SENSE ORGANS 
475 
Care of the Eyes. — The eyes are our most precious sense 
organs, and as such they should receive the best of care. 
Certain imperfections in the lens or other parts of the eye 
can be helped by the use of glasses. If your eyes annoy 
you, or if you cannot see objects as clearly as your school¬ 
mates, have a competent oculist examine and treat them. 
The Ear. — The ear is a sense organ for the reception 
of the stimuli which we interpret as sounds. The ear of 
man consists of the outer, middle, and inner ear. The 
first two carry the stimuli to the 
third, where they are received by 
nerve cells and carried to the brain. 
The diagram of the ear (Figure 
418) shows the several parts and 
their relations. The outer ear 
leads to the tympanic (tim-pan'ik) 
cavity; the middle ear is in com¬ 
munication with the mouth, and 
the complex inner ear is partly 
shown. There is a group of small 
bones in the middle ear which con¬ 
duct the sound vibrations to the delicate inner ear. The 
internal ear receives the various sound waves, and transmits 
them to the brain, where they are explained as sounds. 1 
Hearing. — Sound waves strike the ear drum (tympanic 
membrane), which in turn causes the small bones in the 
middle ear to vibrate. The bones cause the water in the 
internal ear to move, thus stimulating the nerves of 
hearing. 
The pressure of air on each side of the ear drum is nor¬ 
mally the same. This is due to the entrance into the mid¬ 
dle ear of air from the mouth, through the eustachian tube 
Figure 418.— Plan of Ear. 
0. E, outer ear; M. E, middle 
ear; I. E, inner ear; Eu, eu¬ 
stachian tube. 
1 When certain parts of the ear (semicircular canals) are injured, one has 
difficulty in standing or in walking erect. This is because the inner ear 
serves both as a hearing and a balancing organ. 
