EXTERNAL STRUCTURE. Q\ 



The last of these four little bones, is the stapes or stirrup-bone. It 

 closely resembles a stirrup in form, and it is placed on the membrane of 

 the fenestra ovalis, the oval window, or opening into the most interior part 

 of the ear, and the immediate and actual seat of hearing. The stirrup 

 being retained in a perpendicular direction on this membrane,, by the round 

 bone, not only is the full impression which had been communicated to the 

 first membrane conveyed to the other, but it is trebled by the beautiful 

 mechanism of the bones. 



Sound, we have said, is produced by vibrations conveyed to the ear, and 

 exciting similar vibrations in certain parts of the ear. These vibrations, 

 once excited, do not immediately cease. A glass continues to sound, and 

 the prolonged undulations of the deep-toned bell are familiar to every one. 

 The pulses of sound succeed each other with great rapidity. In speaking, 

 the words quickly follow each other, and each syllable produces a separate 

 impulse on the external membrane of the ear. Unless, however, one 

 pulsation or vibration had ceased before the next was communicated, 

 language would be unintelligible, and a confused and endless noise would 

 prevail. The finger placed on the edge of a glass immediately stops the 

 vibration. The damper applied to the piano-forte effects the same purpose, 

 and gives distinctness of sound and tone. 



There is in the ear an admirable contrivance to accomplish the same 

 object. Muscles are attached to these little bones, and particularly to the 

 hammer and stirrup, which are in contact with the membranes. One 

 belonging to the stirrup is given in our cut, g. They are placed there, 

 according to some physiologists, to tighten or relax the chain of bones, in 

 order to produce greater or less intensity of sound. We would rather say 

 that they were intended as dampers, to prevent the otherwise unavoidable 

 confusion of sound. No sooner is an impression conveyed to these bones, 

 or a vibration communicated down them, than the muscles contract, and 

 by that contraction tighten the chain of bones, and by that tightening, 

 destroy, and not increase the vibration. The heads of the bones are 

 pressed one on the other, so that, like the finger on the edge of the glass, 

 the vibration is not only immediately arrested in these bones, but in the 

 membranes above and below to which they are attached. 



The air in the drum of the ear is not always of the same warmth. In 

 fever, or in consequence of inflammation in a neighbouring part, or during 

 the excitement of exercise, the air in the drum may attain a degree of 

 heat far above the natural standard ; the consequence of which would be 

 that it would expand. All bodies expand with heat ; and this air expand, 

 ing would press on every part of the cavity. The bony walls of the 

 cavity would not yield, but the membrane might be so violently distended, 

 as to be incapable of vibrating. Under the cold fit of fever, the air would 

 collapse, or would diminish in bulk. All bodies contract by the application 

 of cold. Then the external air, endeavouring to enter the partial vacuum, 

 and pressing the membranes inward, might produce precisely the same 

 effect. To prevent all this, and to preserve a proper balance between 

 the heat of the air in the tympanum, and that of the other parts of the 

 body, or the atmosphere, there is a passage communicating with the mouth ; 

 and by means of the mouth, with the external air. See p in this cut, and 

 9 in the cut, page 68, which gives the cartilage that covers the entrance 

 of this passage, the Eustachian tube, into the mouth. 



The Eustachian tube commences in the drum of the ear, by a mere slit 

 in the bony wall, which, passing through the stony portion of the temporal 

 bone, and part of the sphenoid bone, becomes cartilaginous, and then expands, 



