512 STUDIES IN ADVANCED PHYSIOLOGY. 



in certain forms of blowing, force the air through the Eu- 

 stachian tube into the middle ear, and thereby push out the 



Fig. 157. THE BONES OF THE MIDDLE EAR, WITH ARROWS INDICATING THE DIRECTION 



OF MOVEMENT OF EACH BONE IN THIS SERIES OF LEVERS. a, am, FULCRUM POINTS. 



tympanic membrane, the result of which would be to pull 

 its chain of bones with it, even at the risk of pulling the 

 stapes entirely out of its moorings. But this is prevented 

 by the arrangement of the teeth between the incus and 

 malleus in the manner just stated, so that while a motion 

 inward is at once communicated to the incus, a motion out- 

 ward affects the malleus alone. 



The exact manner in which these bones work may be 

 easily understood from the accompanying diagram, in which 

 the direction of motion of the various parts is indicated by 

 arrows. It will be noticed that the malleus moves like a 

 lever of the first class, its fulcrum being near its middle 

 at the point where it is fastened by means of the ligaments 

 to the wall of the middle ear. 



4. The Cavity of the Middle Ear. The function of 

 the middle ear is its ability to act as a resonance cavity. 

 But for this fact the tympanic membrane might have been 

 stretched across the oval foramen at once. Sounds are ma- 

 terially strengthened when brought near resonators. The 

 note of a violin is to a very large extent made possible by 

 the resonance of the violin frame beneath the string. It is 

 the piano-board almost as much as the string that gives the 

 tone, while in the case of a horn, tones would be impossible 

 without the resonance cavities in the horn. A tuning-fork 



