218 



THE NERVOUS SYSTEM OF MAN 





The diagram of the ear (Figure 229) shows the several 

 parts and their relations. The outer ear leads to the tym- 

 panic (tim-pan'ik) cavity ; the middle ear is in commu- 

 nication with the mouth, and the complex inner ear is 

 partly shown. There is a group of small bones in the 

 middle ear which conduct 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 mem- 

 brane), 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 (see page 166). This tube is a trifle more than 

 an inch long. When it becomes closed, partial deafness 

 results. 



Defects in hearing may be caused by blows upon the 

 ears, by the accumulation of wax in the ears, and by sore 

 throat. When there is a continued ringing or hissing 

 sound in the ears, consult a doctor at once. 



167. Brain Efficiency. — While the efficiency of the brain 

 depends upon mental training, in order properly to exer- 

 cise the many functions of this organ at least three things 



Figure 229. — Plan of Ear. 



0. E, outer ear ; M. E, middle 

 ear ; /. 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. 



