202 



region, namely tlie atlas-vai-iations, the Pro-atlas uf Albrecht, the 

 concresceiitia atlaiito-ocri|iitalis and the manifestation of the occipital 

 vertebra. 



In this second communication I intend to trace the signification 

 of the existence of the semi-segment for the morphological explanation 

 of the atlas-variations. 



The fact that I wish to discuss in the first place these atlas- 

 variations finds its foundation among otiiers in the circumstance that 

 it is just the study of these variations that has been the nearest induce- 

 ment to state the existence of the semi-segment la described in the 

 pi'evious communication. 



In the description of the human atlas it is always indicated, that 

 the most lateral part of the arcus posterior, namely that part that 

 borders immediately on the massa lateralis is characterized by a 

 notch. This notch, called sulcus arteriae vertebralis, is caused by 

 the arteiia vertebralis, which after having passed through the foramen 

 transversarium atlantis bends behind the massae laterales and crosses 

 the arcus posterior together with the first cervical-nerve, before it 

 pierces the membrana atlanto-occipitalis. The degree of development 

 of this notch shows a great deal of variability. Now it is flat and 

 shallow, now one sees that it has been transformed into a channel 

 shut off from all sides, because an osseous bridge extends itself 

 from the posterior rim of the massae laterale to the upper-rim of 

 the arcus posterior, so that one must then speak of a canalis or 

 foramen arteriae vertebralis. This latter condition occurs frequently, 

 witness the fact, that nearly all text-books call the attention to it 

 in their descriptions of the atlas. 



The nomenclature, however', of this variation, both of the osseous 

 bridge, mentioned above, and of the channel or foramen the cranial 

 border of which is formed by the bridge, varies so very much, tiiat 

 it is almost as arbitrary to find out oneself a name for it, as to 

 make a choice from the numerous existing names. In ray opinion 

 foramen atlantoideum posterius (Boi.k) and foramen arcuale (Gaupp) 

 are the simplest among the denominations of the above-mentioned 

 foramen. 1 shall call the osseous bridge over this foramen ponticulus 

 posterior. 



Beside this variation of the human atlas a second is known, 

 wliicD Dccurs less frequently, li consists herein, that from the lateral 

 side I f ilie up|)er-rini oF the massa lateralis an osseous bridge 

 ■ x eii'ls lo the most lateral |)art of the upfier-rim of the processus 



ii y ■ us atlant -. Here is consequently the arteria vertebralis 



d i)) .1 bone, now uigether with the ramus anterior of the 



