Dwight. . . . 

 Veraglia . . . 

 Regnault. . 

 Le Double. 



Cases. 



1 



3 



15 



2 



21 



Open 



arch. 



13 



RUDIMENTARY SPINA BIPIDA. 105 



plete. Seven other specimens were so damaged that it was impossible to determine 

 whether or not the atlas showed a bifid condition. In 43 intact specimens there was 

 physiological union of atlas and occiput and, among 

 these, incomplete union of the dorsal laminae of the TABLE 5. 



atlas was found in 25 cases (58 per cent), the bifid con- 

 dition (fig. 11) ranging from 1 to 12 mm., with an aver- 

 age of 4.16 mm. There was one exception (an Eskimo 

 specimen) in which the dorsal laminae of the atlas were 

 entirely lacking. In every case of bifid atlas there 

 was a marked asymmetrical arrangement in its fusion 

 with the occiput, one side being more closely united 

 than the other. In the 18 cases in which the dorsal 



atlantal arch was complete there was a much greater degree of symmetry in fusion 

 of the atlas to occiput, and in 14 of these this symmetry was very marked. 



In summing up the various factors which may play an etiological role in these 

 cases of minor incompletion of the posterior vertebral arch, we must first take into 

 account the residual influence of early embryonic spina bifida. We know that 

 localized delayed closures in the embryonic central nervous system occur, and it is 

 very likely that with some of the less severe types of this condition a moderate 

 degree of lagging is set up, resulting in the incoordination of the growing parts, and 

 finally in the lack of bony union of the vertebral dorsal arches. One would suppose 

 this factor to be at work to a greater degree in cases with associated neurological, 

 meningeal, or skeletal malformations, or where a considerable length of spinal 

 column is involved, than in cases where only one vertebra shows the condition. 

 Here other factors may be called upon to explain it. Thus, in addition to the 

 embryonic theory, which undoubtedly brings us closest to thinking of the cause 

 in chemical terms, owing to the knowledge we have of experimental NaCl spina 

 bifida, we must consider also the mechanical theories that appear to contain rational 

 suggestions. However, it must be borne in mind that with any biological theory 

 there is no such thing as real separation of the physical from the chemical they 

 are always in closest association. While possibly mechanical factors are pres- 

 ent in this condition, great care should be exercised in placing confidence in any 

 unproved statements which they may represent, for here, especially, mechanical 

 explanations are rather specious and almost impossible to test directly. The 

 mechanical factor upon which most stress has been heretofore laid is that of longi- 

 tudinal flexion of the vertebral column, and the point has been emphasized that 

 it is in the region of greatest flexion that the bifid condition most frequently arises. 

 The element of lateral torsion has been considered as only a possible, minor factor. 

 It is possible, however, that this may be of importance, especially where its action 

 in individual vertebrae would seem to be combined with factors such as the follow- 

 ing: (1) Position of the last vertebra of a free series, lying next to more rigid struc- 

 tures; i. e., atlas or last lumbar vertebra. (2) Asymmetry; i. e., atlas opposed to 

 asymmetrical condyles of occiput, or asymmetrical fusion occurring between atlas 

 and occiput. 



