can actually see. These 

 are the highest mollusca, the arthropoda, and the vertebrates. 



