314 MAMMALIAN ANATOMY 



sents the line of attachment to the anterior surface of the posterior 

 edge of the mesethmoid and to which is attached the falx cerebri of 

 the dura mater. This bar becomes an elevated ridge in the lower third 

 of the surface, and divides below to form the prominent and sharp 

 biarcuate lower border and then ascends on each side to form the lower 

 part of each lateral border. The lower border and these upward lateral 

 extensions are united with the anterior end of the upper surface of the 

 presphenoid. The lower scrolls of the lateral ethmoid, which project 

 upward behind the lower border, are received into the sinuses of the 

 presphenoid. The outer or lateral borders of this surface are nearer 

 the middle line below than they are above. They are irregular and 

 jagged, and form with the inner wall of the frontal, to which they are 

 attached, foramina of varying size and position. The posterior surface 

 of the cribriform plate is pierced by many small foramina, also seen, 

 of course, on the anterior surface. Their arrangement on each side of 

 the median line is, as a rule, symmetrical, although this symmetry may 

 vary in different specimens. In the lower third they are larger and 

 more numerous ; in the upper two-thirds the large foramina are near the 

 margins and the middle line, and leave an intermediate elongated area 

 either pierced by few foramina or entirely solid. This area is narrowed 

 at the middle by several large foramina near the external margin. 



The anterior surface of the cribriform plate has the form and 

 dimension of the posterior surface, but is convex from above down- 

 ward and convex from side to side above, and nearly flat below. To 

 the lower two-thirds of the middle line is attached the mesethmoid. 

 On each side of this attachment is a row of large foramina. The first 

 foramen is particularly large, and at this point a notch in the posterior 

 edge of the mesethmoid forms a foramen of communication between 

 the two nasal chambers. At the margins of this surface the posterior 

 ends of the scrolls of the lateral ethmoid are attached at inconspicuous 

 points which give no indications of the complicated convolution which 

 they assume further forward. 



Nomenclature. The name ethmoid, from the Greek for a sieve, 

 and eides, like, was given to this bone because of the many perforations 

 in the horizontal plate. The ancients believed it to be a sieve through 

 which humors from the brain were drained away. The French use 

 I'ethmo'ide, the Germans, das Siebbein. Among other names are os 

 ethmoidale, os ethmoideum, os cribrosum, and os spongiosum. 



