
Pods J 
XII. Observations on the Neck of the Three-toed Sloth, Bradypus tridactylus, Linn. By 
Tuomas Bett, Esq., F.R.S., L.S., G.S., § Z.8. 
Communicated August 13, 1833. 
THE laws which regulate the numerical variations in the different systems of organs 
in animals, are perhaps less defined, or at least less understood, than those which relate 
to many other conditions of their existence. In some cases, indeed, these variations 
appear to be wholly anomalous ; but in others the normal number of parts is so strictly 
adhered to, as to be absolutely without any known exception in a whole group. 
Amongst these, one of the most obvious and remarkable is the restriction of the 
cervical vertebre in the whole of the class of mammiferous animals, to the number seven. 
That this number should be found equally in the short interval between the cranium 
and the thorax, scarcely deserving the name of a neck, which we see in the Cetacea, 
and in the long flexile neck of the Camel and the Giraffe, is indeed a striking and in- 
teresting fact, and may be viewed as an important illustration of that law which pro- 
vides for the most considerable variations in the offices or functions of a part, rather by 
a modification, in form or size, or even situation, of organs already existing and es- 
sential to the type of the group, than by the production of new organs on the one hand, 
or, on the other, by the abstraction of any which appertain to the normal form. 
To this normal number, however, the Ai, Bradypus tridactylus, Linn., has for many 
years been considered as an exception ; as by the examination of numerous specimens, 
the neck was found to possess nine vertebre, which were all believed to belong to the 
cervical class. 
An isolated exception to a rule so general, and obtaining in cases of such diversified 
forms as those to which I have alluded, presents itself to the mind of every one accus- 
tomed to look at the general harmony of the established laws of formation, as a vio- 
lation of that unity of design which constitutes one of the most interesting objects of 
our investigation, especially as the exception itself is abrupt and sudden, and without 
any of those intermediate gradations of structure by which the mind is prepared, as it 
were, for considerable diversities of form, and which so generally soften the transitions 
which the different offices of the same organ in different groups may render necessary. 
It was from this consideration, rather than as merely correcting a generally received 
error, that I found, with feelings of no ordinary satisfaction, that in truth this nume- 
rical law is not departed from in the present instance, and that the animal in question 
forms no such exception to the general rule as had been asserted; the two vertebre 
which have hitherto been considered as the eighth and ninth cervical, being in fact the 
