22 Prof, BucKLAND mi the Adaptation of the Structure 



tions of this animal*. The unusual position, however, of these two anterior 

 dorsal vertebrae, so far in advance of the clavicle and scapula, enables them to 

 cooperate with the seven true cervical vertebrae 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 cUnging 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 aftbrd 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 vertebrae 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 wliich it is joined by cartilage :" but he does not pro- 

 ceed, as Mr. BeU has done, to dravir from this fact the important conclusion, that the presence of 

 rudimentary ribs causes the vertebrae 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. 



