302 THE NERVOUS SYSTEM 



The function of tlie tympanum is to transform the 

 vibrations of the atmosphere into mechanical movements. 

 To perform this function adequately it must be aperiodic — 

 that is to say, it must itself have no period of vibration. 

 Owing to the pull of the tensor tympani on the malleus 

 the tympanum is bell-shaped, its apex inwards. It is 

 composed of a series of superimposed and gradually narrow- 

 ing rings. Each ring has its own periodicity, but the 

 bell cannot vibrate as a whole. 



The ossicles form a chain of bones crossing the middle 

 ear. The malleus consists of a head and two processes, 

 the handle attached to the tympanum and the processus 

 gracihs to the wall of the middle ear. The head of the 

 malleus engages with a hollow surface on the next ossicle, 

 the incus. The incus has a long process, directed down- 

 wards, articulating with the third ossicle, the stapes, a 

 stirrup-shaped bone, the base of which is adherent to the 

 fenestra ovahs. The function of the ossicles is to transmit 

 the vibrations of the tympanum to the fenestra ovalis, 

 and so to the fluid perilymph of the internal ear. 



The malleus rotates through a horizontal axis which 

 passes just below the heads of the malleus and incus. 

 When, therefore, the handle of the malleus moves inwards 

 the upper part of the malleus and incus move outwards and 

 the process of the incus moves inwards. The inward move- 

 ment is transmitted through the stapes to the fenestra ovalis. 



It is beheved by some authorities that in the ossicles 

 a magnification of effect is produced owing (1) to the 

 handle of the malleus being larger than the process of 

 the incus, and (2) to the fenestra ovahs being only one- 

 twentieth the size of the tympanum. It is probable that 

 any effect of this kind is to a great extent discounted by 

 the friction and inertia of the system. 



Of the two muscles of the tympanic cavity the tensor 

 tympani exerts a constant pull, as already stated, upon 

 the membrane, and therefore keeps it taut. It is also said 

 to influence by alterations in its tension the receptivity of 



