112 



OSTEOLOGY 



two synchondroses, neurocentral synchondroses, traversing it along the planes of junction of 

 the three centers (Fig. 102). In the thoracic region, the facets for the heads of the ribs lie behind 



the neurocentral synchondroses and 

 FIG. 101. Ossification of a vertebra 

 By 3 primary centers 



1 for body (8th week) 





I /or each vertebral arch (1th or 8th week) 



FlG. 102. 

 By 3 secondary centers 



Neurocentral 



i 1 for each 



trans, process 

 ( 16th year 



1 for spinous process (16th year) 



FIG. 103. 

 By 2 additional plates 



1 for upper surface' 



' 



jj_ ( jTj 1 for under surface 

 of body 



FIG. 104. Atlas. 



>. 16th year 



By 3 centers 



By 7 centers 



1 for anter. arch (end of 1st year) 



.1 for each } lth week 



lateral mass j 



FlG. 105. Axis. 



6th month 



1 for each vertebral arch (7th 



It or 8th week) 

 If 1 for body (4th month) 

 1 for under surface of body 



FIG. 106. Lumbar vertebra. 



2 additional centers for mammillary processes 





are ossified from the centers for the 

 vertebral arch. At birth the vertebra 

 consists of three pieces, the body and 

 the halves of the vertebral arch. Dur- 

 ing the first year the halves of the 

 arch unite behind, union taking place 

 first in the lumbar region and then 

 extending upward through the thoracic 

 and cervical regions. About the third 

 year the bodies of the upper cervical 

 vertebrae are joined to the arches on 

 either side; in the lower lumbar verte- 

 brae the union is not completed until the 

 sixth year. Before puberty, no other 



Additional centers 

 for costal elements * 



At birth 



FIG. 107 



At 4 yrs. 



FIG. 108 



Two epiphysial plates 

 for each lateral surface * 



At 

 25th year 



FIG. 107-109. Ossification of the sacrum. 



changes occur, excepting a gradual increase of these primary centers, the upper and under sur- 

 faces of the bodies and the ends of the transverse and spinous processes being cartilaginous. 



