574 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



parts belonging to the first division ; while all the parts found in the ears of birds 

 and quadrupeds, that are not met with in fish, must belong to the 2d. In fish, 

 the organ consists of a vestibulum and 3 semicircular canals, and these are met 

 with in all fish. In some genera there is an external opening, and substances of a 

 hard nature are found lying loose in the vestibulum: these however cannot be con- 

 sidered as essential parts of the organ, from their not being common to fish in 

 general. Birds have the vestibulum and semicircular canals in common with fish, 

 but they have also a membrana tympani; a slender bone connecting that membrane 

 with the vestibulum ; and an Eustachian tube. In birds, the membrana tympani 

 is convex externally, being pushed forwards by the end of the slender bone above- 

 mentioned. In quadrupeds and man, besides the vestibulum and canals met with 

 in fish, the membrana tympani, the bone connecting it with the vestibulum, and 

 the Eustachian tube, found in birds, there is a cochlea. The membrana tympani 

 is either flat or concave externally ; the bony connection between it and the ves- 

 tibulum is made up of several bones, supplied with muscles to move them in 

 different directions. 



The parts which compose the organ of hearing in fish, must be intended for re- 

 ceiving impressions conveyed through water: those additional parts met with in 

 birds, and the still greater additions which are found in the quadruped and man, 

 must be intended by nature for rendering more perfect the impressions conveyed 

 to the ear through the medium of the external air. Fish, from the structure of 

 the organ, can only hear sounds ^vhich agitate the water immediately in contact 

 with the head of the fish; so that the impulse is conveyed, without interruption, 

 from the liquid in which they live, to the organ of hearing. Man is capable of 

 hearing in a similar manner to fishes, when a communication of solid parts is kept 

 up between the sounding body and the bones of the skull: experiments of this 

 kind must have been made by many members of the r. s. One of the most 

 common is, applying a watch to the forehead, and stopping the ears, which does 

 not prevent the ticking from being heard: the sound is still more distinct when the 

 watch is applied to the mastoid process. Here, as the sound can neither pass 

 through the meatus externus, nor by the Eustachian tube, while the mouth is 

 kept shut, it evidently must be conducted through the bones of the skull. When 

 the sound produced by boiling water is brought to the ear, by one end of an iron 

 rod resting on the side of the kettle and the other kept in contact with the teeth, 

 the sound is conducted in the same way, though in this case it has by some been 

 supposed to pass through the Eustachian tube. In this mode of hearing, the ves- 

 tibulum and semicircular canals are probably the only parts of the organ which are 

 necessary to convey the impression to the expansion of the auditory nerve. 



In hearing in air, the use of the membrana tympani in man and quadrupeds has 

 already been explained. Its office in birds is precisely the same; but as in birds 

 this membrane has no tensor muscle to vary its adjustment, but is always kept tense 

 by the pressure of the end of the slender bone, the scale in birds cannot descend so 



