34 2 ELEMENTS OF PHYSIOLOGY 



by means of a hand mirror the size of the pupil. Turn at 

 right angles to the light, still looking in the mirror, and 

 note the size of the pupils. What have you noticed about 



the eyes of a cat at night or in a 

 darkened room ? How do your 

 own eyes feel when going from 

 the dark into a lighted room ? 

 Can you see as well when you 

 first go from a brightly lighted 

 room into a dim lighted one, as af- 

 ter being in the dim light for a 

 short time ? The iris contains cir- 

 cular muscle fibers, which reduce 



FIG. 220. -Test for Astigmatism. the size Q f the pupil> and mdi _ 



If this defect exists, the lines that run , /- 1-1 i 



in a certain direction will appear atmg fibers, which enlarge the 



d7rSions than the Hnes in ther PUP* 1 ' The arrangement of pig- 

 ment sometimes follows the line 

 of the fibers. Have you ever noticed lines in the iris ? 



618. After-images. Did you ever whirl a stick with a 

 glowing coal on its end ? What was noticed ? Can you 

 notice anything similar if you shake the hand up and 

 down quickly before the face ? If you gaze for a moment 

 at a bright light, then quickly close the eyes, what is noticed 

 after the eyes are closed ? These effects are produced by 

 the bleaching action of light upon the pigment of the retina, 

 an effect which persists for a fraction of a second after the 

 light is removed. 



619. The Yellow Spot, or center for most distinct vision 

 (especially for color), is near the middle of the back part 

 of the retina (Fig. 214). 



620. Blind Spot. Light falling on the optic nerve itself 

 does not give the sensation of light, but gives it only when 

 falling upon the ends of the nerve fibers. Where the 

 optic nerve enters the eyeball, there are none of these end- 

 ings, and the light that falls there does not enable us to 



