20 



VERTEBRAL COLUMN. 



from an additional centre. On the body of each sacral vertebra, epiphysal plate? 

 are formed at a later period, as in other vertebra. To complete the ossification 

 of the sacrum, there are also formed on each side of it two flat and irregular plates, 

 of which one extends over the first three vertebrae, while the other connects the 

 last two. 



Fig. 39. 



Fig. 19. OSSIFICATION OP THE SACRUM. 



A, sacrum of a foetus before six months, seen from the front, showing the ossific 

 nucleus in the body of each vertebra, from 1 downwards. 



B, sacrum of a child at birth, showing three pairs of additional nuclei for the lateral 

 masses, 2, 2, close above the sacral foramina. 



C (one-fourth of the size of nature), sacrum of a person of about twenty-five years of 

 age. At 3 & 3, epiphysal plates are still visible above and below the first vertebral 

 piece, and the fissures still remain between the first and second and the second and third 

 lateral pieces ; at 4 & 4' are shown the lateral epiphysal plates. 



Each of the coccygeal vertebrae is usually ossified from a single centre ; but occa- 

 sionally one of the first three contains two granules placed side by side. 



Progress of Ossification in the Vertebral Column. The deposit of bone in the 

 arches of the vertebrae takes place first in those belonging to the upper end of the 

 column, and gradually proceeds downwards. In the bodies, it first occurs in the 

 lower part of the dorsal region (about the ninth dorsal vertebra) ; and from that 

 the process is extended upwards and downwards, reaching last of all the atlas and the 

 coccyx, neither of which ossify till after birth. But though the nuclei of the lower 

 dorsal vertebrae are the first to appear, they are soon surpassed in size by those below 

 them ; and in the foetus at birth the relative size of the nuclei corresponds with that 

 of the fully-grown vertebrae. 



In the subjoined table are stated the ages at which the osseous centres of the 

 vertebral column appear and the steps of union take place. But it is to be observed 

 that it is only attempted to state these ages approximately, both because of the great 

 difficulty, in most instances, of determining the age of the human foetus with 

 accuracy, and because of a certain amount of variety which probably exists in the 

 rapidity of the growth of bone in different cases. (See Albinus, " Icones Ossium 

 foetus," p. 

 " On the Skeleton, 



; Meckel, in his " Archiv " vol. i. 1815, p. 648, tab. vi. ; Humphry, 

 .eton," p. 132 ; Aitken, " Growth of the Recruit," 1862.) 



PERIODS OF OSSIFICATION OF THE VERTEBRAE. 



In the vertebrae generally 



The three primary centres appear in the 7th or 8th week (foetal life). 

 The epiphyses of the processes appear in the 18th year or later. 

 The epiphysal plates of the body appear in the 18th or 20th year. 

 The two centres of the arch unite in the first year. 



