Page:First course in biology (IA firstcourseinbio00bailrich).pdf/622

 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 convalescing 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 atmosphere, 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.

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 nearest 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