THE TYMPANIC MEMBRANE. 



889 



fro as a whole in accordance with the condensations and rarefactions of 

 the undulating air (in the direction of the sound-waves). The mem- 

 brane, therefore, makes transverse vibrations, for which it is especially 

 adapted, owing to the relatively slight resistance. 



Stretched cords and membranes are set into decided sympathetic vibration 

 only when they are affected by tones that correspond with their own fundamental 

 tone, or whose rate of vibration is some multiple of their own rate (octave, duo- 

 decime, etc.)- If they are affected by other tones, their associated movement 

 will be only inconsiderable. This may be illustrated by a simple example: if 

 a membrane be stretched over a cylinder or a funnel, and a piece of sealing-wax 

 be^ suspended by a silkw T orm-thread, so that it just touches the middle of the 

 membrane, it will remain comparatively quiet when musical tones are struck 

 in its vicinity. As soon, however, as the fundamental tone of the apparatus 

 is sounded, the piece of wax will be greatly agitated by the marked vibrations 

 of the membrane. 



FIG. .516. Tympanic Membrane and Auditory Ossicles (left) 

 viewed from within (from the tympanic cavity): M, 

 manubrium of the malleus; T, insertion of the tensor 

 tympani; h, head of the malleus; 1 F, long process of the 

 malleus; a, incus, with its short (K) and its long (1) process; 

 S, plate of the stapes. A x, A x is the common axis of ro- 

 tation of the ossicles. S the rachet-like arrangement be- 

 tween the malleus and the incus. 



FIG. 317. Tympanic Membrane of a 

 Newborn Infant, viewed from the 

 Outside, with the Handle of the 

 Malleus shining through: At, tym- 

 panic ring, with its anterior (tO and 

 posterior (h) end. 



FIG. 318. Tympanic Membrane and 

 Ossicles (left) viewed from within: 

 Ci, Cm, C/i, chorda tympani; T, 

 pocketlike depression (Urbant- 

 schitsch). 



If these conditions be transferred to the tympanic membrane, this 

 would also be set into marked vibration when its fundamental tone is 

 sounded, but only into slight vibration when other tones are produced. 

 Such a state of affairs would be attended with an enormous inequality 

 in the act of hearing, and provision is, therefore, made in the tympanic 

 membrane for the neutralization of this inequality. This end is attained : 

 (i) Through the great resistance to the vibrations of the membrane 

 that the chain of attached ossicles offers. They act as a damping 

 apparatus, which (as in the case of any damped membrane) prevents the 

 tympanic membrane from vibrating excessively when its fundamental 

 tone is struck. The damping reduces the amplitude of vibration of the 

 membrane for all other tones also. In this way all of the vibrations of 

 the tympanic membrane are moderated, but especially the excessive 



