THE SENSES 



$37 



HEABINGK 



The transverse vibrations of the ether fall upon all parts of the 

 surface of the body, but only find nerve-endings capable of giving 

 the sensation of light in the little discs which we call the retinse. So 

 the much longer and slower longitudinal waves of condensation and 

 rarefaction which are being constantly originated in the air or im- 

 parted to it by solid or liquid bodies that have been themselves set 

 vibrating fall upon all parts of the surface, but only produce the 

 sensation of sound when they strike upon the tiny mechanism of the 

 internal ear. 



But just as the ethereal vibrations, and especially those of greater 

 wave-length, are able to excite 

 certain end-organs in the skin 

 which have to do with the 

 sensation of temperature, so 

 the sound-waves, when suffi- 

 ciently large, are also capable 

 of stimulating certain cuta- 

 neous nerves and of giving 

 rise to a sensation of inter- 

 mittent pressure or thrill. 

 This is readily perceived when 

 the finger is immersed in a 

 vessel of water into which dips 

 a tube connected with a source 

 of sound, or when a vibrating 

 bell or tuning-fork is touched. 

 So far as we know, what takes 

 place in the ear is essentially FlG> 322 .-THE EAR. 



Similar that is tp say, a m> external meatus ; /, head of malleus ; 



mechanical Stimulation of the o, short process of malleus ; g, handle of mal- 



ends Of the auditory nerve, leus; A, incus; /.foot of stipes in oval foramen; 



. , 7 . , ' e, tympanic membrane. 



but a stimulation which acts 



through, and is graduated and controlled by, a special intermediate 



mechanism. 



As the visual apparatus consists of a sensitive surface, the 

 retina, which contains the end-organs of the optic nerve and 

 of dioptric arrangements which receive and focus the rays of 

 light, the auditory apparatus consists of the sensitive end- 

 organs of the eighth nerve and of a mechanism which 

 receives the sound-waves and communicates them to these. 



Physiological Anatomy of the Ear. At the bottom of the external 

 auditory meatus lies the membrana tympani, a nearly circular mem- 

 brane set like a drum-skin in a ring of bone, and separating the 

 meatus from the tympanum or middle ear. Its external surface looks 



