THE ENTIBE SKULL 493 



between the orbital and median plates. This latter part has been 

 called the external frontal sinus, and because of the close union of the 

 partition of the mesial ethmoidal scrolls with the edge of the nasal, 

 and the union of the lower edge of the median plate of the frontal 

 with the upper edge of the nasal septum, it is apparently shut off from 

 the rest of the nasal fossa and is not seen in the ordinary median lon- 

 gitudinal vertical section. It is exposed by breaking away the median 

 plate of the frontal. It is wider behind than in front, where it is con- 

 tinued into the small fossa between the nasal process of the frontal and 

 the excavated upper part of the outer surface of the nasal bone. This 

 fossa receives the anterior end of the upper ethmoidal scroll. 



By the outer wall is meant not the wall formed by the inner sur- 

 faces of the scrolls of the lateral ethmoid and the maxillo-turbinal 

 facing the mesethmoid, but -the wall to which these are attached, and 

 which is exposed only by removing them (Fig. 398). To expose the 

 upper portion, the median plate of the frontal also should be cut 

 away. When this is done, the outer Avail is seen to be triangular in 

 shape, with the lower border straight. The posterior border is not 

 so long as the lower, and is emarginate, but its upper point lies nearly 

 over its lower point. The upper border is arcuate and directed from 

 behind downward and forward. The anterior angle is truncated, and 

 the small anterior border thus formed is the side margin of the ante- 

 rior naris. The surface slopes outward as it descends from the roof, 

 making the nasal fossa widest below. It is concave from before back- 

 ward, and the line of the deepest concavity begins at the upper poste- 

 rior part and arches forward and downward to meet the lower border 

 a little anterior to its middle point. The portion of the surface in 

 front of this line faces backward, inward, and downward ; the portion 

 behind the line faces forward, inward, and downward. Furthermore, 

 the anterior part is concave from above downward and is composed 

 of the inner surface of the nasal processes of the premaxillary and 

 maxillary. The almost vertical premaxillo-maxillary suture is seen 

 a little behind the emarginate anterior border. This anterior part of 

 the surface is bounded above by the prominent, slightly rolled outer 

 edge of the posterior surface of the nasal, to which the partition 

 between the mesial scrolls of the ethmoid is attached. The posterior 

 boundary is the line already mentioned which lies approximately 

 along the suture between the maxillary in front and the frontal and 



