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D.R. Ketten
whale head tissues ... without the usual ear drum/ossicular chain amplification of the aerial middle ear." However, these data could equally be interpreted to mean that there is a middle ear mechanism in D. leucas that
maintains middle ear impedance characteristics independent of depth.
The corpus cavernosum is a prime candidate for regulating the middle
ear space. It is not only distensible, it also contains large bundles of trigeminal nerve fibers (Fig. 2.4). It is not known whether these fibers actually
innervate the cavernous tissue or are simply transiting it, but it has been
suggested (Ketten 1992) that the trigeminal, which is a somato-sensory
nerve with up to 500,000 fibers in cetaceans versus 140,000 in humans
(Morgane and Jacobs 1972) is in the right position to subserve a middle ear
regulatory function.
One other anatomical observation may be relevant to this issue. Beaked
whales (Ziphiidae) are perhaps the deepest diving mammals. Ears from
three ziphiid species were recently examined for adaptations related to
their diving abilities or, more precisely, to their ability to avoid barotrauma
(Ketten 1998b). Among the specialized structures unique to these ears is a
newly described function for a bony strut associated with the anterior edge
of the tympanic bulla. This sigmoid bone attaches to the Eustachian tube
at its entrance to the tympanic bulla and prevents the tube from collapsing.
The question is, of course, why a species, if it does not have air in the middle
ear, would have a mechanism for maintaining Eustachian tube patency? It
could be argued that air is required and therefore maintained in the middle
ear to allow differential motion of the cochlear windows but that the space
per se is acoustically moot. Coordinated oppositional motion of the
windows does not actually require an air cavity if the corpus cavernosum
is compressible, leaving still a need to explain an air pocket that is sufficiently large that it requires equilibration. Finally, a less cogent but anatomically correct argument is that the middle ear structures in whales are
sufficiently complex, organized, and conservative across species that it runs
contrary to basic functional principles to dismiss them as simply auditory
bric-a-brac.
To the extent that information extrapolated from available anatomical
data are reliable, the middle ear anatomy of all Cetacea appears to be tailored to sustain large ambient pressures. The massiveness and complexity
of cetacean ossicles suggest that the middle ear has at least some minimal
conventional impedance matching or energy transfer function. Mysticetes
and odontocetes differ chiefly in the rigidity of the ossicular chain and in
the prospect, based on an elaborate tympanic structure, that mysticetes
receive auditory stimuli primarily laterally from the ear canal or via bone
conduction to the membrane and not from specialized bundles of soft
tissues. If the middle ear space is defined by the volume of the tympanic
shell, then mysticete middle ears are substantially more voluminous than
those of odontocetes, or in fact than of any other extant animal. Functions
for odontocete middle ear cavities and ossicular chains are simply unclear.
D.R. Ketten
whale head tissues ... without the usual ear drum/ossicular chain amplification of the aerial middle ear." However, these data could equally be interpreted to mean that there is a middle ear mechanism in D. leucas that
maintains middle ear impedance characteristics independent of depth.
The corpus cavernosum is a prime candidate for regulating the middle
ear space. It is not only distensible, it also contains large bundles of trigeminal nerve fibers (Fig. 2.4). It is not known whether these fibers actually
innervate the cavernous tissue or are simply transiting it, but it has been
suggested (Ketten 1992) that the trigeminal, which is a somato-sensory
nerve with up to 500,000 fibers in cetaceans versus 140,000 in humans
(Morgane and Jacobs 1972) is in the right position to subserve a middle ear
regulatory function.
One other anatomical observation may be relevant to this issue. Beaked
whales (Ziphiidae) are perhaps the deepest diving mammals. Ears from
three ziphiid species were recently examined for adaptations related to
their diving abilities or, more precisely, to their ability to avoid barotrauma
(Ketten 1998b). Among the specialized structures unique to these ears is a
newly described function for a bony strut associated with the anterior edge
of the tympanic bulla. This sigmoid bone attaches to the Eustachian tube
at its entrance to the tympanic bulla and prevents the tube from collapsing.
The question is, of course, why a species, if it does not have air in the middle
ear, would have a mechanism for maintaining Eustachian tube patency? It
could be argued that air is required and therefore maintained in the middle
ear to allow differential motion of the cochlear windows but that the space
per se is acoustically moot. Coordinated oppositional motion of the
windows does not actually require an air cavity if the corpus cavernosum
is compressible, leaving still a need to explain an air pocket that is sufficiently large that it requires equilibration. Finally, a less cogent but anatomically correct argument is that the middle ear structures in whales are
sufficiently complex, organized, and conservative across species that it runs
contrary to basic functional principles to dismiss them as simply auditory
bric-a-brac.
To the extent that information extrapolated from available anatomical
data are reliable, the middle ear anatomy of all Cetacea appears to be tailored to sustain large ambient pressures. The massiveness and complexity
of cetacean ossicles suggest that the middle ear has at least some minimal
conventional impedance matching or energy transfer function. Mysticetes
and odontocetes differ chiefly in the rigidity of the ossicular chain and in
the prospect, based on an elaborate tympanic structure, that mysticetes
receive auditory stimuli primarily laterally from the ear canal or via bone
conduction to the membrane and not from specialized bundles of soft
tissues. If the middle ear space is defined by the volume of the tympanic
shell, then mysticete middle ears are substantially more voluminous than
those of odontocetes, or in fact than of any other extant animal. Functions
for odontocete middle ear cavities and ossicular chains are simply unclear.