2. Cetacean Ears
83
tion with the apex up, but to be accurate, this is backwards to a whale ear's
normal position in the real, aquatic world.
The contents of the cochlear duct are bathed in endolymph that travels
between the cochlear duct and the endolymphatic sac via the endolymphatic duct, which runs inside a bony canal, the vestibular aqueduct. Scala
tympani and scala vestibuli are filled with perilymph, which is transported
via the perilymphatic duct (periotic duct). In land mammals, the periotic
duct connects scala tympani to the subarachnoid space. The bony passage
that houses the perilymphatic duct is the cochlear aqueduct.
There are significant variations in the structure of the cochlea and its
related ducts and canals among cetaceans. Virtually all cochlear duct structures are hypertrophied in odontocetes. Mysticete ears appear to be less
well endowed, but some of the reported low level of cellular development
may be post-mortem artifact. Both odontocetes and mysticetes have
exceptionally high ganglion cell counts and extreme basilar membrane
constructions.
Comprehensive reports on cochlear duct anatomy in two species of
dolphin (T. truncatus and L. obliquidens) are available in Wever et al.
(1971a,b,c; 1972). More recent studies reported on cochlear structures in
phocoenids, monodontids, and 10 additional delphinid species (Ketten 1984;
Ketten and Wartzok 1990; Solntseva 1990). Although perfusion is not an
option for cetacean tissues, improvements in stranding network communications have drastically reduced post-mortem collection time, and many
specimens can now be obtained with equal or better preservation than the
average human temporal bone. The consensus of available data is that all
cellular elements of the organ of Corti in odontocetes are larger, more
densely packed, and have stronger size gradients than in other mammals.
There is a IS-to 20-fold reduction in the height of the Claudius cells from
base to apex (Fig. 2.6). Boettcher cells are distributed throughout the entire
length of the cochlear duct with double rows in some species. Hensen
cells reinforce the basilar membrane in the lower basal turn. Although
Wever et al. (1971a) reported four rows of outer hair cells in some parts
of the apical region of T. truncatus, all other authors report no more
than three rows. The discrepancy may be due to individual variability or
to oblique sectioning artifacts. Pilleri, Kraus, and Yihr (1987) reported
FIGURE 2.7. Two-dimensional representations of mammalian basilar membranes.
(A) Basilar membranes from high- and low-frequency mammals drawn to a
common scale in orthogonal projection illustrate differences in width, length, and
turns. Scale bar = 1mm. (B) Type I, Type II and a range of Type M basilar membrane systems are drawn using a dual scale to show differences in the thinkness and
width (11m scale) versus membrane length and extent of outer laminar support (mm
scale).
83
tion with the apex up, but to be accurate, this is backwards to a whale ear's
normal position in the real, aquatic world.
The contents of the cochlear duct are bathed in endolymph that travels
between the cochlear duct and the endolymphatic sac via the endolymphatic duct, which runs inside a bony canal, the vestibular aqueduct. Scala
tympani and scala vestibuli are filled with perilymph, which is transported
via the perilymphatic duct (periotic duct). In land mammals, the periotic
duct connects scala tympani to the subarachnoid space. The bony passage
that houses the perilymphatic duct is the cochlear aqueduct.
There are significant variations in the structure of the cochlea and its
related ducts and canals among cetaceans. Virtually all cochlear duct structures are hypertrophied in odontocetes. Mysticete ears appear to be less
well endowed, but some of the reported low level of cellular development
may be post-mortem artifact. Both odontocetes and mysticetes have
exceptionally high ganglion cell counts and extreme basilar membrane
constructions.
Comprehensive reports on cochlear duct anatomy in two species of
dolphin (T. truncatus and L. obliquidens) are available in Wever et al.
(1971a,b,c; 1972). More recent studies reported on cochlear structures in
phocoenids, monodontids, and 10 additional delphinid species (Ketten 1984;
Ketten and Wartzok 1990; Solntseva 1990). Although perfusion is not an
option for cetacean tissues, improvements in stranding network communications have drastically reduced post-mortem collection time, and many
specimens can now be obtained with equal or better preservation than the
average human temporal bone. The consensus of available data is that all
cellular elements of the organ of Corti in odontocetes are larger, more
densely packed, and have stronger size gradients than in other mammals.
There is a IS-to 20-fold reduction in the height of the Claudius cells from
base to apex (Fig. 2.6). Boettcher cells are distributed throughout the entire
length of the cochlear duct with double rows in some species. Hensen
cells reinforce the basilar membrane in the lower basal turn. Although
Wever et al. (1971a) reported four rows of outer hair cells in some parts
of the apical region of T. truncatus, all other authors report no more
than three rows. The discrepancy may be due to individual variability or
to oblique sectioning artifacts. Pilleri, Kraus, and Yihr (1987) reported
FIGURE 2.7. Two-dimensional representations of mammalian basilar membranes.
(A) Basilar membranes from high- and low-frequency mammals drawn to a
common scale in orthogonal projection illustrate differences in width, length, and
turns. Scale bar = 1mm. (B) Type I, Type II and a range of Type M basilar membrane systems are drawn using a dual scale to show differences in the thinkness and
width (11m scale) versus membrane length and extent of outer laminar support (mm
scale).