146 PHYSIOLOGY AND TEMPERANCE. 



17. Color- Blindness. Color-blindness is an inability to 

 distinguish colors. It has been ascertained that about four 

 persons out of every hundred are thus affected. The colors 

 which usually give the most difficulty are red and green, and 

 as these are the colors most frequently used in connection 

 with the signal lights of railroads and steamboats, it becomes 

 a serious matter. 



Color-blindness, in its true sense, is usually an inherited 

 defect in sight, but, as a matter of fact, children and others 

 often fail to recognize colors because they have not been 

 trained to do so. The faculty of distinguishing colors should 

 be cultivated from childhood onward. 



18. The Sense Of Hearing. The organ of hearing is 

 divided into the outer, middle and inner ear. The outer ear 

 comprises that part which stands out prominently on either" 

 side of the head, and the small tube or canal leading into the 

 bone, called the auditory canal. The peculiarly shaped outer 

 part, commonly called "the ear," is so constructed to collect 

 sound and transmit it through the auditory canal. It con- 

 sists of plates of cartilage covered with skin on both sides. 

 The auditory canal is about an inch long, and is lined by a 

 continuation of the skin of the ear. Glands are found in 

 this lining which secrete ear-wax, to moisten and protect the 

 parts. Fine hairs grow at the outer part of the canal, to 

 prevent insects and foreign matter from getting into the ear. 



At the bottom of the canal, stretched across it, is a thin 

 membrane, the drum. It is this thin membrane that receives 

 the sound-waves in the ear. 



19. The Middle Ear. Beyond the drum is a small 

 cavity, the middle ear. Hanging across this chamber, from 

 the drum inward, is a chain of three very small bones, which, 

 from their shape, have been named the hammer, the anvil, 

 and the stirrup. These bones, though so small, are complete 



