n-] THE SPINAL SKELETON. 33 



At the summit of the sacrum is a pair of articular pro- 

 cesses, which join the lower articular processes of the last 

 lumbar vertebra. 



The lower end of the sacrum is devoid of similar pro- 

 cesses, or has them only represented by rudiments. 



On each side of the upper part of the sacrum 'is a laro- e 

 irregular surface, which is coated with cartilage, and articu- 

 lates with the hip or haunch bone. This is called the 

 auricidar 1 - surface. 



14. The last part of man's spine is, as has been said alreadv 

 the COCCYX, which consists usually of four rudimentary 

 vertebre, completely or partially united 



so as to form a small conical bone. 



Its uppermost part (i.e. the first 

 coccygeal vertebra) articulates with the 

 lower end of the sacrum, not only by 

 its centrum, but also by two little arti- 

 cular processes; it has besides two FIG. 45 . -THE COCCYX 

 rudimentary transverse processes and At its end are ^ 



rudimentary pedicles, between which two prezygapophyses. 

 latter membrane alone extends to close 



the neural canal. Below this the coccyx is destitute of pro- 

 cesses, and consists of but smaller and smaller vertebral 

 centra fused together. 



Thus the last vertebra is the very opposite of the first (or 

 atlas), being all centrum, while the atlas has no centrum at 

 all. The coccyx usually becomes anchylosed to the sacrum 

 about or after the middle of life. 



15. The WHOLE SERIES of vertebras are so disposed that 

 the backbone, when seen in profile, forms four sigmoid curves, 

 directed alternately forwards and backwards (see Fig. 38). 

 Thus in the neck it is slightly convex forwards, more 

 convex backwards in the dorsal region, rather strongly 

 convex forwards in the loins, while below this the terminal 

 part of the column sweeps round in a more marked curvature 

 concave in front, the coccyx continuing onwards the vertical 

 curve of the sacrum. 



These gentle curves together form a line of beauty, and 

 give to the vertebral column a strength ten times greater 

 than it would have were it quite vertical. 



The thickness of the front part of the backbone (formed of 

 the vertebral centra) increases slowly downwards to the 



1 From its resemblance to the outline of t'ie external ear. 

 D 



