106 



VERTEBRAL COLUMN OF MAMMALIA. 



the Mammalia this organ is of little use for locomotion, though 

 amongst others it becomes a very powerful instrument for it. 

 Thus, in the Kangaroos, the Jerboas, &c., the tail, with the hind 

 feet, forms a kind of tripod, upon which the animal sits, and makes 

 its spring. Among a great number of the American Monkeys it 

 is prehensile, and serves these animals as a fifth hand, to suspend 

 themselves from the branches of trees ; and, lastly, amongst the 

 Cetacea, it grows to an enormous size, and becomes the principal 

 instrument for swimming. Under the first caudal vertebrae of 



these last animals are certain bones 

 in the shape of a V ; which seem 

 to represent the ribs ; and which 

 are of use in increasing the power 

 of the muscles that bend this part 

 of the body. The length of the 

 neck also varies very much. 

 Amongst the Giraffes, for ex- 

 ample, it is very considerable ; 

 whilst in the Whales it is ex- 

 tremely short, and yet the number 

 of vertebras, which are seven as in 

 Man, is really the same. We only 

 know one exception to this rule 

 the Lamantin, which has only six. 

 The Ai, or Three-toed Sloth, con- 

 stitutes an additional apparent ex- 

 ception ; the number of its cervi- 

 The first and second are the lowest ca i vertebrae seeming to be nine; 



true cervical, the third and fourth the ad- 

 ditional or false cervical, and the lower but the tWO lowest of these are 



ones the dorsal. famished with rudiments of ribs, 



as seen at 3,4, in the accompanying figure, and are therefore 

 really dorsal. 



87. The conformation of the thorax varies but little ; the 

 number of the ribs corresponds with that of the dorsal vertebrae, 

 and is in general from twelve to fourteen pairs ; sometimes, how- 

 ever, it increases still more. Thus, in the Horse, there are 

 eighteen pairs, and in the Indian Elephant twenty pairs. The 



FIG. 60 VERTEBRAE OF Ai. 



