Chap. I X. ] VE R TEBR AL COL UMN. 319 



Edwards, as the region in which the vertebrae have 

 additional (pleurapophysial) centres of ossification 

 for the attachment of the ilium (Fig. 132), and 

 which is defined posteriorly by the point of inser- 

 tion of the ischio-sacral ligament. There are ordi- 

 narily two true sacral vertebrae, but with them there 

 often become connected some of the (5) caudal or 

 tail vertebrae ; the whole fusing to form a single bone 

 of great strength. (See page 321.) The caudal 

 vertebrae vary greatly in number, according to the 

 length of the tail. The 

 greatest known number 

 among mammals is found 

 in the insectivorous 

 Microgale longicauda, 

 which may have as many 

 as forty-eight ; Main's has 

 forty - six. Connected 



with and intermediate to Fig. 132. Anterior Surface of First 

 ,1 i j -i Sacral Vertebra of Man. 



the several caudal ver- 



.. c, Centrum ; ?fff, neural arch ; p, rleuva- 



tebrae are V - shaped popbysfc. 



(chevron) bones, which 



protect the vessels of the tail, and afford a larger 

 surface of attachment for the muscles. 



The neural spines and the transverse processes 

 vary very considerably in length and size, according 

 to the functions and size of the muscles attached 

 to them ; the transverse processes, for example, being 

 long in the lumbar region of active jumping forms, 

 such as the hare or the bandicoot. 



The first vertebra, which in man supports the 

 head, has been on that account called the atlas, while 

 the second, on which the atlas moves, is distinguished 

 as the axis ; the centrum of the atlas is remarkable 

 for either fusing with that of the axis to form the 

 odontoid process, or, as in the Monotremata and 

 many Reptiles, it persists as an independent bony piece. 



