2IO WHALES 



epithelium, rather hke the horns of a cow or our own nails, although their 

 structures are not quite analogous. Wax glands proper are present only 

 in the pronounced evagination of the tympanic membrane of Mysticetes, 

 \vhere a thin layer of ear-wax can in fact be found on the inner side of the 

 horn. 



The external auditory canal terminates in the thick eardrum which is 

 attached to an ossified ring of the 'ear-bone". In Odontocetes, this mem- 

 brane is fairly small and convex, so that it projects like a cone into the 

 tympanic cavity, and is therefore known as the tympanic cone. In Mysti- 

 cetes the corresponding part of the tympanic membrane is a taut ligament, 

 and the remaining part of the drum projects along the external auditory 

 passage as an elongated, hollow structure, resembling the finger of a 

 glove. Vibrations are, however, transmitted direct to the fairly small 

 ligament. Not to make things too complicated, we shall simply refer to 

 all these structures as the eardrum. 



Experts still differ on the way in which vibrations reach the eardrum. 

 Fraser and Purves, who carried out experiments on fresh material, con- 

 cluded that sound travels far better through the external auditory passage 

 and the 'wax plug' of Mysticetes than through the blubber itself. They 

 attribute this difference to the fact that the connective tissue fibres of the 

 blubber run in all directions, while, in the main, those of the auditory 

 canal run longitudinally along it. They also found that the wax plug is an 

 excellent conductor especially of very high tones, its conductive properties 

 being roughly equivalent to those of wood, and that sound travels through 

 it much better in a longitudinal than in a transverse direction. Reysenbach 

 de Haan, on the other hand, holds that the external auditory passage 

 plays no special part in the transmission of sound, since he found that 

 Cetacean blubber and muscle have the same sound-propagating properties 

 as the water outside. Even so, the auditory passage is by no means a 

 rudimentary organ and consists of apparently well-functioning tissue. 

 Moreover, Yamada has shown that its connective tissue is provided with 

 a great many sense receptors, which probably serve to communicate the 

 state of tension in the passage to the central nervous system. Further 

 experiments in this field are clearly desirable, for all the evidence seems to 

 point to the conclusion that the external auditory passage plays an 

 essential part in transmitting sound from the water to the eardrum. 



The eardrum, i.e. the taut membrane dividing the external from the 

 middle ear, is surrounded by an annular part of what, for convenience, we 

 shall call the 'ear-bone', though its proper name is the petro-tympanic 

 bone or the tympano-petro-mastoid. The ear-bone is found on the base 

 of the skull (Fig. i lo) and consists, as the Latin name indicates, of two, or 

 if you like of three, bones: the tympanic, the petrosal and the mastoid. 



