DEVELOPMENT OF VERTEBRAE. 17 



slight lateral curve exists in the dorsal region, having its convexity 

 towards the right side. 



Development. The vertebrae are developed by three primary and 

 five secondary centres or epiphyses. The primary centres are, one for 

 each lamella, and one for the body ; the epiphyses, one for the apex 

 of the spinous process, one for that of each transverse process, and one 

 for the upper and under surface of the body. Exceptions to this 

 mode of development are met with in the atlas, axis, vertebra pro- 

 minens, and lumbar vertebrae. The atlas has four centres : one for 

 each lateral mass, one (sometimes two) for the anterior arch, and one 

 for the centre of the posterior arch. The axis has five : one (some- 

 times two) for the body, two for the odontoid process, appearing side 

 by side in its base, and one for each lamella. The vertebra prominens 

 has two additional centres for the anterior or costal segments of the 

 transverse processes, and the lumbar vertebra two for the posterior 

 segments of the transverse processes. 



The primary centres of the vertebrae make their appearance during 

 the seventh or eighth week of embryonic existence, the lamella being 

 somewhat in advance of that for the body. From the former are pro- 

 duced the spinous, transverse, and articular processes, and the sides of 

 the body ; they unite, to complete the arch, one year after birth, and 

 with the body during the fifth year. The epiphyses, for the extremi- 

 ties of the spinous and transverse processes, make their appearance at 

 fifteen or sixteen, and become united between twenty and twenty- 

 five. The epiphyses of the body are somewhat later in appearance, 

 and are consolidated between the periods of twenty-five and thirty 

 years of age. 



The ossific centres for the lateral masses of the atlas appear at the 

 same time with those of the other vertebrae ; they unite posteriorly at 

 the end of the second year, by the intervention of the centre for the 

 posterior arch. The one or two centres of the anterior arch appear 

 during the first year, and become consolidated with the lateral pieces 

 during the fifth or sixth year. The axis develops its lateral pieces at 

 the same time with the rest of the vertebrae ; they join posteriorly soon 

 after birth, and with the body during the fourth or fifth year. The 

 centres for the body and odontoid process appear during the sixth 

 month, and are consolidated during the third year. The body of the 

 axis is more largely developed at birth than that of the other ver- 

 tebrae. The costal segments of the vertebra prominens appear during 

 the second month, and become united to the body at the fifth or sixth 

 year. These processes sometimes remain permanently separate, and 

 constitute a cervical rib. The transverse process of the first lumbar 

 vertebra has sometimes a distinct centre, which may remain per- 

 manently separate, in that case forming a lumbar rib. 



The ossification of the arches of the vertebrae commences from 

 above, and proceeds gradually downwards ; hence arrest of develop- 

 ment gives rise to spina bifida, generally in the loins. Ossification of 

 the bodies, on the contrary, commences from the centre, and proceeds 



c 



