LUMBAR VERTEBRA. 



43 



the homologue of the rib, and hence sometimes called costal processes (Fig. 8). 

 Although in man these are comparatively small, in some animals they attain con- 

 siderable size, and serve to lock the vertebrae more closely together. 



The Fifth Lumbar vertebra is characterized by having the body much thicker 

 in front than behind, which accords with the prominence of the sacro-vertebral 

 articulation; by the smaller size of its spinous process; by the wide interval 

 between the inferior articulating processes ; and by the greater size and thick- 

 ness of its transverse processes, which spring from the body as "well as from the 

 pedicles. 



Structure of the Vertebrae. The body is composed of light, spongy, cancellous 

 tissue, having a thin coating of compact tissue on its external surface 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 con- 

 verge toward a single large, irregular aperture or several small apertures at the 

 posterior part of the body of each bone. The arch and processes projecting from 

 it have, on the contrary, an exceedingly thick covering of compact tissue. 



Development. Each vertebra is formed of four primary centres of ossification 

 (Fig. 9), one for each lamina and its processes, and two for the body. 1 Ossifica- 



By 4 primary centres. 



f r body (*^ week). 



By 4 secondary centres. 



I/or each lamina (6th week). 

 FIG. 9. Development of a vertebra. 



By 2 additional plates. 



~~ )-! for upper surface " 

 of body, 



.1 for under surface 



of body, 

 FIG. 10. 



21 yearn. 



C 1 for each trans- 

 < verse process, 

 ( 16 years. 



2 (sometimes 1) for spinous process (16 years). 

 FIG. 11. 



tion commences in the laminae about the sixth week of foetal life, in the situation 

 where the transverse processes afterward project, the ossific granules shooting 

 backward to the spine, forward into the pedicles, and outward into the transverse 

 and articular processes. Ossification in the body commences in the middle of the 

 cartilage about the eighth week by two closely approximated centres, which speedily 

 coalesce to form one central ossific point. According to some authors, ossifica- 

 tion commences in the laminae only in the upper vertebrae i. e., in the cervical and 

 upper dorsal. The first ossific points in the lower vertebrae are those which are to 

 form the body, the osseous centres for the laminae appearing at a subsequent period. 

 At birth these three pieces are perfectly separate. During the first year the 

 laminae become united behind, the union taking place first in the lumbar region 

 and then extending upward through the dorsal and lower cervical regions. 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 con- 

 tributed by the pedicles increasing in extent from below upward. Thus the 

 bodies of the sacral vertebras are formed almost entirely from the central nuclei ; 

 the bodies of the lumbar are formed laterally and behind by the pedicles ; in the 

 dorsal region the pedicles advance as far forward as the articular depressions for 

 the head of the ribs, forming these cavities of reception ; and in the neck the 



1 By many observers it is asserted that the bodies of the vertebra are developed from a single 

 centre which speedily becomes bilobed, so as to give the appearance of two nuclei ; but that there are 

 two centres, at all events sometimes, is evidenced by the fact that the two halves of the body of the 

 vertebra may remain distinct throughout life and be separated by a fissure through which a protru- 

 sion of the spinal membrane may take place, constituting an anterior spina bifida. 



