162 NATURAL HISTORY. 



Some small tail bones exist, for that organ is rudimentary in all the Sloths, there being a stump 

 in the Ai, and eleven very small bones ; but in the Two-fingered Sloths the tail is not visible, and there 

 are four little ossicles. There are no long and very prominent spines to any of the back bones, and the 

 whole bony column of the spine is readily curved and bent. The animals so constantly bring the 

 hands and feet close together, when hanging, that a ready bending of the spine is absolutely necessary. 

 Moreover, in sleeping they often rest in the fork of a tree, or on a branch, and place the head 

 between the hind legs, rolling the body up as it were in a ball, and this is facilitated by the peculiar 

 construction of the long chain of back bones with small spines. 



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 naiTOw, 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 (metacafpals) 

 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 



BONES OF HAXD OF TKHEE-TOEI) i i i v i \ i .1 j ,1 , 



SLOTH- seen holding a carrot very firmly between them and the wrist. In the 



case of the Unau Sloth, the outer claw is the longer. 



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 ti'ees 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 tho 

 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 



