516 APPLIED BIOLOGY 



so that by pressure its shape can be changed. Back of the 

 lens is the sensitive membrane (retina), which is closely 



attached to the back 

 wall of the eyeball, 

 and hence is hemi- 

 spherical in shape. 

 From near the center 

 of the retina the optic 

 y nerve extends to the 

 brain. 



The space between 



FIG. 164. Diagram of human eye. w, outer or the lens and the ret- 



sclerotic wall ; y, inner or choroid wall ; r, ma j g filled with a 

 retina (dotted line) ; c, cornea ; t, iris ; I, lens . ,, ,., 



with its suspending capsule; v, vitreous transparent jelly-llKe 



humor; TO, optic nerve; 1-2, a.u object, and substance (vitrCOUS 



ntheretina ' (FromHou h humor). The small 



space between the 

 lens and the cornea is filled with a watery fluid (aqueous 

 humor). If an eyeball, obtained from a meat-market, be 

 punctured, these humors escape, and the eyeball collapses. 

 They are so transparent as not to interfere with the passage 

 of light from the cornea to the retina. 



Between the retina and the outer white coat (sclerotic) 

 is a layer of tissue (choroid) with abundant blood-vessels and 

 black pigment. The pigment in the choroid and iris pre- 

 vents light from entering the eye except through the pupil 

 and the lens. In the same way black pigment in a photo- 

 graphic camera prevents light from reaching the sensitive 

 plate or film except through the lens and the opening of 

 the diaphragm which corresponds in use to the iris and its 

 opening. 



In both a camera and an eye the whole structure is essen- 

 tially a light-proof box with a diaphragm or iris for regulating 

 the amount of light, and a lens for focusing the rays of light 

 upon a sensitive plate or retina. The chief differences be- 



