THE UNIQUENESS OF THE INDIVIDUAL 



region, the small of the back, and backwards in the fused 

 vertebrae that form the sacrum. That is the mature pattern; in 

 development, the neck flexure appears somewhat before birth, 

 and the lumbar flexure between the ninth and eighteenth 

 months of age. 



An upright stance imposes new and peculiar stresses upon 

 the spinal column. The support of weight imposes a force acting 

 down the vertical long axis, which tends to compress the 

 vertebrae upon themselves. The angle of their apposition is 

 responsible for a shearing force between the bottom-most 

 lumbar vertebra and the sacrum; and general flexional strains 

 become very apparent when stooping to pick up weights. To 

 cope with these new forces (for such, in an evolutionary sense, 

 they are) man inherits only the standard outfit of muscles and 

 ligaments, and the muscular bracing of the neck and lumbar 

 region leaves much to be desired. 



What suff'ers from the w^ear and tear of habitual use is not, 

 primarily, the vertebrae themselves, but the tissues lying 

 between them. The bodies of the vertebrae are not set against 

 each other face to face; on the contrary, about one-quarter of 

 the total height of the column (more in the lumbar region) is 

 occupied by peculiar solid intervertebral joints. Each joint 

 forms a so-called intervertebral disk — a central nucleus of semi- 

 fluid consistency, which embodies or represents the remnant of 

 the embryonic notochord; contained within a tough fibrous ring, 

 the annulus, in which the fibres are disposed cylindrically in 

 coaxial rings; the whole being bounded above and below by 

 flat cartilaginous plates. The whole organ has been described 

 by one of its leading students, Ormond Beadle, as a hydro- 

 dynamic ball bearing. 



The bearings may give in a variety of difl'erent ways. Under 

 repeated flexion of the spinal column when it is taking weight, 

 the nucleus may gape through a weakness in the fibrous ring 

 which normally contains it, press against the posterior liga- 



128 



