THE SKULL THE CKANIUM 291 



pendicular plate of the ethmoid. This plate joins the frontals and 

 nasals above and the vomer below, and with its cartilaginous forward 

 extension divides the nasal cavity into two lateral chambers, each 

 almost filled by the lateral ethmoid. 



The lateral ethmoids and the mesethmoids are fastened together 

 above and behind by a thin transverse cribriform plate which sepa- 

 rates the nasal cavity from the cranial cavity and is pierced by 

 numerous small holes for the transmission of the olfactory nerves. 



Below and behind, the lateral ethmoids coalesce with the posterior 

 expanded portion of the vomer. A transverse vertical section through 

 the posterior part of the bone shows the relation of these parts. Such 

 a section may be represented by the Roman numeral III, in which the 

 upper transverse bar is the cribriform plate and the lower transverse 

 bar is the vomer with its expanded wings. The lateral vertical bars 

 represent the lateral ethmoids, and the middle vertical bar is the per- 

 pendicular plate. 



THE LATERAL ETHMOIDS. 



Each lateral ethmoid is a mass of bony scrolls which is flattened so 

 that its width is only half its height. Its widest point is in front and 

 below. Its height is a third less than its greatest length. It presents 

 large internal, or medial, and external, or lateral, surfaces ; a long, 

 narrow, rounded superior surface ; a triangular lower surface more or 

 less incised and wide in front and narrow behind ; a high and narrow 

 posterior surface ; and a triangular anterior surface, the apex of the 

 triangle being above. 



The external surface (Fig. 211) has the outline of a distorted 

 square, the upper angles lying in front of the lower angles. The lower 

 border, somewhat irregular owing to the downward projecting ends of 

 the bony scrolls, is quite straight and horizontal. Its length is about 

 four-fifths of the entire length of the surface. Its posterior end is 

 prolonged as the rounded end of the lower posterior scroll. The 

 posterior border is about as long as the lower border, and meets the 

 lower border at an angle of sixty-seven degrees, being directed forward 

 as well as upward. It is deeply emarginate, inasmuch as it is the 

 lateral border of the cribriform plate, which is concave from above 

 downward. It is jagged for articulation with a curved line on the 

 inner surface of the vertical plate of the frontal. At its upper end 

 is the more or less prominent and rounded posterior end of the upper 



