THE ETHMOID BONE 55 
cranial and nasal cavities. Its margin joins the orbital wings of the sphenoid 
laterally, the body of the sphenoid ventrally, and the cranial plate of the frontal 
bones dorsally. Its cerebral surface is divided into two parts by a median ridge, 
the crista galli, which may be regarded as the intracranial projection of the perpen- 
dicular plate. Each half forms a deep oval cavity, the ethmoidal fossa, which 
lodges the olfactory bulb. The plate is perforated by numerous small foramina for 
the passage of the olfactory nerve filaments, and on either side is the much larger 
ethmoidal foramen. The nasal surface is convex, and has the lateral masses 
attached to it. 
Each lateral mass or labyrinth (Labyrinthus ethmoidalis) is conical in shape and 
its base is attached to the cribriform plate. It projects forward into the posterior 
Supraorbital Frontal Frontal 
bone sinus 
process 
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Or 
Vomer 
Fic. 32—Cross-secTion oF CRANIUM OF Horsp. THE Section Passes THrouGH THE MiIppLE OF THE ORBITS AND 
Is VIEWED FROM IN FRONT. 
1, Lateral mass of ethmoid bone; 2, perpendicular plate of same; 3, common nasal meatus; 4, sphenopalatine 
Sinus; 5, perpendicular part of palatine bone; 6, zygomatic process of temporal bone; 7, zygomatic arch; 8, orbital 
part of frontal bone. 
part of the nasal cavity. The medial surface is separated by a narrow space from the 
perpendicular plate. The lateral surface is convex and faces chiefly into the frontal 
and maxillary sinuses, but is attached behind to the inner wall of the orbital cavity; 
it is covered by a very thin layer of bone, the lamina lateralis. The mass consists 
of a large number of delicate, scroll-like plates of bone, termed ethmo-turbinates.! 
These are attached to the lamina lateralis, and are separated by narrow intervals 
termed ethmoidal meatuses, which communicate with the nasal cavity. In the 
fresh state the ethmo-turbinates are covered with mucous membrane. 
The lateral mass is a very complex structure, the arrangement of which may be understood 
by examination of cross-sections. Each mass consists of six turbinates which extend almost to the 
f ‘The term “ethmoid cell,” borrowed from human anatomy, should not be retained, since 
it connotes an entirely erroneous conception of these structures in macrosmatic animals, such as 
those under consideration. 
