246 ZOOLOGY. 



Again, this difference is chiefly remarkable in the caudal or 

 coccygeal part of the column ; in some, as in bats of the 

 genus Pteropus, there exist no coccygeal vertebrae whatever ; 

 in other mammals, they number forty, fifty, and even sixty; 

 and in these we also find this distinction, that the vertebral 

 canal for lodging the spinal marrow and a portion of its 

 nerves still exists in some of these bones, whilst in the ter- 

 minating ones it is wholly absent. In most mammals the 

 caudal part of the column is but little used for locomotion : 

 in the kangaroo, jerboa, &c., it becomes, with the hinder 

 limbs, a sort of tripod, from which the animal springs with 

 great force (Fig. 103) ; many American apes use the tail as 

 a prehensile organ of great power, as a fifth hand in fact, by 

 which they dexterously cling to branches of trees (Fig. 110) ; 

 finally, in the cetacea (whales) it becomes enormous, and 

 serves as their great moving agent in swimming. It is 

 beneath the bodies of these caudal vertebrse that we find the 

 bones shaped like the letter V, which seem to serve as ribs to 

 strengthen the flexor muscles of the tail (Fig. 210). The 

 length of the neck varies also in mammals, although the 



Fig. 210. Skeleton of a Cetaceous Animal (the Dugong).* 



number of cervical vertebraa is, with few exceptions, the same 

 in all. The exceptions are the ai, which has nine, and the 

 lamantin, which has six. The giraffe and whale, opposed as 

 they are in respect of length of neck, have precisely the same 

 number of cervical vertebrse. 



396. The conformation of the thorax or chest varies 

 little ; the number of ribs is generally from twelve to fourteen 



* I regret that my friend has placed this figure here, for, in point of fact, 

 the Dugong is not a cetaceous animal, as I proved long ago. See Trans. 

 R. S. of Edin. for 1830. R. K. 



