THE SPECIAL SENSES 



267 



the cheeks. This is called crying or weeping. The tears 

 can be made to flow in a reflex way by almost any violent 

 stimulation, as, for example, by irritation of the nasal pas- 

 sages by pepper. 



367. Color Blindness. The inability to distinguish between 

 certain colors is called color blindness. It is sometimes pro- 

 duced by sickness, but it may exist at birth and is often 

 hereditary. It is in- 

 curable. 



This defect of sight 

 is quite common. Out 

 of the many thousands 

 that have been exam- 

 ined, it is found that 

 four men out of every 

 hundred are lacking 

 in the power to dis- 

 tinguish between cer- 

 tain colors. Total 

 color blindness is very 

 rare. 



A person who is 

 color blind cannot 

 match colors. He may 

 pick up red, brown, 

 and orange wools and 

 match them with 

 green of different shades. A person may be color blind and 

 not know it until the defect is accidentally revealed. 



This defect of vision is a matter of the utmost prac- 

 tical importance to those employed on electric and steam 

 railways, vessels, and other places where colored signals 



FIG. 1 66. The Relative Position of the 

 Lachrymal Apparatus, the Eyeball, and 

 the Eyelids. 



A, lachrymal canals, with the minute orifices 

 represented as two black dots to the right; 

 B, tendon attached to a muscle which sur- 

 rounds the circumference of the orbit and eye- 

 lids ; under B is seen the kchrymal sac. The 

 minute openings of the Meibomian glands are 

 seen on the free margins of the eyelids. Below 

 A is seen a small conical elevation, with black 

 dots (the lachrymal papilla, or caruncle). 



