316 



MAMMALIAN ANATOMY 



cribriform plate is almost horizontal, and does not slope from behind 

 upward and forward as it does in the cat. There is reason to believe 

 that this change of position is the result of the pressure of the over- 

 lying anterior lobes of the cerebrum. As a result also of this pressure 

 some of the scrolls which form the lateral ethmoid are so bowed 

 laterally from the middle line that their roofs show on each side of the 

 cribriform plate, where they articulate with the depression around the 

 ethmoidal notch of the frontal bone. It is in the lateral ethmoid 

 itself, however, that the effect of this pressure is most apparent. If 

 we compare the lateral ethmoid of man with that of the cat, we cannot 

 but recognize that the horizontal position of the cribriform plate has 

 been secured at the expense of all the upper long scrolls, by practi- 

 cally cutting off that part which in the cat lies dorsal to a line drawn 

 from the posterior end of the cribriform plate to the lower ejid of 

 scrolls 1 and V. Bearing this in mind, if we examine the human 

 ethmo-turbinal its structure can be readily understood. In the first 

 place, we notice that the external surface (Fig. 244) is greatly modi- 



FIG. 24i. 



FIG. 245. 



73 43 



ETHMOID BONE. LEFT SIDE. 



1, orbital plate; 2, anterior ethmoidal sinuses; 

 3, middle turbinated bone; 4, uncinate process; 

 5, crista galli ; 6, alar process ; 7, mesethnaoid ; 

 8, 9, position of the ethmoidal foramina. 



RIGHT LATERAL ETHMOID. INNER 

 ASPECT. 



1, cribriform plate; 2, superior turbinated 

 bone; 3, middle turbinated bone: 4, supe- 

 rior meatus ; 5, a shorter meatus occasionally 

 present; 6, part of the foramen 



fied by the overgrowth of the os planum, or orbital plate, which con- 

 ceals the outer surface of many of the scrolls. In front of it are seen 

 the ethmoidal cleft and the cleft plate terminating below in a promi- 

 nent uncinate 1 process, which joins the maxillo-turbinal or inferior 

 turbinated bone. 



The medial surface of the human lateral ethmoid (Fig. 245) is 

 divided into two parts by a deep fissure extending from the posterior 



1 From uncus, a hook. 



