THE STRUCTURE OF THE HUMAN SKULL, 135 



Behind the posterior boundary of the squamosal, constituted 

 by the two diverging lines above described (Fig. 55), lies all that 

 portion of the temporal bone which is known as the pars mas- 

 toiclea. But, as I shall have occasion to demonstrate, when ex- 

 plaining the mode of development of the temporal bone, this 

 pars mastoidea is, in reality, made up of extensions of two of the 

 primitive constituents of the pars petrosa, and of a third element, 

 the epiotic. The posterior margin of the squamosal, as above 

 described, may be said roughly to form two sides of a parallelo- 

 gram. The third side is the thick part of the upper edge of the 

 pars mastoidea, corresponding with the termination of the upper 

 and anterior surface of the ptars petrosa on the inner side of 

 the bone. If a fourth side is made bv an imasinarv line con- 

 necting the ends of the others, the bony surface which lies above 

 and in front of the line will, as nearly as possible, belong to the 

 pro-otic element, while that which lies below and behind it, in- 

 cluding the mastoid process, appertains to the epiotic. On the 

 other hand, a certain amount of the pars mastoidea internal to 

 the digastric groove belongs to the opisthotie. 



