522 



Prof. A. M. Paterson. 



[May 19, 



belonged to spines absolutely complete, and 96 to spines complete 

 except for a deficiency of the coccyx. The material has been obtained 

 from many sources : and I am specially indebted to Professor D. J. 

 Cunningham, of Dublin, for the use of a large number of specimens, 

 and notes and drawings of observations made by him on the subject. 

 The conclusions arrived at are as under : — 



1. The examination of a large series of vertebral columns compels 

 one to discard as inadequate the theory of "intercalation" and 

 " excalation " to account for the variations in the number of verte- 

 bras in the several regions. The hypothesis of inherent variability, 

 of shifting of one region at the expense of another, fully explains the 

 cases of individual variation. The changes met with may be re- 

 garded as produced, not by the sudden (and anomalous) interposition 

 or loss of a vertebral segment, but by a conversion of the segments 

 of one region into those of another and contiguous region. 



2. (a.) There is a marked tendency on the part of the first sacral 

 vertebra to be liberated from the rest of the sacrum. It retains its 

 individuality more clearly than the other vertebrae, and frequently 

 approximates in type to the lumbar series. 



(6.) The surface for articulation with the ilium, while usually 

 placed on the first two, and a part of the third, sacral vertebra?, varies 

 considerably in position. The surface may be shifted backwards or 

 forwards ; and the tendency is more marked towards a shifting in the 

 caudal than the cephalic direction. 



(c.) The surface for attachment of the sacro-iliac ligaments is 

 generally subdivided into two or three depressions, of which that on 

 the first sacral vertebra is, in the great majority of cases, the largest 

 and deepest. The inference from this fact is that the first sacral 

 vertebra has usually the greatest responsibility in supporting the 

 ilium. 



(d.) In the vast majority of cases there are five constituent bones 

 in the sacrum. Increase to six is much more common than diminu- 

 tion to four ; and increase by addition at the caudal end is apparently 

 much more common than by addition at the cephalic end. 



(e.) Asymmetry of the sacrum occurs frequently (S"3 per cent.). It 

 occurs in two forms : as either a sacro-coccygeal or a lumbo-sacral 

 vertebra ; and in two ways, by diminution or addition at either end 

 in the number of component bones. A sacro-coccygeal vertebra is 

 more frequent than a lumbo-sacral ; and asymmetry with addition is 

 more common than asymmetry with diminution in the number of 

 bones forming the sacrum. 



(/.) The examples of correlated variations of the several regions of 

 the vertebral column indicate a greater tendency towards increase than 

 diminution in the total number of bones. Increase is more common 

 than diminution in the number of bones in the proesacral, sacral, and 



