Vertebra and Annellus. 63 



conspicuous, that the two lateral neural columns have 

 coalesced into the single spinal cord ; it is in the cranial 

 region, where the bodies of the vertebrae are least con- 

 spicuous, that these lateral neural columns have diverged 

 as crura cerebri, and, by so doing, have put an end to 

 the existence of the central neural axis. 



The heart, also, is not a single and central structure, 

 but a composite structure formed by the coalescence 

 of two lateral arterial and venous trunks ; and therefore 

 the inference which has been drawn as to the double- 

 nature of the body of the vertebra from the history of 

 its development, may derive some support from the 

 history of the development of the heart no less than 

 from that of the development of the great cerebro-spinal 

 neural axis. 



Again : the idea of the non-centrality of the body of 

 the vertebra gains not a little in probability, if an 

 attempt be made to deduce the nature of the body from 

 the office which it is destined to fulfil. The principal 

 office is not peculiar and special. In one point of view, 

 indeed, the body is only one of several processes which 

 serve to connect the vertebrae ; and so it may be ex- 

 pected that a comparison of the vertebral junctures each 

 with the other, will issue in a clearer knowledge of the 

 real nature of each and all. And so it falls out 

 actually. 



In the vertebrae of the cranial region, and also in those 

 belonging to the thoracic region, in chelonians, as in 

 the annelli of the cranial and thoracic regions of insects 

 and many other articulata, the vertebrae are joined 

 together by the complete apposition of the entire edge : 

 but in the trunks of vertebrate animals generally the 

 office of connection is confined to certain points of the 

 vertebral zone, which vary in number and position in 

 different instances. 



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