424 



HUMAN PHYSIOLOGY 



a transparent tough layer called the cornea. A second cop.t, the 

 choroid, is supplied with blood vessels and cells which bear pigments. 

 It is this coat which we see through the cornea as the colored part 

 of the eye (the iris). In the center of the iris is a small circular 

 hole (the pupil). The iris is under the control of muscles, and may 

 be adjusted to varying amounts of light, the hole becoming larger 

 in dim light, and smaller in bright light. Watch the pupil of 



a cat's eye, in 

 changing the ani- 

 mal from a dark 

 to a light room. 

 The inmost layer 

 of the eye is called 

 the retina. This 

 is, perhaps, the 

 most delicate layer 

 in the entire body. 

 Despite the fact 

 that the retina is 

 less than -g^^- of 

 an inch in thick- 

 ness, there are no 

 less than eight 

 layers of cells in 

 its composition. 

 The optic nerve 



Longitudinal section through the eye; Sc, sclerotic coat ; Ch, cho- 

 roid; O, optic nerve ; C, cornea; /.iris; B, blind spot; con, con- 

 junctiva; R, retina; Y, yellow spot; L, lens; A, anterior 

 chamber, filled with aqueous humor; V, posterior chamber, 

 filled with vitreous humor. 



enters the eye from behind and spreads out over the surface of 

 the retina. Its finest fibers end in elongated rodlike and cone- 

 like cells. The retina is dark purple in color, this color being 

 caused by a layer of cells next to the choroid coat. This accounts 

 for the black appearance of the pupil of the eye, when we look 

 through the pupil into the darkened space within the eyeball. 

 The retina acts as the sensitized plate in the camera, for on it are 

 received the impressions which are transformed and sent to the 

 brain as sensations of sight. The eye, like the camera, has a lens. 

 This lens is formed of transparent, elastic material. It is found 

 directly behind the iris and is attached to the choroid coat by 



