2. Cetacean Ears
81
cochlea towards the back of the head and the apex oriented in the same
direction as the nose. In a dog, cat, or seal, a modiolar plane is consequently
a coronal section. In humans, because of the reorientation of the head atop
the spinal column, the modiolar plane is transaxial but in terms of the
cochlea's relation to the planes of the head and normal direction of motion,
the inner ear has the same orientation as in other mammals. In cetaceans,
the modiolar axis runs orthogonal to the common mammal orientation.
That is, in a resting whale on the surface, the cochlear apex points down.
Cetacean cochlear spirals run dorso-ventral/base-apex with the round
window posterior and medial to the oval window (Fig. 2.4). In this chapter,
to ease comparisons of cetacean ears with ears of other animals, cochlear
anatomy and reconstructions are shown in the conventional display posiFIGURE 2.6. Cochlear duct cytoarchitecture. Micrographs of 251lm celloidin sections
illustrate similar basal and apical turn positions in Type I, Type II, and Type M
cochleae. The images are shown in a conventional orientation, although in vivo, the
cochlear apex points ventrally in cetaceans (see also Fig. 2.4). Tissues were collected
5 to 48h post-mortem and have preservation and processing artifacts similar to
human temporal bones, including disrupted Reissner's membrane, necrotic organ of
Corti, and ganglion cell loss. Scale bar = 1001lm. (A) The basilar membrane (bm)
(601lm x 20llm) of Phocoena phocoena, a Type I odontocete, in the mid-basal turn
is stretched between inner (iosl) and outer (oos1) ossified spiral laminae. The outer
lamina is 40 Ilm thick. A distinctive dark cellular layer known only from the basal
turn of odontocetes lines the lateral basilar membrane recess. Although noted by
several authors, these cells are unclassified and their function remains unclear. The
dense collagenous basketwork of the spiral ligament, darkly stained hypercellular
stria vascularis, and tight packing of nerve fibers are classic odontocete basal turn
structures common to both types. The strong development of support cells shown
in this section is characteristic of Type I odontocete ears. (B) In an apical section,
the basilar membrane of an Atlantic white-sided dolphin (Lagenorhynchus acutus,
Type II odontocete) is thin and broad compared to the basal anatomy. Only the
spiral ligament (sl) directly supports the lateral edge of the basilar membrane. Note
the inverse development of the tectorial membrane in comparison to the basilar
membrane. (C) In an adult Northern right whale (Eubalaena glacialis), a mysticete,
the basilar membrane (m) is 51l x 1251l in the lower basal turn. The level of cellular development of the spiral ligament is similar to that of the odontocete apical
region in (B). The inner osseous laminae are also noticeably thin with a large central
lumen. Lack of supporting cells and nerve fibers is probably the result of postmortem necrosis and does not represent a normal density. (D) A mid-apical section
of Eubalaena glacialis shows a characteristically lissome mysticete basilar membrane that is -1,200 Ilm by 31lm. The spiral ligament is intact but is sufficiently acellular that it is difficult to detect in this micrograph. The tympanal plate of the inner
osseous lamina is negligible as a support element. bm, Basilar membrane; esc, external sulcus cells; 1, spiral limbus; nf, auditory nerve fibers; ohc, outer hair cells; rm,
Reissrers membrane; sl, spiral ligament; sp, spiral prominence; stv, stria vascularis;
tC, tunnel of Corti; tm, tectorial membrane.
81
cochlea towards the back of the head and the apex oriented in the same
direction as the nose. In a dog, cat, or seal, a modiolar plane is consequently
a coronal section. In humans, because of the reorientation of the head atop
the spinal column, the modiolar plane is transaxial but in terms of the
cochlea's relation to the planes of the head and normal direction of motion,
the inner ear has the same orientation as in other mammals. In cetaceans,
the modiolar axis runs orthogonal to the common mammal orientation.
That is, in a resting whale on the surface, the cochlear apex points down.
Cetacean cochlear spirals run dorso-ventral/base-apex with the round
window posterior and medial to the oval window (Fig. 2.4). In this chapter,
to ease comparisons of cetacean ears with ears of other animals, cochlear
anatomy and reconstructions are shown in the conventional display posiFIGURE 2.6. Cochlear duct cytoarchitecture. Micrographs of 251lm celloidin sections
illustrate similar basal and apical turn positions in Type I, Type II, and Type M
cochleae. The images are shown in a conventional orientation, although in vivo, the
cochlear apex points ventrally in cetaceans (see also Fig. 2.4). Tissues were collected
5 to 48h post-mortem and have preservation and processing artifacts similar to
human temporal bones, including disrupted Reissner's membrane, necrotic organ of
Corti, and ganglion cell loss. Scale bar = 1001lm. (A) The basilar membrane (bm)
(601lm x 20llm) of Phocoena phocoena, a Type I odontocete, in the mid-basal turn
is stretched between inner (iosl) and outer (oos1) ossified spiral laminae. The outer
lamina is 40 Ilm thick. A distinctive dark cellular layer known only from the basal
turn of odontocetes lines the lateral basilar membrane recess. Although noted by
several authors, these cells are unclassified and their function remains unclear. The
dense collagenous basketwork of the spiral ligament, darkly stained hypercellular
stria vascularis, and tight packing of nerve fibers are classic odontocete basal turn
structures common to both types. The strong development of support cells shown
in this section is characteristic of Type I odontocete ears. (B) In an apical section,
the basilar membrane of an Atlantic white-sided dolphin (Lagenorhynchus acutus,
Type II odontocete) is thin and broad compared to the basal anatomy. Only the
spiral ligament (sl) directly supports the lateral edge of the basilar membrane. Note
the inverse development of the tectorial membrane in comparison to the basilar
membrane. (C) In an adult Northern right whale (Eubalaena glacialis), a mysticete,
the basilar membrane (m) is 51l x 1251l in the lower basal turn. The level of cellular development of the spiral ligament is similar to that of the odontocete apical
region in (B). The inner osseous laminae are also noticeably thin with a large central
lumen. Lack of supporting cells and nerve fibers is probably the result of postmortem necrosis and does not represent a normal density. (D) A mid-apical section
of Eubalaena glacialis shows a characteristically lissome mysticete basilar membrane that is -1,200 Ilm by 31lm. The spiral ligament is intact but is sufficiently acellular that it is difficult to detect in this micrograph. The tympanal plate of the inner
osseous lamina is negligible as a support element. bm, Basilar membrane; esc, external sulcus cells; 1, spiral limbus; nf, auditory nerve fibers; ohc, outer hair cells; rm,
Reissrers membrane; sl, spiral ligament; sp, spiral prominence; stv, stria vascularis;
tC, tunnel of Corti; tm, tectorial membrane.