THE VERTEBRAL COLUMN. 



which correspond to the neural ossifications of the other vertebrae, and which begin to ossify 

 about the seventh week. Their union posteriorly occurs in the third year, and is frequently 

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

 does not take place till the fifth or sixth year. 



In the axis, the arch and processes are formed from two centres corresponding to those of 

 the other vertebras, and appearing about the seventh or eighth week. Ossification begins in 



Fig. 21. OSSIFICATION OF THE AXIS. (R. Quain.) 



A, from a foetus of seven months ; 3, centre for the body ; 

 4, 5, two centres in the base of the odontoid process. 



B, shortly after birth ; 1, 2, neural ossifications ; 3, 

 central ossification ; 6, odontoid ossification. 



the body about the fourth month, from one or sometimes two centres, occupying the lower 

 part of the common cartilage of the body and odontoid process. In the upper part of this 

 cartilage, a little later, two collateral centres appear for the odontoid process ; these soon unite 

 into one, so that at birth the axis is composed of four pieces of bone. About the fourth year 

 the odontoid process becomes joined to the body and the fore part of the neural arch of the 

 axis on each side, and a little later union occurs in front and behind. In the centre, 

 however, a small disc of cartilage remains until advanced age. 1 The apex of the odontoid 

 process is formed from a distinct centre which appears in the second year, and joins about the 

 twelfth year. There is the usual annular epiphysis on the lower surface of the body. 



Sacral vertebrae. Each of the sacral vertebras has three primary centres of ossification, 

 one in the body and a pair in the arch. The centres of the bodies of the first three vertebra; 

 appear about the eighth or ninth week, those of the two following vertebras somewhat later. 



Fig. 22. OSSIFICATION OF THE SACRITM. (R. Quain.) 



A, sacrum of a fcetus before six months seen from the front, showing the ossific centres in the 

 bodies of the vertebrae. 



B, at birth ; 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 ; 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. (Allen 

 Thomson.) 



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



The lamina? begin to ossify about the sixth month, but the time of union with the bodies 

 differs in the different vertebras, taking place as early as the second year in the lowest, but 

 not till the fifth or sixth year in the uppermost. In each of the first three vertebras (sometimes 

 however only in two, sometimes in four) the anterior part of the lateral mass on each side is 

 formed from an additional nucleus which appears at the outer margin of the anterior sacral 

 foramen from the sixth to the eighth month. These unite to the bodies later than the arches. 

 In the case of the lower two vertebras the lateral masses are formed by extension of 

 ossification from the primary lateral nuclei. On the body of each vertebra epiphysial plates 

 are formed after puberty, as in other vertebras ; and two irregular plates of bone are added on 

 pach side of the sacrum, the uppermost of which extends over the auricular surface, and the 



1 D. J. Cunningham, Journ. Anat., xx, 238. 



