'SO 



Mit. I), c. 1-ii.rji: ON Till-; anatomy 



^r.MVJ4. 



ijioove on the inner side of the ptery<,'oitl plate of the alisplienoiil 

 (text-tig. 7 1 , Pt.', p, 777) to open into the alveohiteil Jiasopliai-yngejxl 

 chamber near the jnnction of tlie nares. This tulie i.s abont one 

 foot in lengtli in the larger s|ieeiinens. The niucons membrane, 

 which lines the Eustachian tnbe and is continued into the pterygoid 

 fossa and tympanic cavity, lies directly on the bony walls of these 

 cavities and covers over the ossicles. 



The cavities of the middle ear are probably filled with air throngh 

 the Eustachian tube (text-fig. 72), while the external ear appears to 

 l)e filled with water. The pressure of the water upon the tympanic 

 membrane when the whale dives must be considerable, and it is 



Text (iir. 72. 



Dia^n'iinmiiitic \ icw of tlic outer ami niiddli' par of liolfiiinj>fera. 



fi = extcviial auditory meatus; ft = i)lii<rot ear-wax; r=ty)npanic iiipnihrane attuflied 

 to inallous by lig-.iineut ; rf = malleus fused to tympanic bone; <> = ineus; 

 /■= stapes; </ = fenestra ovalis; /< = tynipanie cavity; J = ]it('ryp:oul fossa; 

 A- = Eustachian tube. 



(•nrious that the meiiibraiic should lie concave on its inner side 

 insteail f)f on the outer side. The sjic-like memi)r:ine. being con- 

 nected by a ligament to the rigid malleus, cannot have ;i vibrating 

 fmu'tion. 'JMie iinimal probably fills the middle ear with air on 

 coming to the surface to Ineathe. When the creature dives, this 

 air is imprisoned by the dosing of the Eustachian tidie. The walls 

 of the latter are ))rovided with a strong sjihincter muscle. The 

 air in the Eustachian ttibe would be forced into the pterygoid fossa 

 :in<l tympanic cavity when tlu^ tube wsis compressed, and would 

 inflate the sac-like tympanic mciidiriine. This air would be 

 appioximjitely :it atmospheric pressure. 'I'lie increase of pressure 



