60 SPECIAL SENSES. 



ball, the eyes may be rolled in every direction, so as to view 

 objects on all sides, without moving the head. The eyes are 

 usually protected by lids, which are two in the mammals, and 

 generally furnished with a range of hairs at their edges, called 

 eye-lashes. Birds have a third, or vertical lid, which is also 

 found in most reptiles, and a few mammals. In fishes, the 

 lids are wanting, or immovable. 



DIOPTEICS OF THE HUMAN EYE. 



[ 130. " The rays of light which attain the retina, and there 

 unite to form images, must of course pass through the whole 

 of the refracting media described in the preceding paragraphs. 

 The refracting powers of these media, which are spoken of 

 collectively as the humours of the eye, differ in conformity 

 with the fashion, structure, density, and chemical constitution 

 of each.* These humours are farther the principal cause of 

 the form of the eye-ball, which not only differs in reference 

 to kinds, but also among individuals of the same kind. In 

 man, the eye-ball, in a general way, presents the form of an 

 ellipsoid open in front, where it is met and completed by a 

 small segment of a sphere engrafted upon it. The axis of the 

 eye corresponds with the optic or visual axis, and extends 

 from the centre of the cornea backwards to the foramen of 

 Soemmerring, a little to the outside of the point at which the 

 optic nerve makes its entrance. This optic axis of the eye 

 measures on an average from 10^ to 11 lines, arid differs from 

 the axis of the optic nerve which passes from the outer third 

 of the cornea, to the middle of the point of entrance of the 

 optic nerve, crossing the optic axis at an angle of about 20 

 degrees. In its general condition, the eye is so fashioned that 

 the rays which arrive from a point divergingly upon the 

 cornea, are immediately made to converge, and this in such 

 measure precisely, that they meet in a focus as they attain the 

 retina. It is of course the central ray alone of a pencil of 

 rays that passes through dioptric media unrefracted ; all the 

 other rays suffer refraction, and are approximated to the 



* [We have various estimates of the refracting powers of the transparent 

 media of the eye, a summary of which is given by Weber in his edition of 

 Hildebrand's Anatomy, IV. 103. The numbers of the several humours of 

 the human eye, according to Brewster, are the following: Cornea, 1,386 ; 

 aqueous humour, 1,3366 ; lens, as a whole, 1,3767 ; middle portion of the 

 same, 1,3786; nucleus of ditto, 1,3999 (according to Young, 1,4025J ; 

 vitreous humour, 1,3394.] 



