Vertebral Column 



37 



processes. The epiphyses on the body only ossify the periphery of the cartilaginous 

 plate, the central part remaining cartilaginous, but in advanced life this also may 

 become ossified. 



The account just given applies to a typical vertebra such as the human dorsal 

 segment. Highly-modified vertebrae, as the first two cervical, show corresponding 

 modifications in ossifications, but even the slighter differences in other bones introduce 

 certain extra centres. Thus in the cervical region the bifid spines have double epiphyses, 

 while the costal processes of the seventh and occasionally of the sixth (and even the 

 fourth) have separate centres of ossification, instead of becoming ossified by extension 

 from the neural arch and transverse process, as in the higher vertebrae. In the lumbar 

 region the mammillary processes have separate centres (like the twelfth dorsal), and 

 there is said to be an occasional " costal " centre in the 

 transverse process of the first lumbar, and occasional double 

 centres for each half of the neural arch in the fifth lumbar 

 (and very rarely in others). 



Of such double centres one forms the pedicle, upper 

 articular and transverse process, and the other forms the 

 lower articular process and lamina : occasionally a suture 

 persists in the adult, passing obliquely between these two _ 



Portions. FIG. 3 o.-Cervical verte- 



The Axis has the ordinary primary centres plus those of bra from a child to show 

 the body of the Atlas (odontoid process). It also has the taSSJSd tatto 

 corresponding secondary centres, with the exception, perhaps, region in the formation 

 of those for intervening discs between its body and odontoid : theVroper centrum.' 

 the disc on the upper surface of the odontoid is represented 



by an apical epiphysis. At birth the bony parts are four, with wide tracts of 

 cartilage intervening, and it does not become consolidated before six years of age : 

 the four pieces are (i) centrum ; (2) odontoid and top of body ; (3) and (4) two 

 halves of neural arch. The odontoid portion unites first with the others, about three, 

 and the remainder join each other between the ages of four and six. 



The odontoid ossification includes the top of the body and the inner part of the 

 upper articular processes. 



The Atlas has the usual two centres for the neural arch, from which it and the 

 greater part of the lateral masses are formed bilaterally. At birth these halves are 

 separate, and are only joined in front by fibrous and cartilaginous tissue of the anterior 

 arch. This begins to ossify in the first year from one central or two lateral centres, 

 there appearing to be great individual variation in this matter : the front part of the 

 lateral mass and upper articular surface is formed in this ossification. 



The neural halves join dorsally about the fourth year, and with the anterior arch 

 usually between the fifth and ninth years, but the consolidation of the bone may be 

 incomplete in some cases even at puberty. 



Secondary epiphyses have been described for the tip of the transverse process, 

 and occasionally for the posterior and even for the anterior tubercle. 



The vertebrae are inlersegmental in position that is, they aie to be considered as situated 

 morphologically opposite the " septum " between two neighbouring segments * of the body wall. 

 This is brought about by a secondary division into anterior and posterior parts of the mesoderm 



The term segment is now being used, not in the descriptive sense in which it has hitherto been employed 

 but in its stricter morphological sense, implying the theoretically fundamental and serially homologous 

 foundation of the body wall, indicated in the embryo by the primitive segmentations of paraxial mesoderm 

 and its extension into the wall, with its " segmental " nerve and vessels. 



