104 



RUDIMENTARY SPINA BIFIDA. 



cabinet at the National Museum 7 atlases showing incomplete posterior arch. The 

 latter have not been included in the series, as they represent material wh ch has 

 been selected on account of this variation, while the 11 cases cited above belong to 

 routine acquisitions. In 2 of the Indian specimens there was absence of practically 

 all arch formation (figs. 7 and 8), while in all of the remaining specimens the two 

 laminse approximate within 0.5 to 4 mm. of each other. 



TABLE 4. 



*One case of incomplete posterior arch of C's (fig. 6). 



Associated with this common type of incomplete arch there was a slight, though 

 appreciable asymmetry in a little less than one-fourth of the specimens observed 

 (4 out of 13 unbroken atlases, exhibit specimens included). This consists appar- 

 ently in a bending of one of the lateral masses forward or backward, and sometimes 

 also outward, with subsequent anterior or posterior projection of the corresponding 

 dorsal lamina of the atlas, so that the two laminse did not meet symmetrically. 

 Figure 9 represents an atlas of this type in the exhibit cabinet at the National 

 Museum, while a symmetrical atlas is shown in figure 10, representing the Egyptian 

 specimen designated in table 4. That this asymmetry is associated with an asym- 

 metrical position of the condyles of the occiput seems probable, as the latter condi- 

 tion is found in all types of crania. It was impossible to follow up this point. 



In attempting to analyze the various factors that seem to influence the closure 

 of the posterior vertebral laminse, another group of cases, in which the atlas and 

 occiput show physiological union, may be introduced. The condition has been 

 frequently reported, and among 21 cases of physiological union found in the lit- 

 erature incomplete dorsal arch occurred 13 times, or 61.9 per cent, as shown in 

 table 5. 



In many of these cases a lateral twisting of the atlas on the occiput has been 

 noted. At the National Museum there are 56 specimens of Dr. Hrdlicka's collection 

 showing fusion between the atlas and occiput. Probably in 6 of these the fusion was 

 the result of a pathological process, and in none of them was the atlantal ring incom- 



