EDENTATA.
49
Fig. 33.
Head of the Megatherium.
sypus. The inferior maxillary bone varies
no less in its form in the different genera of
this incongruous order than the superior. It
is greatly elongated and very slender in the
Edentata proper, particularly in the Ant-eaters ;
the ascending plate is thin and small, the right
and left branches of the bone are united at the
symphysis to a considerable extent, and at a
very acute angle. In the Sloths this bone ex¬
hibits a very different structure ; it is short and
deep, the ascending plate is broad and almost
square, the angular process is very large, and
the two branches of the jaw unite at the
symphysis without an angle, the anterior por¬
tion of each side being curved inwards to meet
its fellow. In the Megatherium the body of
the bone is still higher and shorter, but the an¬
terior part is prolonged into a narrow and de¬
pressed groove somewhat similar to that of the
elephant.
The vertebral column.—The variation in the
form and construction of the vertebræ will be
found to bear an exact relation to the habits of
the different genera. The cervical vertebræ of
the Aï, Bradypus tridactylus, have always, until
very recently, been believed to form an excep¬
tion to the general law, which assigns seven as
the strict number of these bones in the mam-
miferous animals. That this number should
exist equally in the hog and the giraffe is in¬
deed a remarkable fact, and may be considered
as a striking illustration of the law by which
variations in volume in any particular’ system
of organs are provided for rather by the differ¬
ence in volume or in the relative proportions of
the organs themselves, than by any abrupt
change in their number. The supposed excep¬
tion to this law which now comes under our
notice consists in the fact that the neck of the
animal in question, (speaking of the part
rather in reference to its use than in strict ana¬
tomical language,) is formed of nine vertebræ.
Two skeletons in my own possession, however,
have enabled me to demonstrate that the posterior
two of these vertebræ (fig. 34) have attached to
them the rudiments of two pair of ribs in the
form of small elongated bones articulated to the
transverse processes of these bones, which are
therefore to be considered as truly dorsal ver¬
tebræ, modified into a cervical form and func¬
tion, suited to the peculiar wants of the animal.
The object of the increased number of ver¬
tebræ in the neck is evidently to allow of a
more extensive rotation of the head; for as
VOL. II.
Fig. 34.
Neck of the Sloth.
each of the bones turns to a small extent upon
the succeeding one, it is clear that the degree
of rotation of the extreme point will be in pro¬
portion to the number of moveable pieces in
the whole series. When the habits of this
extraordinary animal are considered, hanging
as it does from the under surface of boughs
with the back downwards, it is obvious that the
only means by which it could look downwards
towards the ground must be by rotation of the
neck ; and as it was necessary, in order to
effect this without diminishing the firmness of
the cervical portion of the vertebral column, to
add certain moveable points to the number
possessed by the rest of the class, the ad¬
ditional motion was acquired by modifying
the two superior dorsal vertebræ, and giving
them the office of cervical, rather than in¬
fringing on a rule which is thus preserved
entire without a single known exception.
In the two-toed Sloth there is but one pair of
these rudimentary ribs, and consequently only
the first dorsal vertebra enters into the compo¬
sition of the neck.
The dorsal portion of the vertebral column is
particularly long in the Ant-eaters as well as the
Sloth. These vertebræ are also generally more
numerous in this than in most other groups—the
great Ant-eater having sixteen, the Aï fourteen,
and the Unau no less than twenty-three—a larger
number than is found in any other mammi-
ferous animal. The ribs offer some striking
peculiarities in their construction. In the Ant-
eaters and Armadillos they are excessively broad
with the exception of the first and second. In
the Myrmecophaga jubata and M. didactyla
they overlap each other in an imbricated man¬
ner on the upper part,—a conformation which
gives great solidity to the chest. The Sloths
and the Megatherium exhibit also considerable
breadth of the ribs, but to a much less extent
than that just described, and the latter animal,