STRUCTURE OF TYPICAL VERTEBRA. 613 



Tig. 307. 



Neural spine. 

 Zygapophysis- - 



Neurapophysib. 



Heemapophysis. 



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Zygapophysis. 



Haemal spine. 



(1639.) The names in the above diagram printed in Roman type 

 signify those parts which, being usually developed from distinct and in- 

 dependent centres, have been named autogenous elements. The italics 

 denote the parts more properly called processes, which shoot out as con- 

 tinuations from some of the preceding elements, and are termed exogenous. 



(1640.) The autogenous "elements generally circumscribe holes about 

 the centrum, which in the chain of vertebrae form canals. The most 

 constant and extensive canal is that formed above the centrum (fig. 307) 

 for the lodgment of the main trunk of the nervous system (neural axis) 

 by the elements thence termed neurapopliyses. The second canal, below 

 the centrum (fig. 307), is in its entire extent more irregular and inter- 

 rupted ; it lodges the central organ and large trunks of the vascular 

 system (haemal axis), and is usually formed by the laminae which are 

 therefore called Hcemapophyses. At the sides of the centrum, most com- 

 monly in the cervical region, a canal is circumscribed by the pleurapo- 

 pliysis, or costal process, by the parapopliysis, or lower transverse pro- 

 cess, and by the diapopliysis t or upper transverse process, which canal 

 includes a vessel, and often also a nerve. 



(1641.) Thus, a typical or perfect vertebra, with all its elements, pre- 

 sents four canals or perforations around a common centre ; such a ver- 

 tebra we find in the thorax of Man, and most of the higher classes of 

 vertebrates, also in the neck of many birds. In the tails of most reptiles 

 and mammals the inferior are articulated or anchylosed to the under 

 part of the central elements, space being needed there only for the caudal 

 artery and vein. But where the central organ of the circulation has to 

 be lodged, an expansion of the haemal arch takes place, constituting a 

 thorax. Accordingly, in order to construct the thoracic cavity, the 

 pleurapophyses (fig. 307) are much elongated, and the haemapophyses 

 (fig. 307) are removed from the centrum, and are articulated to the distal 

 ends of the pleurapophyses, the bony hoop being completed by the inter- 



