SENSE OF SIGHT. 849 



protect the eye from external injury, and, although not transparent, is to 

 a certain extent translucent. Anteriorly it passes into the cornea, which 

 though equally thick, is absolutely transparent. The latter membrane 

 rises in thickness in front of the eye like a watch-glass. In other words 

 the radius of curvature of the cornea is that of a smaller circle than that 

 of the body of the eye. The shape of the eye is preserved by two fluids, 

 the aqueous humor filling the cavity behind the cornea, and the denser 

 jelly-like vitreous humor occupying the larger posterior cavity (Fig. 361). 



Between these two chambers is a diaphragm the size of whose aper- 

 ture is capable of being modified — the iris. Behind the iris lies the crys- 

 talline lens and between the vitreous humor and the sclerotic is found 

 the choroid membrane, covered with dark pigment-cells, which are 

 arranged like a mosaic on its inner surface. Between the choroid and 

 the vitreous humor is found the retina, which is the transparent expan- 

 sion, in a number of layers, of the terminal filaments of the optic nerve. 



The most sensitive part of its surface is that which lies in contact 

 with the black pigment. Externally, in most vertebrates, the eyes are 

 protected by eyelids, which are, however, absent in fishes ; in them, the 

 eyes being continually bathed in fluid, the lachrymal apparatus is like- 

 wise absent. Their eyes are, as a rule, but slightly mobile, the crystal- 

 line lens is spherical, the cornea almost fiat, and the iris but slightly 

 contractile. In most fishes the eyes are placed so far back that the fields 

 of vision are distinct. 



In reptiles three eyelids are often found, although in some, as in the 

 serpents, they are entirely wanting. In the latter case, as in fishes, the 

 ocular globe is then covered only b_y the transparent conjunctiva. In 

 many reptiles there is often a rudiment of the lachrymal apparatus. The 

 crystalline lens varies greatly in form. 



In birds the sense of sight is especially developed. Those which are 

 in the custom of flying at great heights in the atmosphere appear to be 

 able to distinguish with the greatest exactness small bodies on the sur- 

 face of the earth. In birds, at the centre of the ocular globe and pos- 

 terior to the crystalline lens, is found a peculiar projection of the 

 choroid, impregnated with the choroid pigment and covered by an exten- 

 sion of the retina, which is termed the pectin. It is not known in what 

 way this structure serves to assist vision. Perhaps it contains muscular 

 fibres which act upon the crystalline lens and aid in accommodation. 



In mammals the ocular apparatus takes about the same form as is 

 seen in the human species, there scarcely being any difference except in 

 the relative volume of the eyeball and the pupillar}^ opening, and in the 

 fact that sometimes the shape of the ej^eball is elongated rather than 

 spherical. Animals which pass the greater part of their time under 

 ground are remarkable for the smallness of their eyeballs. In others 



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