128 



HUMAN ANATOMY. 



original pieces, which become less and less flat and more and more rounded. It is 

 rare to see more than four distinct segments, but very often the last is somewhat 

 elongated and shows signs of subdivision. It is not uncommon for the first piece 

 to remain separate, neither fusing with the sacrum nor the next coccygeal plate. 



STRUCTURE OF THE VERTEBRA. 



The shell of compact bone forming the surface is everywhere very thin. The 

 general plan of the internal spongy bone is one of vertical plates which in a frontal 

 section (Fig. 156, A) are bowed somewhat outward from the middle of the bone, and 

 of transverse plates connecting them near together at the ends and farther apart in the 



Fig. 156. 



B 



Frontal (A) and sagittal {B) sections of body of lumbar vertebra, showing the arrangement of the bony lamellae. 



Natural size. 



middle third where larger spaces occur. The strongest plates spring from the pedi- 

 cles and diverge through the bone, joining, probably, for the most part the hori- 

 zontal system. In the sacrum the same general plan prevails, but in addition there 

 are series of plates, mainly horizontal, in" the lateral parts ; those from the first sacral 

 are the most important. 



DEVELOPMENT OF THE VERTEBRA. 



Presacral Vertebrae. — These vertebrae ossify from three chief centres and 

 at least five accessory ones. The median one of the three chief centres forms the 

 greater part of the body ; while the other two, one appearing in each pedicle, form 

 the postero-lateral part of the body, the arch, and the greater part of the processes. 

 The oblique neiiro-central sutures separate the regions of these centres. The lat- 

 eral centres of the upper thoracic and the cervical vertebrae appear first. It is 

 usually taught that they appear in the sixth or seventh week of foetal life, but Bade ^ 

 with the Rontgen rays found no sign of them at eight weeks. The point is unset- 

 tled. The first median centres to appear are those of the lower thoracic and the 

 upper lumbar vertebrae. In this region and below it the median centres precede the 

 lateral ones ; in the upper part of the spine the growth is much more vigorous in 

 the lateral centres. The median centres of the cervical vertebrae appear in order 

 from below upward. The upper ones (judging from Rontgen-ray work and from 

 transparent foetuses) sometimes have not appeared as late as the sixth month, 

 although we have seen them towards the close of the third. 



At birth the upper and lower ends of the bodies are still cartilaginous, but the 

 arches are well advanced in ossification, although bone does not cross the median 

 line until some months later. The transverse processes of the thoracic vertebrae are 

 farther advanced than those in other regions. The spines are still cartilaginous. 

 The neuro-central suture is lost at from four to six years, disappearing first in the 



* Arch, fiir Mikros. Anat.. Bd. Iv., 1859. 



