THE SKULL THE CEANIUM 



293 



the surface is viewed directly from the side, only the anterior wall of 

 the cleft is visible, the bottom being overhung by the posterior edge. 

 The cleft is subject to considerable variation in depth, shape, and 

 direction. It usually is deeper above than below, and its upper end 

 curves forward. 



The large section of the external surface lying behind and above 

 the cleft is almost square, and is about one-third higher than it is 

 wide. Its upper third faces upward as well as outward, and the lower 

 two-thirds face outward and slightly downward and backward. This 

 part of the external surface consists of a series of narrow scrolls of 

 thin bony tissue placed one above the other and running across the 

 long axis of the surface, that is, downward and forward in the nasal 

 chamber. The scrolls are slightly curved, the convexity facing 

 upward. The topmost scroll is the longest, and they become suc- 

 cessively shorter as they descend. Not all of the surface is occupied 

 by the scrolls. Along and parallel with the posterior border of the 

 cleft which divides this part from the anterior triangular part is a 

 thin bony plate, which, as we shall see, represents the external ex- 

 panded edge of the bar which runs upward from the lower part 

 of the expanded scrolls, forms the posterior wall of the cleft, and 

 gives attachment to the anterior ends of the six upper scrolls. This 

 plate covers the anterior ends of the scrolls ; it is continued backward 

 as the external edges of 



the partitions which spring FIG. 212. 



from the posterior wall of 

 the cleft, lie between and 

 parallel with the scrolls, 

 and end by joining the 

 cribriform plate above and 

 behind. The lower edge 

 of the plate forms the 

 lower border of the sur- 

 face. The partitions, 

 which usually number 



LATERAL ETHMOID, EXTERNAL SURFACE. PARTS NUM- 



seven (Fig. 212), are wider BERED, LARGER SCROLLS SHADED. 



in front than behind, and 



may vary considerably in length, width, and form. The upper are 



the longest, and those near the middle are shorter and more curved. 



