ADDITIONAL TOPICS IN HUMAN BIOLOGY 167 



because the sudden compression of the air is likely to injure the drum. 

 Earache is often relieved by hot applications ; never should lauda- 

 num or other substances be put into the ear without the advice of 

 a physician. 



250. The middle ear. — Beyond the tympanum is a small cav- 

 ity, known as the middle ear. From this cavity a narrow tube 

 (the Eustachian tube) about an inch and a half long, commimicates 

 with the upper part of the throat cavity (Fig. 55). If one were to 

 go up on a high mountain, he would find that the pressure of the 

 air on the outside of the body, and therefore on the exterior of 

 the eardrum, would become less, and if some of the air in the middle 

 ear were not to escape, the eardrums would be forced outward, and 

 hence would be ruptured. If, on the other hand, one should go into 

 a deep mine, the increased pressure on the outside of the drums 

 would force them inward. All these accidents are prevented by the 

 presence of the Eustachian tubes, through which air can pass into 

 and out from the middle ear, and so the pressure on both sides of 

 the tsrmpanum can be equalized. In severe head colds, the opening 

 from the throat cavity into the Eustachian tubes becomes tempo- 

 rarily closed and we are then conscious of a ringing sensation in the 

 ears. Catarrh sometimes closes the Eustachian openings and causes 

 deafness. If the hearing seems to be at fault in any way, a specialist 

 should be consulted. 



251. Sensations of sound. — When a stone is dropped into water, 

 the ripples move outward over, the surface in circular waves. In a , 

 sinular manner sound waves are transmitted in aU directions from 

 a given body, for instance, a vibrating bell. When some of these ^ 

 sound waves enter the tube of the external ear, they cause the ear- 

 drum to vibrate, and this vibration is transmitted across the middle 

 ear by a chain of tiny bones, and so reaches the complicated irmer ear, 

 which is a series of canals imbedded in solid bone. The inner ear 

 contains a large number of sensitive cells which transfer the vibra- 

 tions to nerves communicating with the brain. When the brain 

 cells receive and interpret these impulses, we get sensations of 

 sound. 



