380 ELEMENTARY PHYSIOLOGY less. 



reach it they must, under ordinary circumstances, have 

 become so scanty and weak, that they may be left out of 

 considei'ation. 



The aerial waves which enter the meatus all impinge 

 upon the membrane of the drum and set it vibrating, 

 stretched membranes, especially such as have the form 

 and characters of the tympanic membrane, taking up 

 i'ibrations from the air with great readiness. 



The vibrations thus set up in the membrane of the 

 tympanum are communicated, in part, to the air contained 

 in the drum of the ear, and, in part, to the malleus, and 

 thence to the other auditory ossicles. 



The vibrations communicated to the air of the drum 

 impinge upon the inner wall of the tympanum, on the 

 greater part of which, from its density, they can produce 

 very little effect. Where this w^all is formed by the 

 membrane of the fenestra rotunda the communication of 

 motion must necessarily be greater. All these vibrations, 

 however, may probably be neglected. 



The vibrations which are communicated to the malleus 

 and the chain of ossicles may be of two kinds : vibrations 

 of the particles of the bones, and vibrations of 

 the bones as a whole. If a beam of wood, freely 

 suspended, be very gently scratched with a pin, its 

 particles will be thrown into a state of vibration, 

 as will be evidenced by the sound given out, but the 

 beam itself will not be visibly moved. Again, if a strong 

 wind blow against the beam, it will swing bodily, without 

 any vibrations of its particles among themselves. On 

 the other hand, if the beam be sharply struck with a 

 hammer, it will not only give out a sound, showing that 

 its particles are vibrating, but it will also swing, from the 

 impulse given to its whole mass. 



Under the last-mentioned circumstances, a blind man 

 standing near the beam would be conscious of nothing 

 but the sound, the product of molecular vibration, or 

 invisible oscillation of the particles of the beam ; while 



