SENSES OF ANIMALS 



309 



objects at a distance of many miles, but the keenest vision 

 is probably possessed by birds. A fish hawk at a distance 

 of two hundred feet will spy a fish in the stream when 

 human vision can not detect it fifty feet away. The 

 chimney swift, 

 wheeling through 

 the summer air, no 

 doubt, sees the 

 minute insects con- 

 stituting its food. 

 Most of the lower 

 vertebrates are 

 decidedly short- 

 sighted. A toad 

 does not recognize 

 a fly at a distance 

 of more than two 

 or three feet, and 

 few common fish 

 distinguish food 

 beyond five feet. 



The structure of the vertebrate eye is admirably adapted 

 for its function. The eyeball is composed of the tough, 

 thick sclerotic coat, the thin black choroid coat, and the 

 pinkish retina almost surrounding the vitreous humor which 

 is separated from the aqueous humor by the crystalline 

 lens adjustable for far and near vision. The parts of an 

 eye of a cat, cow, or fish may be easily seen by cutting it 

 in two with a sharp knife. The structure is especially well 

 shown in an eye frozen before being cut. 



Many of the cdeleriterates, echinoderms, worms, and 



rt 



FIG. 345. Section of the cat's eye enlarged twice. 

 aq, aqueous humor ; c, cornea ; ci, ciliary pro- 

 cesses ; cp, capsule surrounding the crystalline 

 lens ; ch, choroid coat ; i, iris ; lg, ligament of the 

 lens ; m, muscle to adjust the lens for far and near 

 sight ; opn, optic nerve leading to the brain ; rt, 

 retina ; sc, sclerotic coat. From Davison's 

 " Mammalian Anatomy." 



