THE SKULL THE CKANTUM 



317 



border upward and forward beyond the middle. The lower triangular 

 part is a single scroll, the concave side of which shows on the external 

 surface as the continuation backward of the ethmoid cleft. This scroll 

 is called the middle turbinated bone ; the cleft above it on the medial 

 surface is known as the superior meatus of the nasal cavity, and the 

 concavity on the lateral surface forms with the adjacent maxillo- 

 turbinal, or inferior turbinated bone, the middle meatus. Hence it 

 corresponds to the smaller scrolls anterior to the ethmoidal cleft (4', 

 4", 4'") in the cat's ethmoid, and the superior meatus corresponds to 

 the depressed area at the posterior part of the medial surface over 

 scrolls 4', 4", and 5 (Fig. 219). 



The upper part of the medial surface, the so-called superior tur- 

 binated bone, in man is often deeply incised behind by a shorter 

 longitudinal fissure just in front of the spheno-turbinal. This corre- 

 sponds in the cat to the medial fissure anterior to the single large 

 inner scroll, 7. 



If the cleft plate be cut off close to the root, it will be observed 

 that it is attached to a flat plate which forms the part of the medial 

 surface above the anterior end of the middle 

 turbinated bone ; this plate may be recognized 

 as a modified mesial scroll articulating by its 

 thin free edge with the nasal bone. 



The lateral surface of the root of the cleft 

 plate bears ridges which are completed into 

 lachrymal cells by the overlying lachrymal 

 bone. 



At the anterior end of the superior surface 

 of the lateral ethmoid (Fig. 246) are three cells 

 which lead down into the cleft. The first ends 

 blindly above and corresponds to the cavity of L orbital P late f th e lateral 



ethmoid; 2, posterior imperfo- 

 SCroll 1 ; the SeCOlld and, SOmetimeS, the third rat e extremity of the cribriform 



plate ; 3, crista galli, on each side 

 Open above into the frontal sinUS, its Cavity the olfactory fossa with foramina 



of the cribriform plate, and, in 

 forming the infundibulum, which Corresponds advance, the alar processes; 4, 



anterior extremity of the mes- 

 With the Cavity Of SCroll 1". ethmoid ; 5, ethmoidal scrolls, or 



sinuses. 



By carefully cutting away the os planum 



and the roof of the lateral ethmoid behind the ethmoidal cleft, the 

 remaining partitions become visible, and the scrolls are seen to be 

 simple irregular cavities. The posterior scroll or pair of scrolls, the 



FIG. 246. 



ETHMOID BONE. UPPER 

 ASPECT. 



