The most distinctive character of the skeleton of the Sloth is the excess of length of the fore limb over the hind one. An examination of the slender bones of the arm shows that they are more solid than those of most Mammals. The arm bone (humerus) has a hole through it in the inner expanded part, just above the elbow (inner condyle), in the Ai; but this is not found in the two-toed kinds. The wrist and hand are long and narrow, and this is produced by the union of some bones which are separate in other Mammalia, and the slight development of others. Thus there are six bones in the wrist instead of eight (the scaphoid and trapezium, and the os magnum and trapezoid have coalesced). In the Ai there are three clawed fingers, and the bones of the thumb and of the little finger are absent, and their corresponding hand bones (metacarpals) are very small, and are joined on to the next, that is, to the metacarpal of the index and third finger.
The three fingers are, moreover, strengthened for their peculiar uses, the first two joints being united, and the tip or last joint being very long, and supporting the claw. Moreover, as the long claws are constantly half closed in the hand, and they are never required to be widely open, the tip of the finger is so made that flexion is possible, but not unclasping widely. The skin comes up to the base of the claws, and encloses the fingers, and the base of each claw is protected by a bony sheath. They form capital hooks; they grasp, and although there is no opposable thumb, they hold the food; and a tame Sloth may be seen holding a carrot very firmly between them and the wrist. In the case of the Unau Sloth, the outer claw is the longer.
BONES OF HAND OF THREE-TOED SLOTH.
The Sloths walk on the outside of the extremities of the fore and hind limbs, and their claws are always curved in, and, as it were, retracted. Consequently, the animal cannot place the soles flat on the ground, and it cannot open its foot-claws to a great extent. This fixing of the claws assists in the clasping and hanging, which are the usual and commonest attitudes. The claws surpass the foot in length, and are so sharp and crooked that they readily seize upon the smallest inequalities in the bark of the trees and branches upon which the animals habitually reside. They and those of the fore limb are no mean weapons of offence and defence, for, situated at the end of long and muscular arms, they can drag, cling, and hold with great tenacity. The thigh bone (femur) of the Sloth is straight, and is thicker and shorter than the arm bone (humerus); it has no ligament to unite it to the joint (no ligamentum teres). On examining the lower bones of the leg (the tibia and fibula), they will be found to be bent, so as to leave a space between them, and they are shorter than the bones of the fore arm. The bones of the ankle joint, are united together immovably—that is to say, the usual bones seen in other Mammalia are there, but are united by bone. Moreover, this union includes the complete and ill-developed feet bones (metatarsals), and the first bones of the second, third, and fourth fingers. One bone is not included in this strange union. It is the astragalus, or the bone immediately jointed with the ends of the bones of the legs. The outer or small bone of the leg (fibula) fits into a pit in the outer part of the upper surface of this bone, and thus prevents any movement of the foot like a twisting outwards, and favours, but does not produce, the usual position of twisting inwards. Moreover, there are two powerful muscles in the front of the leg which are not opposed by others as strong, and they, by their contraction, keep the foot twisted inwards, as in club-foot (the anterior tibial and the long extensor of the great toe).
In the Unau, or Two-fingered Sloth, there is the same general arrangement of the bones and muscles, with some important differences, which result in there being a greater amount of bending and extending of the foot, although the foot rests on its outer edge.
A Sloth’s face is short, and there is a broad snub nose, with round nostrils, which are widely open. The cheeks become wide suddenly, and the forehead slopes rapidly backwards, the eyes being wide apart and small, but looking forwards. The head is small and round, and as it is covered with hair behind, it cannot be distinguished well from the upper part of the back of the neck. The expression of the face is always the same, and the method of masticating and eating is disagreeable to observe. The animal having no front teeth, and moving its jaws usually only upwards and downwards, and not from side to side, places the morsel, such as lettuce leaf or carrot, well into its mouth, and chews at it, dragging out the food every now and then, when it is covered with moisture. On examining the skull, the short cut off or truncated appearance of the face is very evident, and it will be observed that the teeth are wanting in the front bones of the face (the pre-maxillaries), and that only the palatal part of these bones exists. The lower jaw is strongly jointed to the upper, and the back part is large: there are teeth at the sides, but there are none in the front part of it. A very singular-looking cheek bone (zygoma) exists on either side. It is not attached behind to the ear bone, so as to cover the jaw muscles, but it has two processes behind—an upper and a lower—which differ in shape and size according to the species. The central bone of the nose does not reach to the nasal outlet, and there is a system of air-cavities which is continued from the nose into the forehead bone. In some kinds, the lower jaw ends abruptly in front, as in the Ai; but in the Unau Sloths it is slightly angular, and projects.
The back teeth of the Sloths are very simple, and consist of three structures, called vaso-dentine, hard dentine, and cement, there being no proper enamel. The vaso-dentine is a kind of bony substance in the centre of the tooth, in which there are the passages and tubes of blood-vessels. The dentine is outside this, and consists of more earthy particles than the vaso-dentine, and of fewer tubes; it is all the denser and more resistant. Wearing away more slowly than the vaso-dentine, it forms a ridge which grinds easily. The cement is a kind of bony structure on the outside of the tooth. The teeth of the Sloth continue to grow from below as they are worn above, and there is no entire milk set which are replaced by those of a permanent kind.
SKULL OF SLOTH.[57]
(From the Proceedings of the Zoological Society.)