RUDIMENTARY SPINA BIFIDA. 



101 



as many of the X-ray plates did not show the exact number of lumbar verte- 

 brae. The last lumbar was the only lumbar vertebra found to be so affected, except 

 in the case of one female, where the fourth, as well as the fifth, and also the entire 

 sacrum, showed the condition. (See case S. R.) 



TABLE 3. Occurrence of incomplete ossification of spinous process nf last lumbar vertebra. 



That the ratio obtained, representing the variation of bifid L 5 , is a close approxi- 

 mation of its occurrence in any white population of mixed nationalities would seem 

 to be a reasonable assumption. The X-ray plates constituted a consecutive series, 

 the patients coming to the hospital for various causes, and in general the slightest 

 irregularity of the last lumbar vertebral process shows very clearly in the plates. 

 There are two modifications of the above statement, however, that must be taken 

 nito consideration as unknown factors, in spite of the fact that they probably repre- 

 sent only a small source of error and would tend to balance one another. On the 

 one hand it must be admitted that a hospital population would be apt to include, 

 somewhat in excess of an ordinary population, such cases of spina bifida as present 

 clinical manifestations. On the other hand, a certain number of L 5 with defective 

 arch would not show on an X-ray plate, as the split occasionally occurs close to the 

 median line, but shadowed from the antero-posterior view by a bulky spinous pro- 

 cess. In any event, since only 4 of the 23 cases where L 5 was involved gave clinical 

 symptoms (q. v. clinical), and as the additional group of L 5 cases where irregularity 

 of the spinous process was noted numbered only 7, the ratio of occurrence would not 

 be greatly changed by these factors. Figures 1, 2, 3, and 4, taken from the X-ray 

 plates, show the various types of incomplete closure encountered. 



FIGS. 1 to 4. Tracings taken from X-Ray plates, showing various types of incomplete dorsal closure of the fifth lumbar, 

 vertebra. Fig. 1, J. H. H. No. 36595. Fig, 2, J. H. H. No. 45489. Fig. 3, J. H. H. No. 35870. Fit?. 4. 

 J. H. H. No. 43888. 



