The outlines of anatomy, physiology, and hygiene Being an edition of The essentials of anatomy, physiology, and hygiene, rev to conform to the legislation making the effects of alcohol and other narcotics upon the human system a mandatory study in public schools . colors with ease, but can pick out themost delicate shades of any particular color withunfailing accuracy, others are unable to discrimi-nate, for example, between red and green. Suchpersons are called color-blind, and can not see anydifference between the fruit and the leaves of acherry-tree, excepting by the shape. 385. Near-Sight
The outlines of anatomy, physiology, and hygiene Being an edition of The essentials of anatomy, physiology, and hygiene, rev to conform to the legislation making the effects of alcohol and other narcotics upon the human system a mandatory study in public schools . colors with ease, but can pick out themost delicate shades of any particular color withunfailing accuracy, others are unable to discrimi-nate, for example, between red and green. Suchpersons are called color-blind, and can not see anydifference between the fruit and the leaves of acherry-tree, excepting by the shape. 385. Near-Sight and Far-Sight.—In the naturaland healthy eye the ravs of light are brought toa focus on the retina, but in some eyes they are 304 ORGANS OF PERCEPTION. not. In near-sighted persons the eyeball is usuallya little too long, and the rays of light, being broughtto a focus before they reach the retina, cross eachother, and the image is blurred. In far-sightedeyes the eyeball is too short, or the lens is too flat,and the rays are not brought to a focus at all, sothat the effect upon vision is to make everythinglook blurred, as in the former case (Fig. 71). Bothof these defects can and should be corrected byproper glasses, for the constant straining to see, of. Fig. 71.—Diagram showing- the point at which the rays of hght are broughtto a focus in different eyes. short-sighted and far-sighted persons who try to getalong without glasses, eventually injures the Muscles of the Eyeball.—To the outside ofthe eyeball are attached six muscles, by means ofwhich it can be moved in any direction. Four ofthem pass from the four sides of the eyeball straightbackward to the bone at the rear of the orbit, andmove the eye up or down, and to the right or other muscles are attached, one to the upperand the other to the lower surface of the eye-ball,their other ends being attached to the bones on theinner side of the orbit in such a way that, by theircontraction, the eyeball is
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