EXTERNAL STRUCTURE. 81 



The last of the four little bones, is the stajjes, or stirrup bone. It closely 

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

 stra ovalis, the oval window, or opening- into the most interior part of the 

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

 tained in a perpendicular direction on this membrane, by tlie round bone 

 not only is the full impression which had been communicated to the fir«t 

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

 chanism 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 vibra- 

 tions, once excited, do not immediately cease. A g-kss 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 pro- 

 duces a separate impulse on the external membrane of the ear. Unless, 

 however, one pulsation or vibration had ceased before the next was commu- 

 nicated, 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 particidarly to the 

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

 longing to the stirrup is given in our cut, g. They are placed there, ac- 

 cording 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 unavoid- 

 able 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 bv that tightenino-' 

 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 expanding would press on every part of the cavity. The bony 

 walls of the cavity would not yield, but the membrane might be so vio- 

 lently 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, mighl 

 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 communi- 

 cating with the mouth ; and by means of the mouth, with the external air. 

 See 2^ in this cut, and 9 in the cut, page G8, 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, 



