398 



ORGAN SYSTEMS OF MAN 



eye muscle 



suspensory liqarwent 



ciliary muscle 



Fig. 16-4. A longitudinal section of the human eye to show its internal structure. 



Nearly all animals from the simplest to 

 the most complex are sensitive to light. Not 

 that they all are sensitive to the same wave 

 lengths that are recorded by the human 

 eye, but nearly all have evolved some sort 

 of receptor which is sensitive to light. It has 

 been proved, for example, that the bee sees 

 shorter light waves than we can see. Dogs, 

 on the other hand, appear to be color-blind. 

 It is thus abundantly clear that different 

 animals see, hear, smell, and so forth, quite 

 differently from man. Indeed one must 

 carefully avoid an anthropomorphic atti- 

 tude with respect to all behavior. 



Euglena orients itself with respect to 

 light by means of the stigma, and without 

 this photo-sensitive organelle it would be 

 unable to seek proper illumination for 

 photosynthesis (Fig. 16-3). The delicate 

 jellyfish {Gonionemus) possesses photore- 

 ceptors that help orient it to light in the 

 ocean. Planaria has rudimentary eyes which 

 produce no images but detect direction. 

 Neanthes has a set of four eyes which may 

 be superior to those of planaria. The lobster 

 has excellent compound eyes, the details of 

 which were discussed earlier. The octopus, 

 as well as some of its relatives, possesses a 

 remarkably perfect eye. Strangely enough, 

 although it has evolved along an entirely 



different path, it resembles the vertebrate 

 eye very closely in most respects. This is 

 one of the very interesting cases of conver- 

 gent evolution, where very similar organs 

 have evolved along two entirely different 

 routes (p. 660). 



The vertebrate eye. Although there are 

 some minor differences in the eyes of vari- 

 ous vertebrates, the human eye will serve 

 as representative of the group in our dis- 

 cussion (Fig. 16-4). It is first necessary 

 to consider briefly the way light behaves 

 before an understanding can be had of the 

 function of the various parts of the eye, par- 

 ticularly the lens. 



Light travels in straight lines at a speed 

 of 186,000 miles per second in air, but it 

 travels at different rates in other media such 

 as water, glass, and transparent tissues such 

 as the lens. Therefore, when light passes 

 from one medium to another it bends (re- 

 fracts). It is a familiar fact to those who 

 have observed it that when a stick lies at 

 an angle partially in water, it appears bent. 

 Actually, of course, the light is coming to 

 the eye through two different media, water 

 and air, and at different speeds, hence the 

 bending at the juncture of the two media. 

 Light coming through glass is bent in a 

 similar fashion, and when the glass is 



