124 



THE SKELETON. 



STRUCTURE AND DEVELOPMENT OF THE VERTEBRA. 



The structure of a vertebra differs 

 in different parts. The body is com- 

 posed of light spongy cancellous tis- 

 sue, having a thin coating of compact 

 tissue on its external suface perforated 

 by numerous orifices, some of large 

 size, for the passage of vessels; its 

 interior is traversed by one or two 

 large canals for the reception of veins, 

 which converge towards a single large 

 irregular or several small apertures 

 at the posterior part of the body of 

 each bone. The arch and processes 

 projecting from it have, on the con- 

 trary, an exceedingly thick covering 

 of compact tissue. 



Development. Each vertebra is 

 formed of three primary cartilaginous 

 portions (Fig. 75); one for each lami- 

 na and its processes, and one for the 

 body. Ossification commences in the 

 laminae about the sixth week of foetal 

 life, in the situation where the trans- 

 verse processes afterwards project, 

 the ossifio granules shooting back- 

 wards to the spine, forwards to the 

 body, and outwards into the trans- 

 verse and articular processes. Ossi- 

 fication in the body commences in the 

 middle of the cartilage about the 

 eighth week. At birth these three 

 pieces are perfectly separate. During 

 the first year the laminas become 

 united behind, by a portion of carti- 

 lage in which the spinous process is 

 ultimately formed, and thus the arch 

 is completed. About the third year 

 the body is joined to the arch on each 

 side, in such a manner that the body 

 is formed from the three original 

 centres of ossification, the amount 

 contributed by the pedicles increasing 

 in extent from below upwards. Thus 

 the bodies of the sacral vertebrae are 

 formed almost entirely from the cen- 

 tral nuclei, the bodies of the lumbar 

 segments are formed laterally and 

 behind by the pedicles ; in the dorsal 

 region, the pedicles advance as far 

 forwards as the articular depressions 

 for the heads of the ribs, forming 

 these cavities of reception; and in 

 the neck the whole of the lateral 

 portions of the bodies are formed by 

 the advance of the pedicles. Before 



Fig. 75. Development of a Vertebra. 

 1 3 primary centrei 



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Fig. 70. 

 If. Secondary Centres 



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 Fig. 77. 



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Fig. 78. Atlas. 

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Fig. 80. Lumbar Vertebra. 



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