VARIATIONS OF THE VERTEBRA. 131 



sooner than between the bodies. By the fifteenth year the lower three vertebrae are 

 generally fused, the second joining them from eighteen to nineteen. The five pieces 

 are united by the twentieth year. In some cases several of the sutures are still to be 

 seen, but all may have disappeared. The union of the bodies, as shown by sections, 

 in the case of the Upper ones, may not be complete internally till a much later period. 

 Two thin epiphyses appear on each side of the sacrum about the eighteenth year, 

 one for the auricular surface and the other below it. The lines of union of these 

 plates may be visible after twenty-one. 



The Coccyx. — Our data concerning the ossification of the coccyx are very un- 

 satisfactory. Each segment has one centre, but the first may have two, one on each 

 side, and, according to some, secondary centres for the cornua. Ossification begins 

 in the first piece at about birth, and successively in the others, from above down- 

 ward, until puberty. The lower three or four pieces fuse within two or three years 

 after birth, and join the first at perhaps about twenty ; there is, however, great 

 diversity, and frequently the first unites with the sacrum instead of with the others. 



The Atlas, — The atlas is almost 

 wholly formed from two centres which r\G. 161. 



appear in the seventh week of foetal life 

 in the root of the posterior arches ; from 

 these points ossification spreads most 

 rapidly backward. In the course of the 

 first year a centre is found in the middle 

 of the anterior arch. The lateral masses 

 meet behind in the fourth year and join 

 the median anterior nucleus in the fifth. 

 Sometimes the union of the posterior 



arches does not occur. The anterior nu- unique case of absence of the anterior arch of the atlas. 



cleus may be absent, and the front arch 



may show a median suture or be represented by ligament or cartilage. In one in- 

 stance the anterior arch was wholly wanting, the lateral masses being fastened to the 

 odontoid by ligament' (Fig- 161). 



The Axis. — The ossification of the axis begins by two lateral points appearing 

 by the eleventh week. The median one, which does not come till the fifth month, 

 is at first double, but the two points speedily fuse. At about the same time two 

 nuclei appear side by side in the odontoid process, which join together before birth, 

 leaving a space between them at the tip. This may be closed by the extension of 

 ossification, or a centre may appear in it at the second year, which fuses by the 

 twelfth. The piece thus formed has been held to represent the epiphyseal plate for 

 the top of the atlas. The odontoid process joins the body at the periphery, the union 

 beginning in the third year and being complete a year or two later ; a piece of car- 

 tilage in the middle of the juncture is said to persist under the odontoid until old 

 age. Very rarely the odontoid remains distinct. The arches join the body in the 

 third year, and usually meet behind at the same time ; the latter union, however, 

 may be delayed. 



Variations of the Vertebrae. — The commonest and most interesting variations are those 

 of number. These are very frequent in the coccyx, since there are originally more elements than 

 persist, and indeed we are not sure even of the normal number in this bone. Numerical varia- 

 tions are also often observed in the sacral, less so in the lumbar, still less so in the thoracic, and 

 extremely rarely in the cervical region. The pumber of vertebrse above the sacrum (twenty- 

 four) is usually unchanged, but, owing to differences in development of the costal element, one 

 region is not rarely increased or diminished at the expense of the next one. Thus the very com- 

 mon condition of six lumbar vertebrae is due to the want of development of the costal element 

 (the rib) of the last thoracic, and implies only eleven vertebrae in that region. Conversely, thir- 

 teen thoracics imply an undue development of the costal element of the first lumbar, and con- 

 sequently only four lumbar vertebrae. Often the costal element of the last cervical is free and 

 over-developed, making a cervical rib. But even if this be large enough to reach the sternum, 

 which is exceedingly rare, the number of cervical vertebrae is usually considered unchanged. 

 Other changes are due to variations in development of the costal element in the last lumbar 

 and the first sacral. Transitional forms are here very frequently met with. The last lumbar 



^ Dwight : Journal of Anatomy and Physiology, vol. xxi., 1887, 



