RUDIMENTARY SPINA BIFIDA. 



99 



was completely open in 15, or 2 per cent. He further found closure of the hiatus 

 sacralis to be more frequent over S 4 . This occurred in the six-segmented sacra 

 in 57.3 per cent of the specimens, and in the five-segmented sacra in 70 per cent. 

 In a group of 265 sacra with five vertebrae there were 9 (3.4 per cent) with com- 

 pletely open canal, but none among 113 sacra in which the first coccygeal segment 

 was fused with the sacrum, nor among 150 six-segmented sacra. Thus, in a total 

 of 528 specimens representing these three types the open sacral canal exhibited this 

 variation of 1.7 per cent. 



Frets's findings in regard to the hiatus sacralis and open sacral canal in this 

 group of 528 cases are shown in table 1 (adapted from table iv in his article in the 

 Morphologische Jahrbuch, 1914). Here a definite relation appears to exist between 

 the number of segments in the sacrum and the region of closure of the hiatus. In 

 sacra formed of 5 vertebrae it was open higher than in those formed of 6. In the 

 six-segmented sacra the hiatus sacralis closed over the fifth vertebra in 34 per cent 

 of the cases, as opposed to 5 per cent in those with 5 segments; while in the latter it 

 closes over S 3 in 20 per cent of the specimens, as opposed to 7.3 per cent in the six- 

 segmented sacra. 



TABLE 1. Form of Hiatus canalis sacralis, 52S cases (adapted from Frets). 



In 22 Australian sacra Wetzel (1915) found 2 with completely open canal. He 

 also mentions a study of 257 specimens from the "Breslau Anatomy" (which I have 

 not been able to locate) in which this variation was found in 3.5 per cent of the 

 cases. He noted that the hiatus sacralis closed over S 4 in 8 cases and between S 4 and 

 S s in 8 cases, and that there were numerous instances where the arches of Si either 

 did not unite at all or united so low as to correspond to the junction of Si and S 2 . 



At this point it may be well to call attention to the fact that in such reports 

 rarely is any information given, or even an estimate made as to how much of the 

 material presented was originally acquired because of the presence of such varia- 

 tions or anomalies. The chances are that some of the material in almost any col- 

 lection has been selected on this account, and therefore ratios based upon these 

 cases are exaggerated. Very probably the rather wide range of variation of the 

 open sacral canal (5.0 to 1.7 per cent) as presented by the three authors quoted 

 above, Radlauer, Frets and Adolphi, is due as much to differences in collection 

 as to true representative differences in populations or races. 



