154 HUMAN BIOLOGY 



light fades so gradually that you will surely be straining 

 the eyes before you know it. Do not work with the light 

 in front ; the glare of the light makes objects appear dim. 

 The light should come from above, and (for right-handed 

 people) from the left. Do not read papers or books 

 printed in fine type. We should not read when convales- 

 cing from illness ; with the head bent down ; when the 

 eyes are sore ; in jolting cars. Heating the eyes by a 

 burner, or drying the eyeballs in a dry, stove-heated at- 

 mosphere, using a light without a shade, cause trouble 

 with students' eyes. Of what are blood-shot eyes often a 

 sign ? Our eyes are best suited for seeing at a distance 

 because primitive man had no houses, books, sewed 

 clothes. Effort is required to shape the lens for seeing 

 near objects. Most cases of nearsightedness begin when 

 children are taught to read under eight years old. The 

 eyes are sometimes injured by the use of tobacco. 



THOUGHT QUESTIONS. The Eye. 1. The eye is shielded from 



blows by bony projections of , , and . 2. The hairs of 



the eyebrows lie inclined toward , in order to turn from the 



. 3. I find by trying it that I (can or cannot ?) see the position 

 of a window with my eyes closed. 4. The pupil appears to be black, 



because no is from the interior wall of the eye. I know that 



the iris is partly muscle, because it the size of the . 



Sound. Anything that is sending off sound does so by vibrating, 

 or shaking to and fro, very rapidly. For instance, a vibrating violin 

 string sets every particle of air near it swinging to and fro. The near- 

 est particles of air strike the next ones and bounce back, these in turn 

 strike against others, and thus vibrations called sound waves are sent 

 through space in all directions from the sounding body. We feel these 

 waves with the ear. 



The ear consists of three portions : the external ear, the 

 middle ear (or drum), and the internal ear (or labyrinth, 

 see Fig. 128). The cranial nerve connecting the ear with 

 the brain is called the auditory nerve. The outer and 



