20 Prof. Buckuanp on the Adaptation of the Structure 
peculiarity that occurs in almost every bone of the skeleton, and these of course 
corresponding with peculiar structures in the muscles and soft parts of the 
animal*, Cuvier observes with respect to the articulation of the hind-feet, 
that it seems contrived expressly to deprive the animal of the power of using 
them in the act of walking ; that whereas, in most animals, the articulation of 
the ankle with the leg is effected by a hinge more or less pliant, which permits 
the foot to play upon the leg upwards and downwards, in the Sloth a pointed 
pivot at the bottom of the small bone only of the leg, enters a conical cavity on 
the upper surface of the astragalus or chief bone of the instep, rendering it 
impossible for the foot to move vertically in the ordinary manner, and allow- 
ing it only to turn horizontally upon the pointed pivot : it follows further, that 
the sole of the foot is in the same vertical direction with the bone of the leg, so 
* Sir Anthony Carlisle, in a paper published in the Philosophical Transactions, 1800, notices a pe- 
culiarity in the arteries of the limbs of slowly moving animals. The axillary and iliac arteries which 
are distributed on the muscles of the upper and lower limbs, are suddenly divided at their entrance to 
these limbs into a number of equal-sized cylinders, which occasionally anastomose with each other, 
and are exclusively distributed on the muscles of the limbs, whilst the arteries of all other parts of the 
body divide in the usual arborescent form. He first observed this structure in the Macauco (Lemur 
tardigradus, Linn.), and subsequently found a similar distribution of the arteries of the limbs in two 
species of Sloth; in the Bradypus tridactylus he counted 42 separate cylinders on the surface of the 
brachial fasciculus, besides about 20 more, which were concealed within; he found 34 similar branches 
in the middle of the thigh. In the Bradypus didactylus, whose movements are quicker than those of 
the B. tridactylus, he found a similar distribution of the arteries, but to a less degree. As the effect 
of this subdivision of the arteries is to retard the velocity of the blood passing to the muscles of the 
limbs, he points out the importance of these phenomena in relation to the physiological question, 
“ whether the slow movement of the blood sent to these muscles be a subordinate convenience to 
other primary causes of their slow contraction, or whether it be of itself the immediate and principal 
cause.' 
Sir Anthony Carlisle also notices the existence of an analogous arrangement of blood-vessels in the 
carotid artery of the Lion, and suggests, that this peculiarity may be subservient to the long-continued 
exertion of the muscles of his jaws whilst holding a powerful animal, such as a Horse or Buffalo, and 
thus enable him to retain his prey. 
Kircher in his Musurgia states that he received a description of the Sloth from Father Torus, Pe 
vincial of the Jesuits in America, who had animals of this kind in his possession, and made many 
experiments in relation to their nature and qualities. He put a long pole under the feet of one, which 
it seized upon very firmly, and would not let go again: the animal thus voluntarily suspended was 
placed between two beams along with the pole, and there it remained without meat, drink or sleep 
forty days. At last, being taken down, they let loose a dog on it, which after a little while the Sloth 
seized with his feet, and held him four days, till he died of hunger. 
