4 y ATOMY OF THE SLOTHS. 161 



Leading thus a very unusual kind of life, up amongst the dense foliage, and having some very 

 unusual peculiarities of construction, much debate occurred many years since regarding the general 

 conformation as well as the .special anatomy of the Sloths. One school of anatomists considered the 

 Sloths incomplete and abnormal animals, moving with " pain " on the ground, and another regarded 

 their unusual and peculiar anatomy as singularly beneficent. 



But whilst it is perfectly evident that the long limbs and their joints, and the peculiar turning in 

 of the ankles, and the structure of the clawed hands and feet, are all admirably adapted for the 

 peculiar life which the animal leads, it appears to be consistent with anatomical reasoning to believe 

 that the Sloth is an instance of retrograde development ; that, in fact, the peculiar formation of the 

 skull, neck, wrists, and ankles, is the result of the laws of disuse and adaptation operating on ancestral 

 animals, which once had their anatomy more consistent with a perfect mammalian type. 



When the Sloths were first carefully watched and studied, their length of neck and their ability 

 to turn the head, so as to look at a person standing directly behind or beneath them, without swerving 

 the body, struck Mr. Burchell especially. This curious peculiarity led to a careful examination of the 

 skeleton of the different kinds, and much discussion followed, for it was found that in the Sloth 



SKELETON OF THE SLOTH. 



examined (the Three-clawed Ai) there were more neck bones (vertebrae of the cervical region) than in 

 other Mammalia. Instead of the common number of seven neck bones, there were nine. This elonga- 

 tion of the neck of course permitted a greater amount of twisting than could occur in an animal with 

 fewer neck bones. But there are other reasons why the head can be so much twisted round, for the 

 spines on the neck bones are small, and the joint between the skull and the first vertebra is so fashioned 

 that this remarkable motion is possible. There was a great deal of discussion about the extra neck 

 bones, and as the last two had rib-like projections fi'om their sides, some anatomists considered them 

 to belong to the time rib-bearing vertebrae, or those of the back (the dorsal). But when the other 

 Sloths were examined it was found that the number of the bones of the neck in all the two-fingered 

 kinds was not as great as in other animals. There are only six neck vertebrae in one well-known 

 species (Cholcepus Ho/manni, for instance), whilst there are seven in another two-toed Sloth. 



Eating largely and of bulky substances, the Sloths require a large digestive cavity, and the ribs 

 are numerous, and the body is long and broad. There is much variation, however, in the number of 

 the back bones in the dorsal and lumbar regions. Thus in the Ai there are sixteen dorsal and three 

 lumbar vertebrae, whilst in the Two-fingered Sloth there may be twenty-three or twenty-four dorsal bones, 

 and two, three, or four lumbar vertebrae. The ribs are close together and are broad. As the hind limbs 

 require strong muscles, for the animal hangs on by them whilst it is feeding itself with the fore hands, 

 the pelvis is large and is strengthened by having the hip and haunch bones (ilia and ischia) united to 

 the conjoined sacral vertebrae, which may be six, seven, or eight in number. Moreover, all the strength 

 of the pelvis is behind, the fore part or pubic bones being slender and united in front 



