202 THE EAR 



thick, it is constructed of three layers. On the outer surface 

 there is a layer of epithelium protecting the membrane proper 

 which is of fibrous tissue and is covered on the inner side by a 

 layer of mucous membrane. The fibres of the fibrous layer are 

 arranged partly circularly and partly radially the circular fibres 

 being most marked near the rim. To the inner surface is attached 

 the handle of the malleus, the first of the chain of three auditory 

 ossicles. This attachment to the malleus, which is pulled inwards 

 by the tensor tympani muscle, gives the tympanic membrane the 

 form of an eccentric funnel opening outwards. The membrane 

 is highly elastic and responds very readily to very slight variations 

 in the pressure of the air waves entering the external ear. The 

 peculiar form of the membrane contributes to its value as a sound 

 transmitter. In the first place, it acts synkinetically, i.e. moves 

 passively with the vibrations of the sound-waves. It begins and 

 ends its vibrations synchronously with the impact of the sound 

 vibrations. There is no latent period, no waiting for a summation 

 of impulses before it can get into its swing, having no swing to get 

 into. It does not continue to vibrate after the sound vibrations 

 have ceased. It is dead-beat. Further, it does not vibrate 

 sympathetically to any special overtone present in a compound 

 tone reaching the ear. This is brought about by (i) the damping 

 effect of attachment to the ossicles and (ii) by the dragging inwards 

 at the point of attachment (umbilicus). On this account the 

 fibres vary in tension as well as in length, so endowing each bit 

 of the membrane with a different period of vibration resulting, in 

 toto, in an aperiodic membrane. It is obvious that such a property 

 is valuable in rendering hearing distinct. In the second place 

 the arched sides of the membrane act as a lever of the 1st class. 



" As the outward curvature of the radial fibres is slight, each 

 fibre may be regarded as the long arm of a lever, while the handle 

 of the hammer is the short arm. This mechanism secures that a 

 slight pressure of the air corresponding to a sound wave, exerts 

 a considerable force upon the malleus. To aid in understanding 

 the mechanism, it will be easier to consider, first, the effect of 

 pressure upon a single radial fibre. The fibre may be regarded as 

 inextensible and slightly curved outwards ; hence variations in 

 pressure on the convexity of the curve will cause the degree of 

 curvature to change, while the length of the arc will remain the 

 same. 



In other words, the radius of the arc and the chord of the arc 

 will change, while the length of the arc remains constant. But 



