S8 PRODUCTION SECT. XIV. /; , 



The immediate object however of the fenfe of vifion is light , 

 this fluid, though its velocity is fo great, appears to have no per- 

 ceptible mechanical impulfe, as was mentioned in the third 

 Section, but feerns to ftimulate the retina into animal motion 

 by its tranfmiffion through this pan of the fenforium : for 

 r hough the eyes of cats or other animals appear luminous in ob- 

 fcure places ; yet it is probable, that none of the light, which 

 falls on the retina, is reflected from it, but adheres to or enters 

 into combination with the choroide coat behind it. 



The combination of the particles of light with opaque bodies, 

 and therefore with the choroide coat of the eye, is evinced from 

 the heat which is given out, as in other chemical combinations. 

 For the fun-beams communicate no heat in their paflage through 

 tranfparent bodies, with which they do not combine, as the air 

 continues cool even in the focus of the largeft burning-glaires, 

 which in a moment vitrifies a particle of opaque matter. 



IV. Of the Organ of Hearing. 



IT is generally believed, that the tympanum of the ear vi- 

 brates mechanically, when expofed to audible founds, like the 

 ilrings of one rnufical inftrument, when the fame notes are 

 ilruck upon another. Nor is this opinion improbable, as the 

 mufcles and cartilages of the larinx are employed in producing 

 variety of tones by mechanical vibrations : fo the mufcies and 

 bones of the ear feem adapted to increafe or diminifh the ten- 

 iion of the tympanum for the purpofes of fimilar mechanical 

 vibrations. 



But it appears from difleclion, that the tympanum is not the 

 immediate organ of hearing, but that, like the humours and cor- 

 nea of the eye, it is only of ufe to prepare the object for the 

 immediate organ. For the portio mollis of the auditory nerve 

 is not fpread upon the tympanum, but upon the veftibulum, and 

 cochlea, and femi-circular canals of the ear ; while between the 

 tympanum and the expaniion of the auditory nerve the cavity is 

 laid by Dr. Cotunnus and Dr. Mechel to be rilled with water ; 

 as they had frequently obferved by freezing the heads of dead 

 animals before they diflfecled them ; and water being a more 

 denfe fluid than air is much better adapted to the propagation 

 of vibration. We may add, that even the external opening of 

 the ear is not abfolutely neceflary for the perception of found : 

 for fome people, who from thefe defects would have been com- 

 pletely deaf, have diflinguifhed acute or grave founds by the tre- 

 mours of a flick held between the teeth propagated along the 

 bones of the head, (Hailer. Phyf.T. V. p. 295). 



Hence 



