558 THE TYMPANUM. [BOOK m. 



anterior, and external, also serve to keep the malleus in place. The 

 whole series of ossicles may be regarded as a single-armed lever, 

 moving on the ligamental attachment of the short process of 

 the incus to the posterior wall of the tympanum, the weight 

 being brought to bear at the end of the long process of the incus, 

 and the power at the end of the handle of the malleus. The 

 long, malleal arm of this lever is about 9 \ mm., the short, stapedial, 

 6J mm. in length; hence the movements of the stapes are less 

 than those of the tympanum; but the loss in amplitude is made up 

 by a gain of force, which is in itself an obvious advantage. 



Thus every movement of the tympanic membrane is trans- 

 mitted through this chain of ossicles to the membrane of the 

 fenestra ovalis, and so to the perilymph of the labyrinth; the 

 vibrations of the tympanic membrane are conveyed with increased 

 intensity, though with diminished amplitude, to the latter. That 

 the bones thus move en masse has been proved by recording their 

 movements in the usual graphic method. A very light style 

 attached to the incus or stapes is made to write on a travelling 

 surface; when the membrana tympani is thrown into vibrations by 

 a sound, the curves described by the style indicate that the chain 

 of bones moves with every vibration of the tympanum. On the 

 other hand, the comparatively loose attachments of the several 

 bones is an obstacle to the molecular transmission of sonorous 

 vibrations through them. Moreover, sonorous vibrations can only 

 be transmitted to or pass along such bodies as either are very long 

 compared to the length of the sound-waves, or, as in the case 

 of membranes and strings, have one dimension very much smaller 

 than the others. Now the bones in question are not especially 

 thin in any one dimension, but are in all their dimensions ex- 

 ceedingly small compared with the length of the vibrations of even 

 the shrillest sounds we are capable of hearing ; hence they must 

 be useless for the molecular propagation of vibrations. 



The tensor tympani muscle even in a quiescent state is of use 

 in preventing the membrana tympani being pushed out far. When 

 it contracts it renders the membrana tympani more tense and 

 hence has been supposed to act as a damper lessening the 

 amount of vibration of the membrane in the case of too powerful 

 sounds; it is said to be readily thrown into contraction at the 

 commencement of a sound or noise, but to return to rest during 

 the continuance of a musical note. Efferent impulses reach it 

 through fibres of the fifth nerve, and its activity is regulated by 

 a reflex action. In some persons the muscle seems to be partly 

 under the dominion of the will, since a peculiar crackling noise 

 which these persons can produce at pleasure appears to be caused 

 by a contraction of the tensor tympani. 



The so-called laxator tympani is considered to be not a muscle 

 at all, but a part of the ligamentous supports of the malleus. 



