258 SKULL AND. BRAIN IN MAN. 



is important in the investigation of the varieties of human 

 crania. 



/. The spheno-basilar angle is the angle formed by the 

 upper surface of the basilar process, and the floor of the Sella 

 Turcica. It increases from man downwards, and is a most 

 important element in the construction of the human skull. 



g. The spheno-ethmoid angle is the angle between the 

 anterior sphenoid and the cribriform plates. It exists in man 

 in a more or less developed form ; but so that the cribriform 

 side of the angle is, in well-formed heads, horizontal. In the 

 descending animal series the ethmoidal side of the angle rises 

 so as at last to assume the rectangular position. 



h. As the nasal bones rise with the cribriform plates in 

 the descending series, the ethmoido-nasal angle diminishes. 

 The extent of the ethmoido-nasal angle is a marked feature of 

 the human head. 



i. There are three series of curvatures in the construction 

 of the skull : — 



1st, The primordial curvature. 



2d, The secondary or permanent curvatures of the base, 

 which are represented in the series of occipito-basilar, spheno- 

 basilar, spheno-ethmoidal, and ethmoido-nasal angles, and the 

 resultant of which is represented in the cephalic axis. 



3d, The antero-posterior and transverse curvatures of the 

 vault of the cranium. 



h. In comparing the secondary or permanent curvatures of 

 the base of the skull in man and the animal, it is evident that 

 the amount of curvatures is much greater in man than in the 

 descending series of mammals. 



• Moreover, in the greater curvature of his cranium, man 

 retains in great measure the primordial or embryo curvature, 

 whereas the lower we go in the series of vertebrata, the more 

 completely do the primordial curvatures disappear or become 

 masked. 



