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accordance with the pretty- well constant occurrence of the above- 

 mentioned ligaments and the not less constant pulsation of the A 

 vertebralis. Tiiere are however still other considerations that, in our 

 opinion, make Le Double's explanation appear less acceptable. 

 Suppose even that the stimulus of the periost caused by the pulsation 

 of the A. vertebralis should in reality be the cause of the occurrence 

 of the ponticuli posteriores and laterales, then it would at all events 

 be at least astonishing that tlie results of this process, naturally some- 

 what slow, could already be observed at a youthful age, and yet this 

 is the case, as I have been able to ascertain with several atlases of 

 the collection 1 have examined. The extraordinarily powerful way, 

 in which in many cases both the ponticuli posteriores and the 

 ponticuli laterales can be developed make us likewise doubt the 

 correctness of Le Double's explanation of the discussed variations, 

 the more so, as it is generally known, that osseous tissue reacts 

 on the pulsations of the vesselwall rather with atrophy than with 

 hypertrophy. 



This doubt becomes still greater if we also consider the results 

 of comparative anatomical investigation which were also known 

 to Le Double. For then it appears that with many groups of mam- 

 mals, and among these also primates, the ponticuli and foramini, 

 occurring with man only as variations, are constant and normal 

 parts of the atlas. 



BoLK has laid, as far as it regards Primates, a stress upon this 

 fact, which was already known to Merkel. He demonstrates that 

 namely the normal human atlas has been developed by reduction 

 from the more complete form, as it is met with among Primates 

 a.o. with Cynocephalides. This reduction regards in the first place 

 the topmost limitation of the canalis arteriae vertebralis, with 

 Cynocephalides still completely extant, of which first the most lateral 

 part (the [lonliculus lateralis) afterwards also the medial part (the 

 ponticuius posterior) disappears, by which process the channel is 

 changed into a notch. 



The repeated occurrence of these ponticuli must consequently most 

 probably be regarded as a common atavism; ponticuius posterior 

 and ponticuius lateralis are with the human atlas regressive variations. 

 According to this notion the signification of this variation is in 

 compai'ison with Le Double's view a quite difierent one. The prin- 

 cipal cause of its occurrence is now not to be found in outward 

 circumstances, however favourable their influence may for the rest 

 be upon the process, but in a generally occurring inclination of 

 reproducing phylogenelically older forms. 



