OF SIGHT. 59 



its peculiar colour, and is called the iris (ff). The iris readily 

 contracts and dilates, so as to enlarge or diminish the open- 

 ing in its centre, the pupil, according as more or less light 

 is desired. Sometimes the pupil is circular, as in man, the 

 dog, the monkey ; sometimes in the form of a vertical ellipse, 

 as in the cat ; or it is elongated transversely, as in the sheep. 



125. The third membrane is the retina (d). It is formed 

 by the optic nerve, which enters the back part of the eye by 

 an opening through both the sclerotic and choroid coats, and 

 expands into a whitish and most delicate membrane upon the 

 vitreous humour (h). It is upon the retina that the images of 

 objects are received, and produce impressions, which are con- 

 veyed by the nerve to the brain. 



126. The fluids which occupy the cavity of the eye are 

 of different densities. Behind, and directly opposite to the 

 pupil, is placed a spheroidal body, called the crystalline 

 lens (e). It is tolerably firm, perfectly transparent, and com- 

 posed of layers of unequal density, the interior being always 

 more compact than the exterior. Its form varies in the differ- 

 ent classes. In general, it is more convex in aquatic than in 

 land animals ; whilst with the cornea, it is the reverse, being 

 flat in the former, and convex in the latter. 



127. By means of the iris, the cavity (z) in front of the 

 crystalline is divided into two compartments, called the anterior 

 and posterior chambers (i). The fluid which fills these cham- 

 bers is a clear watery liquid, called the aqueous humour. The 

 portion of the globe behind the lens, which is much the largest, 

 is filled by a gelatinous liquid, perfectly transparent, like that 

 of the chambers, but somewhat more dense. This is called 

 the I'itreous humour (A). 



128. The mechanical structure of the eye may be 

 imitated by art ; indeed, the camera obscura is an instru- 

 ment constructed on the same plan. By it, external objects 

 are pictured upon a screen, placed at the bottom of the 

 instrument, behind a magnifying lens. The screen repre- 

 sents the retina ; the dark walls of the instrument represent 

 the choroid ; and the cornea, the crystalline and the vitreous 

 humour combined, are represented by the magnifying lens. But 

 there is this important difference, that the eye has the power of 

 changing its form, and of adapting itself so as to discern, with 

 equal precision, very remote, as well as very near objects. 



129. By means of muscles which are attached to the 



