SENSE ORGANS. ! eg 



Fishes. — In these the lids are wanting, the eyes be- 

 ing kept moist by the water. The lens of fishes is very 

 peculiar. It is perfectly spherical and much denser than 

 in land animals. Both of these 

 qualities give greater refractive 

 power. This is necessary on ac- 

 count of the medium in which 

 they live, for the refractive pow- 

 er of the eye is the difference 

 between that of the medium and 

 of the lenses. This is well illus- 

 trated in the case of the diver. 

 Even in the most transparent Fig. 9 8.— Lizard's eye show- 

 water vision is very imperfect if 5°|, th fJ d f ro " c . bo ? es - 



J c (After Wiedersheim.) 



the eye is immersed. If the diver 



wishes to see distinctly under water he must supplement 

 the refractive power of the eyes by strong double 

 convex lenses, or else by double concave air spectacles. 

 Such spectacles may be easily extemporized by putting 

 two watch glasses back to back and cementing imper- 

 meable paper about the margins. It is evident that 

 these would act precisely like two convex water-lenses 

 in air. 



The ciliary muscles are wanting in fishes. They first 

 appear in amphibians — i. e., in the lowest land verte- 

 brates. Fishes, therefore, can not accommodate the eyes 

 for various distances by changing the form of the lens, 

 for it is already spherical. Their eyes are passively ad- 

 justed for near objects. They probably accommodate 

 for distant objects by drawing the lens back nearer to 

 the retina. 



Binocular Vision in Vertebrates. — There are three 

 points of structure which throw light on this subject — 

 viz., (i) the optic chiasm, (2) the position of the optic 

 axis, and (3) the fovea. 

 12 



