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 

 process 



Frontal 

 bone 



Vomer 



Fig. 32. — Cross-section of Cranium of Horse. The Section Passes Through the Middle 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. splienopalatine 

 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 



' 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. 



