LUMBAR VERTEBRA. 



153 



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

 the bodies are formed entirely by the advance of the pedicles. Before puberty 



By 4 primary centres. 



By 4 secondary centres. 



1 for each lamina ( 6th week). 

 FIG. 116. Development of a vertebra.' 



By 2 additional plates. 



J 1 for upper surface ' 

 of body, 



for under surface 



of body, 

 FIG. 117. 



SI years. 



1 for each trans- 

 verse process, 

 16 years. 



(sometimes 1) for spinoux process (16 years). 

 FIG. 118. 



By 3 centres. 



Ifor anterior arch (1st year), 



not constant. 



FIG. 119. Atlas. 



By 6 centres. 



for odontoid process 

 ( 6th month). 



Ifor each lateral mass. 



no other changes occur, excepting a gradual increase in the growth of these 

 primary centres ; the upper and under surfaces of the bodies and the ends of the 

 transverse and spinous processes being tipped with cartilage, in which ossific 

 granules are not as yet deposited. At sixteen years (Fig. 118), four secondary 

 centres appear, one for the tip of each transverse process, and two (sometimes 

 united into one) for the end of the spinous process. At twenty-one years (Fig. 

 117), a thin circular epiphysial plate of bone is formed in the layer of cartilage 

 situated on the upper and under sur- 

 faces of the body, the former being 

 the thicker of the two. All these 

 become joined, and the bone is com- 

 pletely formed between the twenty- 

 fifth and thirtieth year of life. 



Exceptions to this mode of de- 

 velopment occur in the first, second, 

 and seventh cervical, and in the 

 vertebrae of the lumbar region. 



The Atlas (Fig. 119). The num- 

 ber of centres of ossification of the 

 atlas is very variable. It may be 

 developed from two, three, four, or 

 five centres. The most frequent ar- 

 rangement is by three centres. Two 

 of these are destined for the two 

 lateral or neural musses, the ossifica- 

 tion of which commences about the 

 seventh week near the articular pro- 

 s, and extend backward; these 

 portions of bone are separated from 

 one another behind, at birth, by a 

 narrow interval filled in with carti- 

 lage. Between the second and third 

 years they unite either directly or 

 through the medium of a separate centre developed in the cartilage in the middle 

 line. The anterior arch, at birth, is altogether cartilaginous, and in this a sepa- 

 rate nucleus appears about the end of the first year after birth, and, extending 

 laterally, joins the neural processes in front of the pedicles. Sometimes there are 

 two nuclei developed in the cartilage, one on either side of the median line, which 

 join to form a single mass. And occasionally there is no separate centre, but the 



1 for body (6th month . 

 FIG. 120. Axis. 



8 additional centres. 



for tubercles on superior articular procefs. 

 FIG. 121. Lumbar vertebra. 



