302 ANIMAL MECHANISM. 



sary minutiae. After leaving the inner extremity of the 

 vestibule, commences one canal of the cochlea, which 

 becomes smaller and smaller, till it terminates under the 

 cupola. Now, supposing the reader were travelling in 

 this canal, he could step from the termination of the one, 

 we are describing over the broad opening of the modiolus, 

 shaded above by the cupola, into the mouth of the second 

 canal. By following its turns, increasing in diameter, as 

 he proceeds, till he has gone twice and a half round the 

 modiolus, he would arrive at the fenestra rotunda or 

 round window. This being like parchment, semi-trans- 

 parent, he could look into the tympanum where the little 

 bones are lodged. 



Thus it is, that one canal is in reality a prolongation 

 of the vestibule, and the other opens into the tympanum. 

 A fluid fills the canals, which is prevented from escaping 

 by the oval window, in the vestibule, in one direction, 

 and by the round one at the other. In the centre of this 

 liquor, floating, are the finely organized threads of the 

 acoustic nerve. 



Those animals having the power of combining sounds 

 to produce song, have a cochlea, and generally, a corres- 

 ponding vocal apparatus. Birds, particularly, have a 

 cochlea, but it consists only of two tapering tubes, united 

 at one extremity, but diverging at the other, as in man. 

 A musical ear was once thought by physiologists, to de- 

 pend exclusively on a cochlea ; but common sense 

 teaches us, and the fact is notorious, that singers as well 

 as those who cannot sing, have ears constructed precisely 

 alike ; and therefore, the whole mystery depends on the 

 peculiar development of the brain. 



Explanation of Figure 9. FIG. 9. 



Let it be remembered by the reader, that that part 

 of the last as well as the following diagram, which js| 

 has a sort of shell like turn, is denominated the 

 cochlea. 



The object in this drawing, is to show the soft 

 contents of the labyrinth, of their natural size and 

 in their natural situation. All the rock like portions 

 of the temporal bone have been broken away. 

 aa, the spiral plate of the cochlea; b the round sac, or sac of the 



cochlea ; c alveus communis ; g the posterior ; k the superior, 



and I exterior semicircular canal. 



