260 Life and Health 



\g 407. The Tears. If the eyelids are irritated the sensory 

 nerves are stimulated and the impression is carried to the 

 brain. Thence the nerve impulses travel 

 to the lachrymal glands, leading to an 

 increased flow of their secretion. Thus 

 the irritation of the sensory nerves in 

 the nasal passages by smelling such 

 substances as onions, or pungent salts, 

 may cause a copious flow of tears. 



Various emotions, as joy and grief, 

 may produce similar results. In these 

 cases the glands secrete the fluid in 

 such quantities that it cannot escape 



FIG. 136. Lachrymal , ., , , , , . . 



Canals, Lachrymal b 7 the lachrymal canals, and the excess 



Sac, and Nasal Duct, rolls over the cheeks as tears. Exces- 



opened by their Ante- s i ve grief sometimes acts on the nerve 



centers in exactly the opposite manner, 



so that the activity of the glands is arrested and less fluid 



is secreted. This is the reason why some people do not 



shed tears in times of deep grief. 



Experiment 119. Gently turn the inner part of your lower eyelid 

 down. Look in a mirror, and the small lachrymal point, or opening 

 into the nasal duct, may be observed. 



408, Color-Blindness. The power of discriminating be- 

 tween different colors is impaired in color-blindness. Experi- 

 ment shows that ninety-six out of every one hundred men 

 agree as to the identity or the difference of color. The 

 remaining four show a defective perception of color and are 

 said to be color-blind. 



In its true sense, color-blindness is always congenital, often 

 hereditary. This condition of abnormal vision is totally in- 

 curable. A person may be color-blind and not know it until 



