MECHANICS OF THE INNEK EAR 5 



tion to the function of the tube. It is the movement of the 

 stirrup -which causes the disturbances in the fluid of the tube 

 which we have soon to study in detail. And this motion of 

 the stirrup is made possible only through the mediation of 

 solid bodies, the auditory ossicles. 



The bony connection between the stirrup and the tym- 

 panum would serve its purpose of causing movements in 



the fluid of the tube v^rhatever might be 

 The auditory ^^^ special structure of this connecting 



ossicles are a link. As a matter of fact, it is arranged 



system of levers in such a particular manner that it acts as 



a lever (or system of levers), the large 

 arm, so to speak, being attached to the tympanum, the small 

 arm to the stirrup. This effect, however, is produced in dif- 

 ferent animals in dififerent ways. In birds, for example, (Fig. 

 2) there is no chain of three little bones, but only a single 

 bone, a rod bearing an oval plate. The leverage of this sim- 

 ple connection is explained by the fact that the tympanum 

 and the window plate are not in parallel planes. The far 



Fig. 2. Schematic representation of 

 the leverage in birds 



more complicated connection by means of three links of a 

 chain of bones in most of the mammals has been theoretically 

 studied by various investigators and found to result in a sim- 

 ilar, but probably more delicately adjustable leverage than 

 the simpler arrangement in birds. The advantage of the lev- 



