OSSIFICATION OF THE VERTEBRiE. 



21 



seventh usually presents o. separate osseoiis nucleus, and small nuclei have also 

 heen observed by Meckel in those of the second, fifth, and sixth vertebne. These 

 epiphyses generally appear from the eighteenth to the twentieth year, and are 

 not wholly united to the rest of the vertebra before the twenty-fifth year. 



The Atlas & Axis. — The ossification of the atlas and axis differs consider- 

 ably from that of the other vertebraj. In the atlas, the body bemg aljsent, the 

 anterior arch is formed by a strip of cartilage in which ossification, commencing 

 by one or two centres, only appears in the course of the first year after bu-th. The 

 neui-al arch, together with the processes, is formed from two lateral centres cor- 

 responding with those of the other vertebrae, and which begin to ossify about the 

 7th or Sth week. Their union posteriorly occui-s in the IJrd year, and is frequently 

 preceded by the formation of a distinct spinal nucleus. Their union with the 

 nucleus of the anterior arch does not take place till the oth or 6th year. 



In the axis the arch and i^rocesses are formed from two centres correspondmg 

 to those of the other vertebraj. and appearing about the 7th or Sth week. Ossifi- 

 cation begius in the body about the 6th month, from one or sometimes two 

 centres, occupying the lower part of the common cartilage of the body and odon- 

 toid process. In the upper part of this cartilage, a little later, two collateral 

 centres appear for the odontoid process : these soon unite into one. and become 

 ossified to the body of the axis in the 3rd year. There is frequently also a dis- 

 tinct centre m the apex of the process. (Humphry.) This separate ossification 

 of the odontoid process is important in connection with the view that it is 

 the displaced body of the atlas. 



Sacral Vertebrfe. — Each of the sacral vertebras presents three primary centres 

 of ossification, one in. the body and a pan- in the arch. The centres of the bodies 



Fij;. 20 





^ Xs^,'ii^ 



■^^ 



^o 



Fig. 20. — Ossification op the Sacrum. (R. Quain.) 

 A, sacrum of a foetus before sis; months, seen from the front, showing the ossific centre 

 in the body of each vertebra, from 1 downwards. 



I*>, at bu-th ; 2,2, additional centres for the lateral masses. 



C, about twenty-three years ; 3, 3, epiphysial plates still visible above and below the first 

 vertebral body, the fissures still remaining between the first and second and the second 

 and third lateral masses ; 4, 4', lateral epiphysial plates. 



D, upper surface of first sacral vertebra at four or five years, 1, and 2, as in A and B. ( A. T. ) 

 A & B, nearly full size ; C, one-fourth ; D, one-third. 



of the fii-st three vertebra3 appear about the Sth or 9th week, those of the two 

 following vertebrte somewhat later. The laminaj begin to ossify about the 6tli 

 month, but the time of union with the bodies dilfers in the different verteln-aa ; 

 taking place as early as the 2nd year in the lowest, but not till the 5th or 6th 

 year in the uppei-most. In each of the first three vertebra, and sometimes only 

 of two. the anterior part of the lateral masses on each side is formed from an 

 additional nucleus which appears at the outer margin of the anterior sacral 

 foramen from the 6th to the Sth month. These unite to the bodies later than 

 the arches. In the case of the two lovv-er vertebrte, the lateral masses are formed 

 "by extension of ossification from the x>rimary lateral nuclei. On the body of 



