296 



ANIMAL MECHANISM. 



As these bones are placed in the drum barrel, ^>ne 

 joined to the extremity of the other, they make a Com- 

 pound lever, the object of which is, to have the freest 

 and longest extent of motion, in a little space, unlike the 

 muster drum, which is continually referred toon account 

 of familiar illustration, the sticks of this are fixed on the 

 inside, and' though no hands are there to beat them on 

 the head, they are connected to little cords, which jerk 

 them down with a sort of conscious independence, when- 

 ever there is the least noise abroad, to give the brain 

 intelligence as it were of what is going on without. ( 4 ) 

 Explanation of Figure 7. FIG. 7. 



In this drawing, the little bones are represented 

 of their natural size. 



There is some resemblance in the motion to be effected 

 by this chain of bones, to the up and down motion of the 

 hand at the extremity of the arm, viz. carrying one 

 end of the lever through considerable space, while the 

 other, to which the power is applied, lias no perceptible 

 motion. 



(4) There are some diseases, familiar to medical gentlemen, 

 beside local affections of the ear, which fix upon the bones, particu- 

 larly about the face. Under such circumstances, a sanious dis- 

 charge washes these little bones entirely away nothing is mere 

 certain, than this fact, that the three first bones may be corroded 

 and floated from their connexions indeed, extracted with forceps, 

 and the patient hears, to all intents and purposes, nearly if not quite 

 as well as he did before. Thus the membrane, (drum head) and 

 three out of four bones are unnecessary, it seems, in the auditory 

 apparatus of man. Stripped thus.it fulls before the frog's being 

 deficient in an external covering or vibrating membrane. The 

 current or vibrations, in this case, act directly on the foot piece of 

 the stapes which is broad enough to offer a resistance to the 

 sonorous column. Being connected with the membrane of the 

 fenestra ovalis, it produces a motion in it, and that is propagated (o 

 the fluid beyond, and thus the nerve becomes agitated. If the 

 stapes could be detached without rupturing the membrane of tho 

 fenestra ovalis, then hearing could be effected independent of the 

 little bones. Their use is merely to strengthen the vibrations 

 within, just in the proportion that they have a tendency to become 

 faint, as the distance increases whence they had their origin. 



