2iÖ 



line uninterrupted osseous arch. As a rule the boundary -line between 

 the regions belonging to Sla and Sib cannot be observed here, 

 no more as with the massae laterales, for the simple reason that no 

 passage required for nerve or bloodvessel keeps the regions separated. 



It seems to me to be here the place to fix the attention to 

 peculiarities occurring rather frequently at the ossification of the 

 posterior arch of the atlas. In some cases namely one sees either 

 in the median line, or immediately on either side of if, openings in 

 the arcus posterior. The occurrence of these foramina is not entirely 

 unknown. Lk Double mentions them in his repeatedly cited work, 

 when he says on p. 88 that sometimes the tnberculum posterior 

 atlantis is replaced "par une depression plus ou moius profonde, 

 dans laquelle on trouve par exception un foramen minuscule, qui 

 est I'origine d'une canalicide, qui s'ouvre en avant dans la cavité 

 rachidienne". The author does however not attach any signification 

 to it, nor does he try to give an explanation of it. 



The mentioned opening, which might be distinguished as foramen 

 arcuale medianum or mediale, occurs rather frequently in those 

 atlases, where the process of ossification is not yet completed, but 

 it is not entirely wanting in the normal, well developed atlas, as 

 I could ascertain in the material examined by me. Usually, as 

 likewise Lk Double indicates, the variation remains restricted to a 

 depression lying in the region of the tubei'culum posterius, now of 

 a fantastical shape, now, and this rather frequently, in the form of 

 a rather deep notch running transversally, the two extremities of 

 which are still a little deeper. In fig. 3, 4, and 5 I have represented 

 some forms of this variation, as I foiind them in full-grown atlases 



Fig. 3. 

 Alias with foramen arcuale medianum. 



among the material examined by me. Fig. 3 represents an atlas, in 

 which the for the rest strongly developed arcus posterior shows in 

 the mediau-line a round opening (foramen arcuale medianum) lying 

 in a little cavity. In fig. 4 we find the representation of an atlas, 

 tlie posterior part of which is characterized by a transversal notch 

 extending over a rather large distance. In the bottom of this notch 

 we find on either side of the median-line an opening (foramen 



