22 Prof. Buckuanp on the Adaptation of the Structure 
tions of this animal*. The unusual position, however, of these two anterior 
dorsal vertebrze, so far in advance of the clavicle and scapula, enables them to 
cooperate with the seven true cervical vertebre in increasing the rotatory 
motion and flexibility of the neck. Hence the animal has the unusual power 
of looking backwards over its own shoulders. We see a final cause for this 
arrangement in the peculiar habits:of the Sloth; being always engaged in the 
act of climbing and clinging with its face towards the trunk or branches of a 
tree, with its eyes also almost hidden in long hair, to defend them from insects, 
it could not easily see without a greater flexibility of neck than quadrupeds 
usually possess. Mr. Burchell has observed that this animal can in a remark- 
able manner and with great facility twist its head quite round, and look in the 
face of a person standing directly behind it, while at the same time the body 
and limbs remain unmoved; as the creature, thus embracing and attached 
to the trunk or branch of a tree can keep no look-out in front, the increased 
flexibility arising from the unusual disposition of these two anterior dorsal 
vertebrae may be considered as a compensation, enabling it to see and guard 
against the approach of its enemies in flank and rear, as well as to see the 
position of its food; the habits of the Sloth are unique among quadrupeds, and 
so also is this compensation. Another advantage resulting from this unusual 
flexibility may be to afford ease to the neck under the peculiar position which 
the Sloth assumes in taking its repose. In the case of an animal, great part 
of whose life, when not engaged in eating, is spent in sleeping on trees, an easy 
attitude for repose is most essential to its comfortable existence; and accord- 
ingly we find, that the auxiliary vertebree at the base of the neck contribute 
to produce that flexibility of this organ which allows the head of the animal to 
incline forward and rest upon its bosom. Mr. Burchell observed that his cap- 
tive Sloths assumed during sleep a position of perfect ease and safety on the 
fork of a tree, their arms embracing the trunk, their backs resting in the angle 
* Dr. Harlan, in a highly interesting and admirable memoir on the Anatomy of the Sloth, which did 
not come under my observation until this paper was passing through the press, states, ‘‘ that in a Bra- 
dypus tridactylus which he dissected, the 9th cervical vertebra supported at the extremity of the trans- 
verse process an osseous rudiment of a rib, to which it is joined by cartilage;" but he does not pro- 
ceed, as Mr. Bell has done, to draw from this fact the important conclusion, that the presence of 
rudimentary ribs causes the vertebre to which they are attached to be dorsal and not cervical.—See 
Featherstonhaugh's American Journal of Geology and Natural Science, page 501, May 1832. 
