internal organ of hearing in Cetacea is largely endowed with nervous supply, •
whatsoever otherwise may he the faculty of hearing in this aquatic group of
the Tna.Tnma.lia.. The second smaller branch of the nerve will, of course, correspond
to the facial.
The eighth pairs (8) (pneumogastric, glossopharyngeal, and spinal accessory),
ninth pairs (hypoglossal), and the roots of the anterior spinal nerves, form a successive
series of loose bundles of large roots, issuing in close succession along the lateral
borders of the medulla and top of the cord. Their filaments are encased and interwoven
with the thecal coverings of the cord and spinal vascular plexuses, but
nevertheless their ganglia are pronounced. The nerve strands of the pneumogastric
pair are apparently slightly the thickest; otherwise, it is difficult to distinguish
between them.
I may here remark that in this preserved brain any division between the pons
Varolii (jpv) and medulla oblongata (m) is rather indefinite. The obscure line of
demarcation, the want of transverse fibres at the upper part of the latter, and a kind
of intermediate piece between them I presume to be Treviranus’ so-called “ trapezium?'
If so, it is thus quite appreciable in Platanista. Erederick Cuvier states, a
“ trapezium " is wanting in the dolphin (JD. delphis), himself quoting Tiedemann;
on the other hand, Huxley mentions of the porpoise (Phocaena), “ the medulla
oblongata has eorpora trapezoidea."
The cerebral hemispheres are highly convoluted, though questionably if as much
as in the porpoise, but, before referring to the convolutions, I shall point out the main
fissures and certain of the primary or rather secondary sulci.
The longitudinal fissure is very deep posteriorly, but somewhat less so towards
the frontal region, though nevertheless, on pressing aside the lips of the fissure, the
corpus callosum is by no means near the surface. The Sylvian fissure (sy) is also
remarkably distinct, runs very much upwards on the lateral face of the cerebrum, or
has a pronounced vertical direction, and in its course there is a widish deep valley
partitioning the fronto-parietai from the temporo-parietal region. Even in the plaster
of Paris cast of the cranial cavity, this median depression, or broad indentation of
the parts opposite the lower end of the Sylvian fissure, is strikingly evident. It
is this sunken area at the position in question which gives a marked feature to
the brain of JPlatanista as compared with that of Orcella and such other brains
of the Delphmida that have been figured. I could not satisfy myself of the
presence of gyri forming the island of Beil in the Sylvian cleft, but they may
be present.
On the right side, I could readily distinguish what may be regarded as corresponding
to an antero-parietal sulcus. This commenced on the side of the frontal
lobe and above, the orbital region, and ran along the upper but outer cerebral face, in
a winding manner, but with a tolerably arching curve, across the parietal region and
towards the posterior border of the occipital lobe. I t is highest just above the top
end of the Sylvian fissure. Throughout its course various subsidiary short sulci join
it at different points.
With somewhat of a direction parallel to the former, is another even more
highly contorted sulcus, which, beginning within the Sylvian fissure’s anterior border,
has its termination quite on the opposite side, or right round to the base of the
temporal lobe (I). I t likewise has many subsidiary sulci or spurs. This corresponds
with the fissure of Bolando or postparietal sulcus.
Here, not below, but on the fore face of the anterior lobe of the cerebrum are
one or two sulci, which ascend and trend inwards towards the longitudinal fissure.
These may be regarded as representatives of orbitofrontal sulci. Quite on the
top of the brain and with parallel sinuosities, carried from frontoparietal to the
postoccipital lobe are several sulci separating the gyri, of which more shall be said
presently.
As to the convolutions on this right hemisphere, apparently three orbital exist
and are, less or more, in continuity with the frontal gyn. An anteroparietal fold,
with several loops, follows the course already assigned to the correspondingly named
sulcus. Beneath this, and with the same kind of arched direction, is a long post-
parietal fold, which extends to behind the temporal lobe and outer postoccipital face.
Besides these, there seems to be a marginal gyrus girding the fissure of Sylvius, and
the loops and lobule of which, about the middle of the Sylvian cleft, may in part
answer to the so-called central lobe or insula (island of Beil), although here certainly
freely exposed. Of temporal gyri there are a few, those of the supra-
temporal region at least having, in the main, a perpendicular and not a horizontal
direction.
On the vertex, besides a portion of the gyrus bounding the longitudinal fissure,
that known as marginal, there are at least three broad elongated folds, placed
parallel to each other and extending from the suprafrontal to the postoccipital lobe.
Secondary longitudinal furrows occur within these, giving them an insular or looped
character, wriggling of the folds being most frequent at the occipital region. When
in the fresh condition, doubtless, subsidiary cross and oblique sulci give a more varied
aspect to this area.
On the left hemisphere neither gyri nor sulci quite agree with what has been,
described as occurring on the right half. Those longitudinal parieto-occipital folds
just spoken of present a modified pattern. The same holds good with the orbital
frontal, antero and post parietal and other gyri mentioned. Still, the main features
and general pattern have a certain likeness, the departures being in the secondary
turns and indentations.
I t is to be remembered that in the sketches, (PL XXX,) some of these last are
not represented, and hence a smoother appearance is given than in reality the
brain possessed.
I may sum up this much of the cerebral-anatomy by stating that, so far as the
convolutions and sulci are concerned, this species of dolphin has a brain of a considerably
simpler type than in the porpoise or common dolphin, tending perhaps to
some of the Carnivora, though in such a slight degree as still to impress it with all
the attributes of the complex convoluted cerebrum of the Cetacea.
l 3