DEVELOPMENT OF VERTEBRAE. 55 



directed upwards, and if they were prolonged, they would come into con- 

 tact with a small tubercle which is found at the base of the posterior trans- 

 verse process (in strongly- marked vertebrse) in the vertebra above. This 

 junction would form a posterior intervertebral foramen, as actually occurs 

 in the sacrum. In brief, the lumbar vertebras exhibit those transitional 

 changes which are calculated, by an easy gradation, to convert separate 

 vertebrae into a solid bone. The transverse processes of the eleventh and 

 twelfth dorsal vertebrse are very interesting in a transcendental point of 

 view, as exhibiting a tendency which exists obscurely in all the rest, 

 namely, to trifurcate. Now, supposing these three branches to be length- 

 ened in order to fulfil their purposes, the anterior would constitute the 

 articulation or union with a rib, while the superior and inferior would join 

 similar branches in the vertebra above and below, and so form the poste- 

 rior intervertebral foramen. 



The intervertebral foramina formed by the juxtaposition of the notches 

 are smallest in the cervical region, and gradually increase to the last lum- 

 bar. On either, side of the spinous processes, and extending the whole 

 length of the column, is the vertebral groove, which is shallow and broad 

 in the cervical, and deeper and narrower in the dorsal and lumbar regions. 

 It lodges the principal muscles of the back. 



Viewed from the side, the vertebral column presents several curves, the 

 principal of which is situated in the dorsal region, the concavity looking 

 forwards. In the cervical and lumbar regions the column is convex in 

 front ; and in the pelvis an anterior concave curve is formed by the sacrum, 

 and coccyx. Besides the antero-posterior curves, a 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 deve- 

 lopment are met with in the atlas, axis, vertebra prominens, 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 poste- 

 rior arch. The axis has five : one (sometimes 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 ante- 

 rior 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 some- 

 what in advance of that for the body. From the former are produced 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 extremities 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 an-L 



