426 THE MECHANICAL FUNCTIONS. 



seven vertebrsp. Whatever be the leiigtii or short- 

 ness of the neck, whether it be compressed into a 

 small space, as in the Elephant and the 31ole, 

 whether it be lengthened to allow the head to 

 reach the ground, as in the Horse and the Ox, or 

 whether it be excessively prolonged, to allow the 

 animal to reach the tops of trees, as in the Carne- 

 lopard, still this same constant number is preserved 

 in the vertebrae which it contains. When the neck 

 is long, each individual vertebra must necessarily 

 be lengthened in the same proportion. Thus in 

 the Camelopard, the vertebrae of the neck consist of 

 seven very long tubes, joined together endwise, 

 with scarcely any developement of spinous pro- 

 cesses, lest they should impede the bending of the 

 neck. The greatest contrast to this structure is 

 met with in the Dolphin and other Cetacea, which 

 present externally no appearance whatever of a 

 neck, but whose skeleton exhibits cervical vertebrae, 

 closely compressed together, and exceedingly thin, 

 and most of them united together ;* every bone 

 thus formed, hov/ever, retains the marks of having 

 originally consisted of separate vertebrae ; and still, 

 in this extreme case, the number of primary pieces 

 is constantly seven. -f^ 



* In the Cachalot, the whole of these seven vertebrse are usually 

 anchylosed into one bone. 



t The Bradypus tridactylus, or three-toed sloth, was, till very 

 lately, thought to constitute a notable exception to this law, being 

 described as having nine, instead of seven, cervical vertebrae. It 

 is now found, however, that the two last of these vertebrse, which 

 appeared to be supernumerary, ought properly to be classed among 

 the dorsal vertebrse, of which they possess the distinctive characters, 

 not only from the form and size of their transverse processes, but 



