the dimensions of the former, are only very slightly larger. I t may he that this
difference in size may have no structural or functional significance; but it should
he borne in mind that the habits of the two forms are distinct, the smaller animal
occurring near the coast and frequenting the muddy water of estuaries, and the
larger whale living in the clear open sea. I t will be further observed, however,
that the canals of Monodon, although the animal is smaller than Globicephalus
deductor, has slightly larger canals, and therefore still larger than those of Orcella,
but, according to the proportional size of the animal, probably relatively smaller
than those of Orcella.
I must again revert to the eye of JPlatamsta, which I have shown to be rudimentary,
and, in connection with this, call attention to the great development of
the semicircular canals, as compared with other Cetacean genera possessing well
developed eyes, because it has been pointed out by Crum Brown that, when a
person is laid flat on a horizontal rotating table with the eyes closed, every muscle
at perfect rest, and thus all the ordinary means of ascertaining the position of the
body removed, the person is still able to form a tolerably accurate judgment as to
the angle through which his body is moved, and, according to Crum Brown,1 he
does this by means of the sensation instituted by the pressure on the ampullae
brought about by the movement of the body. He further supposes that the effect is
different according as the flow is from the ampulla into the canal or from the canal
into the ampulla, and that thus we are able to recognize the direction of the rotation,
whether positive or negative, ex. gr., to the right or to the left, and so on. Hence
the existence of six ampullae, two for each of the three axes of rotation; and
Crum Brown asserts that in man and all animals which he has examined, as in
these Cetacean labyrinths, the two exterior canals of the two ears are very nearly
in the same plane, and the superior canal of the one ear very nearly in the same
plane as the posterior canal of the other. Tlatcmista being practically blind,
and having its semicircular canals largely developed, the supposition is permissible,
in view of Crum Brown’s theory, that the great development may somehow compensate
for defective vision.
I must, however, further remark that this development of the canals does
not appear to be associated with a keen sense of hearing (unless the animal
on which I experimented was deaf), because I fired a pistol over it close -to its
ear, on two occasions, without its exhibiting any sign of having received any
impression.
Tympanic (figs. 8, 9, and 10, PI. XL).—I t is a pointedly conical bullate
bone lying below the periotic, with its sharp tubular apex directed forwards and
inwards between the external and nasal plates of the pterygoid, impinging on the
posterior nares in the angle formed by their external and posterior walls. Prom
the Eustachian extremity, the bone is traversed longitudinally, in its outer half, by a
wide and deep cavity, which terminates posteriorly in a semi-circular expansion of the
1 Forster, Phys., p. 495.
a
outer half of the hinder margin, and which projects backwards beyond the inner
half.
The superior border of the bone overlooking the cavity is much arched, rising
from either extremity and culminating in a transverse bullate eminence, to the base
of which posteriorly there is another and smaller bullate process separated by a deep
rounded notch, the external auditory meatus, from a low ’pillar bearing the expanded
rough surface to which the tympanic is attached to the post-auditory process. At
the inner end of the large transverse bulla, the malleus is suspended by its neck
from a firm anchylosis above the processus gracilis. Besides these attachments, a
ridge runs outwards and forwards from a pit on the neck of the malleus above the
processus gracilis, and seems to be the equivalent of the laxator tympani. Before
the ridge and processus gracilis is a deep notch, the Glasserian fissure, anterior to
which is a large thick downward and forwardly projecting process like an enormous
malleus. Its outer surface fofms an upwardly projecting portion of this margin of
the tympanic, and is flattened externally with an irregular concavity at its base; its
lower inner border nearly resting on the inner margin of the cavity of the tympanic;
and as the tip of'the malleus is close behind it, they define a vertical fissure, a continuation
of the Glasserian fissure. Its internal surface is marked by two broad
furrows and a central ridge which fit into the external surface of the anterior division
of the periotic, the posterior margin of the external surface defining the anterior
wall of the Glasserian fissure. In separating the tympanic, this process is usually
left attached to the periotic. The two bones in adult skulls are slightly connected
at the anteriot margin of the external surface of the process, but so very minutely
that I could only detect their presence by the aid of a lens. In young skulls there is
no trace of anchylosisj but this process is so thoroughly wedged into the periotic, the
mastoid so rooted like a tooth in the post-auditory process, and the vaginal process of
the tympanic so locked in between the mastoid, exoccipital, and squamosal, that the
bones never separate from the skull except by fracture.
■ The large and small bullae require to be described in detail, as they are important
structures in the economy of the ear. The former is compressed antero-
posteriorly, and occupies the highest point of the upper border of the tympanic,
being placed transversely to its longitudinal axis. I t is closed anteriorly, but there
is a sma.ll semi-circular opening into its cavity posteriorly, its hinder wall being
incompletely closed; in the recent skull this opening is external to the tympanum;
it is placed above the level of the posterior and smaller bulla, the cavity of which
is internal to the tympanum. The inner extremity of the larger bulla is curved
downwards and inwards, and has the malleus attached by its neck to its extremity.
The external surface is curved downwards, forwards, and then backwards on the
outer side of the tympanic as a prominent bullate band, its anterior border being
the larger; but both it and its posterior border are defined by a deep furrow.
Above, where it bends backwards, there is a slight concavity, but the remainder is
convex. The posterior furrow is prolonged into the hinder opening before the
smaller bulla, which is also an antero-posteriorly compressed transverse smooth