1892.] 



The Human Sacrum. 



523 



caudal regions respectively. Increase in the sacral region is more 

 •common by abstraction from the caudal than from the lumbar series. 

 Liberation of the first sacral vertebra is more common than assimila- 

 tion of the fifth lumbar vertebra ; and assimilation of the first caudal 

 vertebra is more common than liberation of the fifth sacral. With 

 regard to the sacrum particularly, there is found to be a certain limited 

 and inherent variability in the position of the ilium, causing it to be 

 shifted backwards or forwards in relation to the vertebral axis, and 

 more frequently backwards than forwards. There appear to be three 

 separate influences acting upon the sacrum, and producing the differ- 

 ences in number of bones, correlated variations, and asymmetry : — 

 (1) fusion of the first caudal vertebra ; (2) liberation of the first 

 sacral vertebra, by a backward shifting of the ilium, along the 

 vertebral axis ; and (3) fusion of the last lumbar vertebra with the 

 sacrum, by a forward shifting of the iliac attachment. The first in- 

 fluence is most commonly seen, and may be exerted alone or along 

 with the second. The second and third influences are opposed to one 

 another. The former is more frequent than the latter, producing an 

 additional lumbar, or a lumbo-sacral vertebra ; the latter gives rise 

 to a diminution in the number of free lumbar vertebrae, and may be 

 accompanied by the conversion of the last sacral into a sacro-caudal 

 or caudal vertebra. 



(g.) A study of the ossification of the vertebral column leads to 

 similar conclusions, and indicates the existence of inherent variability 

 in the several regions, and a greater tendency to elongation than con- 

 traction of the vertebral column as a whole. The process of ossifica- 

 tion also shows that the ala of the first sacral vertebra (25th 

 spinal segment) is usually the first to ossify ; which vertebra may 

 therefore be regarded as the one primarily responsible for the attach- 

 ment of the ilium. The exceptional cases occur in sacra showing 

 correlated variations or asymmetry, and indicate a greater tendency 

 on the part of the ilium to be shifted backwards than forwards. 



(h.) The evidence derived from a consideration of the vertebral 

 column in other vertebrates is unsatisfactory. The human spine holds, 

 with regard to correlated variations, a position intermediate between 

 anthropoid apes (in which they are very frequent) and quadrupeds 

 generally (in which they are rarely present) ; while asymmetry, 

 especially of the sacrum, may be looked upon as an essentially human 

 characteristic. 



(i.) The examination of the correlation of the spinal nerves and 

 limb-plexuses with the vertebral segments shows both specific and 

 individual differences in this respect. The individual differences 

 may be classified under three types : — (1) a variation in the arrange- 

 ment of the nerves without any concomitant variation in the vertebral 

 column ; (2) a variation in the vertebral column without any con- 



vol. li. 2 N 



