116 MR. T. BELL ON THE NECK OF THE THREE-TOED SLOTH. 
that these vertebre are dorsal instead of cervical, does not in the least affect the question 
of their office. The object of the increased number of vertebre in the neck is evidently 
to allow of a more extensive rotation of the head; for as each of the bones turns to a 
small extent upon the succeeding one, it is clear that the degree of rotation of the ex- 
treme point will be in proportion to the number of moveable pieces in the whole series. 
When the habits of this extraordinary animal are considered, hanging, as it does, sus- 
pended from the under surface of boughs with the back downwards, it is obvious that 
the only means by which it could look down 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 necessary additional motion was 
acquired by modifying the two superior dorsal vertebre, and giving them the office of 
cervical, rather than by infringing a rule which is thus preserved entire, without a 
single known exception. 
PLATE XVII. 
Fig. 1. The two last cervical and four first dorsal vertebre of Bradypus tridactylus. 
a. the first rudimentary rib; b. the second rudimentary rib. 
Figg. 2 and 3. The two rudimentary ribs enlarged to three times their natural mag- 
nitude. 
I 
