The opening for the aqueduct of Eallopius looks along the groove of the
posterior process, and with the fenestra ovalis lies as usual in a hollow in which
they are separated from each other by an interval of only 1*08 of an inch, the
former being the most internal, looking outwards and downwards. Anterior
and slightly external to these is the almost circular hollow for the head of the
malleus.
Cochlea is flattened on the under or apical surface, somewhat globular in
form and relatively of large size compared with the semi-circular canals, PL XL,
figs. 1 and 2. Immediately posterior to the internal opening of the aqueduct of
Eallopius, there is the orifice of the canal of the vestibule opening into the anterior
wall of the vestibule, immediately above the external margin of the fenestra ovalis.
In close relationship with the internal auditory meatus is the aqueduct of the
cochlea, which is rather large with a wide internal orifice.
. The aqueduct of Eallopius has a length of 0"-24 and is curved from above
downwards and backwards, near its termination having a slight inward curve.
Near its lower end its anterior wall is marked by a depression perforated by
the minute canal which probably transmits the large, superficial, petrosal
nerve.
The fenestra ovalis is transversely oval, and within its outer and inner margins
there is the rim on which the stapes rests. The vestibule is 0"-16 in length and is curved
upwards, backwards, inwards and downwards, but its two extremities are nearly on
the same level. Its inner wall, immediately above the fenestra ovalis, is deeply concave,
while the portion posterior to it is more contracted, the inner wall terminating
in the commencement of the la.mina spiralis which curves backwards and outwards.
In the anterior extremity of its outer wall, there is a deep pit, in the bottom of
which is the anterior end of the superior semi-circular canal, and in the floor of
the vestibule immediately internal to the fenestra ovalis is another depression continuous
with the former, but at a considerably lower level, and in which is the
anterior extremity of the external semi-circular canal. In the roof of the anterior
extremity of the vestibule the canal of the auditory nerve opens, being immediately
posterior in position to the aqueduct of Eallopius: The vestibule is rather highly
arched at the middle of its roof, into which opens the minute aqueduct of the vestibule,
transversely to the long axis of the vestibule, protected below by an inwardly-
projecting shelf of bone. The posterior extremity of the vestibule bends downwards
and inwards to the scala oestibuli as a narrow tube compressed from above
downwards.
In the fenestra rotimda which has the position this opening occupies in
Cetacea, the ridge corresponding to the canal described by John Hunter1 as peculiar
to this group of mammals is distinctly visible, in addition to the ridge of the
scala oestibuli which is the uppermost of the two. This secondary canal appeals
faintly as a ridge to the left of the canal of the cochlea in fig. 1, Pl. XLI, also in
Globicephalus and Monodon, figs. 7 and 10, close to the protuberance corresponding
* Phil. Trans., 1787, p. 436.
to the fenestra rotimda. Tracing the extent of the cochlea from the somewhat
oval apical chamber, it would appear to consist of a little more than one and a
half turns. The canal of the cochlea, in transverse section, is seen in its first coil
from the vestibule to be divided into the two distinct portions, the scala 'oestibuli
and scala tympa/ni, in a more marked manner than in other mammals, by the
existence of a prominent osseous ridge, or almost lamina, along the inner wall of the
periphery of the cochlea opposite to the lamina spiralis, the two being separated
from eacfi other by only a narrow space. This ridge begins at th & fenestra rotunda,
and is almost lost beyond the first turn of the canal. The lamina spiralis does not
reach the apex of the cochlea, where there is a considerable chamber with perfectly
smooth walls, and instead of terminating in a hamular process, it ends in a rounded
extremity. I t is much striated, indicating the tracts of the nerves; and along its
under-surface, in close relation to the modiolus, runs a wide spiral cylindrical canal
with very delicate walls, its outer and inner walls being pierced by many foramina,
the remainder being smooth. Its upper wall is formed by the lamina spiralis
itself, and its inner wall by the modiolus. I t commences at the foramen rotundum,
and the other c£taal, beginning at the fenestra rotimda, runs below the osseous
ridge of the periphery of the cochlea, and is partly membranous, and it is lost
about the end of the first turn. I t will be observed (PI. XLI, figs. 1 and 2) that
the coils of the cochlea are separated from each other by a considerable osseous
interspace.
Semicircular canals (Plate XLI, figs. 1 to 12).—The characters and position
of the semicircular canals and their small size relatively to the cochlea, in the Cetacean
genera Tlatamista, Orcella, Globicephalus and Monodon, are well brought out
in the comparative series of ear-labyrinths figured, natural size, on Plate XLI, with
the semicircular canals themselves magnified twice their natural size.
In connection with these figures, I have to mention that they are exact
reproductions of a series of metal moulds made from the ear-labyrinths of these
genera by my accomplished fellow-student and friend Professor Crum Brown, whose
ingenious researches1 into the relations and functions of the semicircular canals are
now well known to every physiologist.
In the plate in question, it will be observed that this blind dolphin has semicircular
canals greatly exceeding in size Globicephalus, an animal more than double
its dimensions, and that the same holds good of these structures and the semicircular
canals of Monodon, a whale which attains to 16 feet in length. These casts also
bring out this circumstance that the semicircular canals of Orcella are very much
smaller than those of this dolphin, although the animals are of nearly equal size;
but whether other Cetacea of the same dimensions as Orcella have similarly-sized
canals I am not in a position to say .
Although Orcella and Globicephalus are Wo forms allied to each other, it is
important to note that the semicircular canals of the latter whale, an animal twice
1 Proo. Roy. Soc. Edin., 1874, p. 255; Jour. Anat. Phys., 1874, p. 327; see also Mach. Lehre v. d. Beweg.
Empfind., 1875; Breuer, Wien. Med. Jahrb., 1874, p. 72;-1875, p. 87.