ORGAN OF HEARING IN MAMMALIA 



228 



Tympanum and eustachian tube. 



XCVIII". 



168 



Human, nat. size. 



or ' lenticular' ossicle. To maintain an equable pressure on both 

 sides of the membrana tympani, and facilitate the movement* of 

 the otosteals on each other, 

 atmospheric air is admitted 

 into the cavity, as in other 

 air-breathers, by the tube 

 called ( eustachian,' fig. 167, 

 c, continued from the back 

 of the nose or mouth to the 

 tympanum. In passing 

 through this tube the air is 

 warmed, and a proper at- 

 mosphere maintained in front 

 of the membranous parts of 

 the walls of the labyrinth. 



The structure of the ear-organ in Cetacea is highly suggestive 

 and interesting : it is, as Hunter remarks, ( upon the same prin- 

 ciple as in the quadruped ; ' 

 yet the outer opening and 

 passage leading therefrom 

 to the tympanum can rarely 

 be affected by sonorous vi- 

 brations of the atmosphere, 

 and indeed they are re- 

 duced, or have degenerated, 

 to a degree which makes it 

 difficult to conceive how 

 such vibrations can be pro- 

 pagated to the ear-drum 

 during the brief moments 

 in which the opening may 

 be raised above the water. 

 In a full-sized Cachalot it 

 is a longitudinal slit one 

 inch in length, admitting 

 with difficulty the end of 

 the fore-finger. In our com- 

 mon porpoises and dolphins 

 this opening is so small as 

 to require search in detect- 

 ing, fig. 168, a : it leads to 

 a flexible membranous canal 



capable of receiving, in Organ of hearing, Delphinas,nat.alae. xx. 



