400 SENSITIVE SYSTEM. 



labyrinth we find a quantity of aqueous Jlu id, bedewing the ex- 

 pansion of the auditory nerve*. 



VESTIBULE. — This is a small roundish, cavity, hardly so 

 much as a quarter of an inch in diameter, situated between the 

 cochlea and the semicircular canals; to the outer side of it is 

 the tympanum, with which it communicates through the fenestra 

 ovalis. In its roof we find five openings, leading into the semi- 

 circular canals; besides which we notice two particular pits or 

 fovea, containing membranous sacs, the sacculi vestibuH, filled 

 with fluid, and furnished with expansions of nerve. Anatomists 

 have been misled, by their examinations of these depressions in 

 the dried bone, in supposing that they reverberated the sound : 

 this shews the danger of forming conclusions from such artificial 

 inquiries. 



Semicircular Canals. 



These canals are three in number, placed side by side, behind 

 the vestibule, opposite to the cochlea; but they are entered by 



Jive openings through the upper part of the vestibule. They 

 are distinguished as the superior or vertical canal, the posterior 

 or oblique, and the exterior or horizontal. The superior and ex- 

 terior canals possess one opening common to both, and one pe- 

 culiar to each, besides ; while the posterior canal opens into 

 the vestibule by two distinct orifices ; thus making altogether 



Jive apertures. The separate orifice of the superior canal opens 

 nearly perpendicularly upon the fenestra ovalis. 



From the sacculi vestibuli branches of nerves are sent into 

 the semicircular canals, in which they float loose and unattached 



* This cavity is provided with a watery (in place of an aeriform) fluid, 

 for three sufficient reasons. In the first place, water adds to the intensity 

 of a vibration in a very much greater deoree than air. Formerly it was 

 imagined that sound could not be conducted through so dense a medium as 

 water ; but the Abbe Nollet overturned this hypothesis by direct and sim- 

 ple experiment. Every schoolboy knows that two stones struck together 

 under water emit a sound, so far greater than the one created in air, as to 

 be in a degree insupportable to the hearer. Consequently, by the water in 

 the labyrinth the impression made upon the auditory nerve is so much the 

 more intense. 



A second reason why the labyrinth should contain water (and not air) is, 

 that sound is so much more quickly propagated by one medium than the 

 other. Through air it vibrates at the rate of 1 132 feet in a second ; through 

 water, at the rate of 4000 feet in that time. Tims the auditory impression 

 is more suddenly and perfectly disseminated over a cavity filled with water 

 than it would be through one that contained air. 



A third reason for placing water here, consists in its not being nearly 

 so expansible a fluid as air, and consequently not subject to that rarefac- 

 tion and increase of volume that air is ; and which might be attended with 

 serious consequences in a cavity so confined as the labyrinth is. 



