94 INNERVATION. [CHAP. XVIII. 



to the tension of the membrana tympani, and, on the other hand, 

 the contraction of the internus mallei depresses the stapes, and con- 

 sequently increases the tension of the membranes of the two fenestrse. 

 The cessation of muscular action restores all three membranes to 

 their original laxity, nor does it appear that they admit of any fur- 

 ther degree of relaxation through the influence of any vital process. 

 The incus forms a bond of union between the two other bones, and 

 its motions depend entirely upon theirs in consequence of its arti- 

 culation with both, while from the fixedness of its connexion with 

 the mastoid cells, as well as from its intermediate position and 

 want of muscular attachment, its motions must obviously be much 

 more limited than those of the other bones. Its use seems to be 

 to complete the chain in such a manner, that by reason of its 

 double articulation with the malleus on the one hand and the 

 stapes on the other, the tension of the tympanic membranes may 

 be regulated without any sudden or violent motion, which could 

 scarcely be avoided were the conductor between the membranes of 

 the tympanum and fenestra ovalis but one piece of bone. 



The mobility of the tympanic bones has, however, a further use, 

 as Miiller suggests ; namely, to favour the oscillations of the mem- 

 brana tympani, by allowing the approximation of the two extremi- 

 ties of the chain of bones. And this opinion is strengthened by 

 the facts of comparative anatomy, for the ossicles have moveable 

 articulations in the frog, as in man, although they have no muscles 

 attached to them. 



The addition of a cavity filled with air outside the labyrinth has 

 a twofold use. First, to preserve uniformity of temperature in the 

 air immediately in contact with the fenestral membranes. Were 

 these membranes situate on the exterior of the head, and exposed 

 to the surrounding atmosphere, they would be constantly under- 

 going changes in their elastic state, under the influence of atmo- 

 spheric vicissitudes. The air which accumulates in the tympanum 

 and mastoid cells, finds its way into them only through the Eus- 

 tachian tubes, and as it does not readily change its position, it is 

 well placed for maintaining a temperature equal to that of the 

 body. Secondly, the action of the chain of ossicles as conductors 

 is materially enhanced by their being completely surrounded by 

 air. The insulation of a solid body by a different medium renders 

 it a better propagator of sound: for the surrounding medium will 

 obviate the dispersion of sound, and will favour its retention in the 

 solid conductor. 



It results from Savart's experiments, that tension of the mem- 



