THE VERTEBRAL COLUMN. 



[chap. 



always become distinct, independent, and moveably arti- 

 culated bones ; after their original segmentation they can 

 never be properly said to constitute part of the vertebra. 

 But it frequently happens that in certain of the vertebrae 

 anterior to the thoracic region, and in certain of those 

 posterior to it, there are bony elements formed at an early 

 period, which, though very different from ribs in the ordinary 

 sense of the word, occupy a somewhat similar position 

 in relation to the vertebrae to that which the ribs do in the 

 thoracic region. These have hence been considered as 

 modified conditions of the same part, and have been called 

 pleurapophyses by Professor Owen. 



Perhaps the clearest case of the presence of rib elements 

 in the vertebrae in any Mammal is afforded by the cervical 

 vertebrae of the Ornithodelphia, where the greater part of each 

 transverse process ossifies separately from the rest of the ver- 

 tebra, and remains for a long time only suturally connected 



F"iG. 5. — Third cervical vertebra of a nearly full-grown Echidna {E. hystrix), the 

 different pieces of which it is composed being slightly separated from one another. 

 na neural arch ; c centrum ; t transverse process ; v veitebrarterial canal ; nc\ 

 neuro-central suture. 



with it (Fig. 5). They thus closely correspond to the cer- 

 vical ribs of reptiles, which are unquestionably homologous 

 serially with the thoracic ribs. 



The anterior, or more properly inferior, bar of the trans 

 verse process of the seventh, ard occasionally of some of the 

 other cervical vertebrae in Man, is autogenously developed, 



