s,
the external being vascular. A remarkable vascular substance connected
with this layer covers the back part of the brain and cerebellum,
extending into the spinal canal, and even into the chest. At the base of
the brain the vascular plexus was about 2 inches in thickness. It is, as
is well known, a sort of erectile tissue, of whose functions we are
wholly ignorant. It is not confined to this course, but extends to the
neck, and, passing through the foramina intervertebralia, fills the
intercostal spaces exterior to the pleura.

There was evidently a canal in the centre of the spinal marrow. Wherever
the nerves of the lungs and stomach were traced, they terminated in
loops. We did not observe in the Great Rorqual any tracheal pouch like
that in the smaller; but it may have escaped notice: if absent in the
Great Rorqual, it would be another proof of the distinctness of the
species.

The doubts raised by M. St. Hilaire, as to the Whale being a mammal in
the true sense of the term, were set aside long ago by an appeal to
facts. The young of the Whale tribe suckle like the young of all
mammals; nevertheless I showed, in 1834, that the lactiferous glands in
the _Balaenopterae_ differ in structure from the same organs in most
mammals.

I do not find in my notes anything to add to the description of the
Great Rorqual already published in the 'Transactions of the Royal
Society of Edinburgh' for 1827, to which I beg leave to refer the
reader.

A single remark must be added regarding the nature of the vascular
plexus which, in the Cetacea, surrounds the spinal marrow, and extends
into the chest. On selecting the artery which seemed to form the plexus,
which was, if I rightly recollect, in this instance an intercostal
artery, and dissecting it under water, I found, to my surprise, that the
artery, so long as I followed it, never gave off any branches, but
continued of the same calibre throughout, making innumerable
flexuosities or turnings. Thus, on a plexiform mass of this kind being
cut across, the first impression is, that a great number of arterial
branches or arteries have been divided, whilst in fact the entire plexus
seems to be formed of one artery.

As was to be expected of animals so much withdrawn from human
observation, there is but little to say on the natural history of the
Cetacea properly so called. Their food, no doubt, is various, and seems
to have little or no relation to the character of their dentitio