ELEMENTAR Y ANA TOMY. 



[LESS. 



Metapophyses and Anapophyses, visible on the first lumbar 

 vertebra, disappear on lower ones. 



13. Below the lumbar vertebrae comes a solid complex bone 

 before mentioned the SACRUM, of a roughly triangular form, 

 with one angle downwards. Its front surface is strongly con- 

 cave vertically, less so transversely. It is really made up of 

 five vertebrae fused together, and plain traces of its original 

 composition remain in the fully ossified bone of the most 

 aged individuals. 



The centra diminish in size from above downwards through 

 the sacral series. 



The neural canal, completed as usual (i.e. by bone) above, 

 remains unossified and closed by membrane only at the lower 

 end of the sacrum, through the imperfect development of the 

 neural arches of the inferior sacral vertebras. 



Neural spines form a median, backwardly projecting ridge 

 behind the sacrum, which ridge projects most above. 



Transverse processes are largely developed, especially 

 above. By their anchylosis they form the lateral masses of 

 the sacrum. 



On both the anterior and posterior surfaces of the sacrum 

 four apertures, one oeiow the other, are visible on each side. 

 This appearance is produced by coalescence of the sacral 

 transverse processes, thus : 



FIG. 44. FRONT AND BACK ASPECTS OF THE SACRUM. 



, auricular surface ; z, prezygapophys : s at upper end of posterior surface 

 of sacrum. 



Nerves, as before said, pass out on each side between the 

 pedicles of adjacent vertebrae. Now the coalescence of the 

 sacral transverse processes necessarily changes each such 

 intervertebral opening into a pair of openings, of which one is 

 dorsal and the other ventral. 



