292 



MAMMALIAN ANATOMY 



outer scroll. The superior outline of the external surface of the 

 lateral ethmoid is slightly longer than the antero-posterior length 

 of the surface itself. It is formed behind by the upper border of 

 the superior external and internal scrolls, in the middle by the thin 

 longitudinal partition which separates them, and in front by the plate 

 on the inside of the upper ends of the upper anterior scrolls. This 

 outline is gently arcuate from behind to the anterior fifth, where it is 



FIG. 211. 



With Nasal Prnrfss of Frnnfnl. 



In Frontal Sinus. 



With Maxillary, j 



With Orbital Plate of 

 frontal. 



With Palatine. 

 With Presphaioid. 



Free Lower Anterior Portion. 



Vomer. 



Os Planum. Vomer, with Maxillary. 

 THE ETHMOID AND VOMER, LEFT SIDE, OUTER SURFACE. 



interrupted by an abrupt step-like drop. From this point it is con- 

 tinued straight to the end. The upper outline is directed downward 

 and forward at right angles to the posterior border. The anterior 

 border is only about half as long as the superior border. It is straight, 

 and is formed by the lower edges of the outer lower anterior scrolls. 

 It is not parallel with the posterior border, but inclined more forward 

 at the upper end, making approximately an angle of sixty degrees 

 with the lower border. 



The external surface is divided into a small triangular anterior 

 third and a larger quadrilateral posterior two-thirds by a deep moder- 

 ately wide ethmoidal cleft, which begins below, behind the anterior 

 lower angle, and runs nearly parallel with the posterior border ob- 

 liquely upward and forward and ends just before it reaches the 

 superior border. This cleft cuts half-way through the lateral ethmoid. 

 It is not perpendicular to the plane of the external surface, however, 

 but slopes obliquely inward and backward in such manner that, when 



