PERCEPTION OF COLORS 705 



Color-blindness is an abnormal condition in which the power of dis- 

 crimination between different colors is impaired or lost. Some persons 

 are insensible to all colors and others to certain colors only. Red-green 

 blindness is the most common form. Cases of disease of the brain, in 

 which ordinary visual perception remains while the sense of color is 

 absent, seem to show that parts of the visual centres are specially con- 

 cerned in the appreciation of colors. That this defect, however, may 

 depend on the retina, is shown in cases in which one eye is color-blind 

 while the other is normal. There are cases, also, in which these defects 

 seem to be due exclusively to atrophy of the disk and contraction of 

 the visual field, the acuteness of vision not being much impaired ; but 

 unfortunately even these striking pathological conditions throw little 

 light on the physiology of normal color-perception. 



PARTS FOR THE PROTECTION OF THE EYEBALL 



The orbit, formed by the union of certain of the bones of the face, 

 receives the eyeball, the ocular muscles, the muscle of the upper lid, 

 bloodvessels, nerves and a part of the lachrymal apparatus. It con- 

 tains, also, a certain quantity of adipose tissue, which latter never dis- 

 appears, even in extreme marasmus. The bony walls of this cavity 

 protect the globe and lodge the parts enumerated above. The internal, 

 or nasal wall of the orbit projects considerably beyond the external 

 wall, so that the extent of vision is greater in an outward than in an in- 

 ward direction. As the globe is more exposed to accidental injury from 

 an outward direction, the external wall of the orbit is strong, while the 

 bones that form its internal wall are comparatively fragile. The upper 

 border of the orbit (the superciliary ridge) is provided with short stiff 

 hairs (the eyebrows) which serve to shade the eye from excessive light 

 and to protect the eyelids from perspiration from the forehead. 



The eyelids are covered with a thin integument and are lined with 

 the conjunctival mucous membrane. The subcutaneous connective tissue 

 is thin and loose and is free from fat. The skin presents a large number 

 of short papillae and small sudoriparous glands. At the borders of the 

 lids, are short, stiff, curved hairs arranged in two or more rows, called 

 the eyelashes, or cilia. Those of the upper lid are in greater number 

 and longer than the lower cilia. The curve of the lashes is from the 

 eyeball. They serve to protect the globe from dust, and to a certain 

 extent to shade the eye. 



The tarsal cartilages are small, elongated, semilunar plates, extend- 

 ing from the edges of the lids toward the margin of the orbit, between 

 the skin and the mucous membrane. Their length is about an inch 



