38 Anatomy of Skeleton 



of each primitive segment as it begins to spread inwards round the notochord. The posterior half 

 of one segment joins with the anterior part of the next segment on its caudal side, and the chon- 

 drification that makes the rudiment of the vertebra takes place in the composite mass thus produced ; 

 it contains, therefore, mesodermal tissue from two segments, so is situated opposite the inter- 

 segmental line, and its neural arch and processes, costal, transverse, and spinous, are all situated 

 in the intersegmental plane. 



The ingrowing cells (sclerotome) of the primitive segments form a sheath for the notochord, 

 representing the membranous stage in the phylogenetic development of the column ; later chon- 

 drification takes place here, and later still ossification. The intervertebral discs are remains of 

 the cartilaginous structure, and the anterior arch of the Atlas is looked on as a persisting and 

 ossified hypochordal bar, which is the cartilaginous portion ventral to the sheath of the notochord 

 in the intervertebral regions, disappearing in the other parts of the column. 



The notochord is destroyed in the vertebrae by the formation of the bones, but persists in the 

 nucleus pulposus of the intervertebral discs. 



SACRAL VERTEBRA. 



Five (or sometimes six) vertebrae below the lumbar region are fused in the adult 

 into a single bone, the Sacrum, which forms the back wall of the bony pelvis, being 



surface for S* lumbar- 1/. 



s./v 



Inferior 

 tatenlangle 



Svrfccefor 

 Codyx 



FIG. 31. Front aspect of sacrum. A . F. anterior sacral foramina ; 

 S.N. lateral sacral notch ; P. sacral promontory. Apex at 

 lower end. 



situated between the two innominate bones and articulating with the upper and back 

 parts of their inner surfaces. 



The Sacrum is a wedge-shaped bone, with its base uppermost. Looked at from 

 the front it presents an anterior surface (Fig. 31) concave from above downwards and 

 slightly from side to side, of which the central part is evidently composed of the fused 

 bodies of the sacral vertebrae. Outside these are four anterior sacral foramina on each 

 side, in nearly parallel rows, for the exit of nerves, and again outside these are the lateral 

 masses of the bone, marked by neural grooves leading out of the foramina. 



The surface is bounded externally by the lateral margin, the edge of the lateral 

 mass : this margin has a depression, the sacral notch, opposite the second segment, 



