132 ON THE VERTEBRATE SKULL. 



ing into any details which were not strictly necessary ; but there 

 remains one part of the cranium — the temporal bone — the struc- 

 ture of which must be carefully and thoroughly investigated, if 

 we desire to understand the modifications undergone by the 

 bones which correspond with its constituent elements in other 

 Vertebrata. 



Viewed from without, the temporal bone presents the well- 

 known pars squamosa (Sq.) and pars mastoidea (ill), in the 

 re-entering angle between which, the tympanic element (Ty.) is 

 fixed (Fig. 55). 



No suture separates the pars squamosa from the pars mas- 

 toidea, but the posterior limits of the former are indicated, in the 

 first place, by the curved ascending portion of the posterior root 

 of the zygoma (a b), which bounds the attachment of the tempo- 

 ral muscle ; and secondly, by a curved ridge, convex backwards 

 and differently defined in different subjects — the mar go tijmpa- 

 nieus of Henle — which passes downwards, behind the auditory 

 meatus, until it cuts the contour of the tympanic bone. Near 

 the upper end of this ridge, or " post-auditory process," is an 

 elongated " post -auditory fossa " (b), more marked in old than 

 in young subjects. 



The portion of the squamosal element, the free edge of which 

 terminates in this ridge, forms an arch, of which the posterior 

 pillar constitutes the posterior and upper wall of the auditory 

 meatus, while the anterior pillar forms the front boundary of the 

 glenoid cavity. The centre of the arch is interrupted by the 

 middle root of the zygoma (e), or " the post-glenoidal process " of 

 the squamosal, which runs, as a wedge-shaped ridge, transversely 

 to the span of the arch. 



The upper edge of the anterior wall of the gutter-shaped 

 tympanic bone (which forms the hinder boundary of the glenoid 

 cavity), unites with this riclge, crossing its direction obliquely 

 inwards and forwards. Beyond the ridge it is no longer united 

 with the squamosal, but, keeping its oblique direction, crosses 

 rather to the inner side of the lower edge of that bone, and 

 leaves the Glaserian fissure between the squamosal and itself. 



A section taken through both the external and the internal 

 auditory meatuses (Fig. 56) shows that this arched plate of the 



