THE SPECIAL SENSES 135 



canal and is in contact with the air on both sides on 

 the outer side with the atmosphere and on the inner 

 with the inspired air as it reaches the back of the nose. 

 This arrangement secures equal pressure on the two 

 faces of the membrane and its importance is readily 

 seen in the impaired sense of hearing when the nose is 

 blocked up by a cold "in the head." Attached to the 

 inner face of this drum and stretching across the middle 

 ear (tympanic cavity) is a chain of minute bones (audi- 

 tory ossicles) so arranged that they act as a bent lever 

 which moves with every vibration of the drum and con- 

 veys these waves to a second drum which closes an 

 opening by which the middle would otherwise com- 

 municate with the inner ear. The latter is the part in 

 which the nerve terminals are located. Briefly de- 

 scribed the organ consists of a basement membrane 

 stretched from the flanges of a screwlike center to the 

 surrounding Avail, on which rest the ends of nerve cells 

 which terminate in hairlike processes in contact with 

 which is a very delicate membrane which is moved by 

 the waves created in the fluid (endolymph) which bathes 

 the organ. The movements of the overlying membrane 

 are communicated to the hairs and these in turn stimu- 

 late the nerve cells and the impulse or sensation is con- 

 veyed to the brain. 



Not all sound is conveyed through the external ear. 

 The sound of our own voices is, in part at least, con- 

 veyed through the bones of the head. A tuning fork 

 placed on the teeth will be heard so long as its vibra- 

 tions can excite movements of the ossicles. A simpler 

 experiment is to stop the ears and apply the teeth to a 

 watch, whose ticking can be heard plainly through the 

 teeth. 



The auditory, or eighth nerve is not entirely given 



