418 CONTRACTION AND DILATATION OP THE PUPIL : RETINA. 



534. By the contraction and relaxation of certain fibres in 

 the Iris, the size of the Pupil is changed according to the 

 degree of light to which the eye is exposed ; the aperture being 

 made to diminish in a strong light, in such a manner as to 

 exclude the rays that would be superfluous, and to prevent 

 too many from falling upon the expansion of the optic nerve ; 

 whilst it dilates in a faint light, so as to admit as many rays 

 as possible. If we notice the pupil of a Man who is looking 

 towards the mid-day sun, we shall see that it is contracted to 

 a small round speck ; but the pupil of a Sheep would be con- 

 tracted, in similar circumstances, into a horizontal slit ; and 

 the pupil of a Cat into a vertical one. The alteration in the 

 size of the pupil in accordance with the degree of light, may 

 be easily observed by stationing oneself at a window pro- 

 vided with shutters, and holding a looking-glass in the hand : 

 if the light be at first strong, the pupil will be seen in a con- 

 tracted state ; but if the shutters be gradually closed, so as to 

 diminish the amount of light that falls upon the eye, the 

 pupil will be seen to enlarge; and it will diminish again 

 when the shutters are re-opened. The blackness which the 

 pupil always presents, in the healthy state of the eye, is due 

 to our seeing the black lining of the back of the eye through 

 it. In many Quadrupeds, the black pigment is replaced, in 

 one portion of the eye, by a layer of a blueish colour, having 

 an almost metallic lustre ; and from this we see the light 

 brilliantly reflected, when we look at their eyes in certain 

 directions. 



535. On turning back the choroid coat, we come to the third, 

 r, of the layers of which the wall of the eye is composed : this 

 is an extremely delicate film, chiefly consisting of nerve-cells 

 and nerve-fibres that spread-out from the optic-nerve, n ; and 

 it is called the Retina (or net). It advances nearly as far as 

 the iris ; but it is deficient in the front of the eye. The part 

 of the retina which lines the globe at the point exactly 

 opposite to the centre of the pupil, is distinguished from the 

 rest by a peculiar yellow opacity, which causes it to be 

 designated " the yellow spot/' In this spot, visual sensibility 

 is more acute than elsewhere. On the other hand, the part 

 of the retina that covers the entrance of the optic-nerve 

 (which is below " the yellow spot," and nearer to the nose) is 

 so much less sensitive to light than the rest, that under 



