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,