Some Considerations respecting thé Parietal Bone. 



By 



E. J. Anderson, 



Pi'ofessor of Biology in Galway. 



(With 83 Fig.) 



The form and relations of the parietal bones are of so much 

 interest that some references to details of relationship in special groups 

 will not be wanting in importance. The size, absolute and relative, 

 the extension of the borders and angles, and the thickness of these 

 bones in some animals, point to various controlling influences during 

 their growth. The condition of the nervous organs beneath seems 

 foi' a longer or shorter time to influence the growth of the cranial 

 bones proper. It may be conceded that the centres of ossification 

 may count upon subjacent vascularity and nerve influence as potent 

 factors. The other factors are the nature of the tissues, the muscle 

 development, the growth of other bones and the nature of the pressures 

 and impulses and the amount and distribution of the formative material. 

 The fluctuations from time to time in the various factors make it 

 difficult to say that bones of the same name exactly correspond in all 

 cases. Indeed it is probable that they serve chiefly as keys to the 

 situation, in each particular case. A bone indeed may be produced 

 or altered by forces from within or from without and the adjustment 

 of these forces may give rise to bones of the same shape, which are 

 resultant of different forces. 



It follows, therefore, that we can only approximately gain some 

 knowledge of the position of the parietal bones. Commencing with 

 a reference to the prehistoric Batrachia! 



