. Outlines of the comparative physiology and morphology of animals. Anatomy, Comparative; Physiology, Comparative. 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 t
. Outlines of the comparative physiology and morphology of animals. Anatomy, Comparative; Physiology, Comparative. 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. 98.—Lizard's eye show- water vision is very imperfect if 5°|, thfJdfro" 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original LeConte, Joseph, 1823-1901. N
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