Chap. I X.] VER TEBR&. 317 



there are only occasional rudiments in the tail, and, 

 much more remarkably, in the Sirenia) disc-like plates 

 of bone, the so-called epiphyses, which, on the 

 arrival of maturity, fuse with the centra, and obscure 

 the line of union (neuro-central suture) between the 

 centra and the neural arches. 



On the dorsal surface of these arches a spinous 

 process (neural spine) is often developed, and from 

 these muscles may have their origin ; forming only a 

 feeble ridge in the Amphibia, though prominent in 

 many fishes, they are often large in the Sauropsida, 

 and are of considerable importance in many Mammals. 



It is in the highest forms that we can best distin- 

 guish the several so-called regions of the vertebral 

 column. In such a Mammal as the rabbit, it is, for 

 example, possible to make out (1) a cervical region, 

 in which the laterally placed ribs are never more 

 than rudimentary ; the vertebra of this region, 

 whether the neck is as long as in the giraffe, or as 

 short as in the porpoise, is always composed of seven 

 vertebra?, with the exception of the three- toed sloths 

 (Bradypus) which have nine ; of another Edentate 

 (Manis), which has sometimes eight (W. K. Parker) ; 

 and of one two-toed sloth (Cholcepus hoffmani), and 

 the Manatee, which have six. (2) A thoracic 

 region, with which are connected ribs that are movably 

 articulated with them, and some of which join the 

 veiitrally placed sternum, and so form a kind of pro- 

 tecting cage for the thoracic viscera, and points of 

 attachment for the important costal muscles. (3) A 

 lumbar region, where the ribs are not movably 

 articulated, but, being shorter, leave space for the 

 coils of the intestine and the distension of the abdo- 

 men which occurs in gravid females of this group. 

 (4) A sacral region, the definition of which is sur- 

 rounded with considerable difficulties, but which is, 

 perhaps, best defined, with Gegenbaur and A. Milne- 



