214 



WHALES 



Figure iij. Highly simplified cross-section through a part of a Cetacean head [right side), 

 showing position of ear. O = external auditory orifice; X = external auditory meatus; 

 B = bulla; P = petrosal; S = bones of skull; E = Eustachian tube; T = tympanic 

 cavity; Ph — pharynx; F — foam-filled cavities. {Greatly changed after Reysenbach de 



Haan, 1956.) 



by a well-developed venous plexus. Variations in the amount of blood in 

 this plexus maintain an equilibrium between the hydrostatic pressure, 

 the volume of the gas in the cavities and the blood pressure, when the 

 animals dive. 



Because of all the above factors, sound vibrations can reach the middle 

 ear in only one way, i.e. through the eardrum. In other words, the situa- 

 tion is precisely the same as in all terrestrial mammals, though it is achieved 

 by different means. Clearly there is no resemblance here to the auditory 

 system offish, and hence whales, just like other mammals, can locate the 

 direction of sounds very accurately. In Archaeocetes, however, the ear- 

 bone was probably connected to the rest of the skull by real bone, though 

 we know little about its exact nature. It seems possible, therefore, that 

 these most ancient of Cetaceans may have had difficulties with directional 

 hearing, although there are indications that they had air sinuses (evagina- 

 tions of the tympanic cavity) . 



Just as in other mammals, the vibrations of the Cetacean eardrum are 

 transmitted through three auditory ossicles to the oval window of the 

 internal ear. The auditory ossicles themselves are thick, short and very 

 heavy, their weight in porpoises being five times as great as it is in man. 

 Their thickness is best appreciated by examining the stapes, which lacks 

 the characteristic stirrup form which it has in man (and from which it 



