i6 Mr. Home's Lecture 



quadrupeds, that are not met with in fish, must belong to the 

 second. 



In fish, the organ consists of a vestibulum and three semi- 

 circular 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, how- 

 ever, cannot be considered as essential parts of the organ, from 

 their not being common to fish in general. 



Birds have the vestibulum and semicircular canals in com- 

 mon 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 abovementioned. 



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 

 vestibulum 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 receiving 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 

 which agitate the water immediately in contact with the head 

 of the fish ; so that the impulse is conveyed, without inter- 



