ORGAN OF HEARING IN MAMMALIA. 



Tympanum and eustachian tuiir 



XCVIII". 



Human, nat. M 



168 



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



sides of the membraiia tympani, and facilitate the movements 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, 



<?, continued from the back 



of the nose or mouth to the 



tympanum. In passing 



thro ugh this tube the air is 



CJ 



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 i it leads to 

 a flexible membranous canal 

 capable of receiving, 



in 



Organ of hearing, Dclplmms, nat. size. xx. 



