54 OSTEOLOGY. 



cavities separated by a thin bony plate, sometimes perforated, 

 which is continuous with the perpendicular plate of the ethmoid 

 bone ; these cavities are the sphenoidal cells, and help to form 

 the sphenoidal sinuses. The sphenoid bone articulates with 

 thirteen bones the occipital, ethmoid, and vomer, single bones ; 

 and the superior maxilla, frontal, parietal, squamosal, and ptery- 

 goid, pairs. 



ETHMOID. 

 (FiG. 13. 7.) 



The ethmoid, or sieve-like bone, is placed between the frontal 

 and sphenoid bones ; it separates the nasal fossae from the 

 cranium, and consists of a perpendicular plate and two lateral 

 portions. 



The perpendicular plate or lamella, continuous with the carti- 

 laginous septum of the nose, is covered, excepting in the cranium, 

 with pituitary membrane ; it divides the sphenoidal sinuses, is 

 continuous with the septum dividing the frontal sinuses, and it 

 joins the vomer below. Posteriorly, it forms the crista galli 

 process, which projects upwards in front of the cranial cavity, is 

 narrow inferiorly, where it joins the sphenoid, and broad sup- 

 eriorly, where it supports the frontal bone. 



The ethmoid cells constitute the lateral parts of the bone. 

 These are numerous and very fragile plates of bone rolled up 

 into conchae or volutes, which contain the ethmoid sinuses, and 

 are attached to transverse plates. The cribriform plates, which 

 are perforated for the passage of the olfactory nerves, separate the 

 cranial from the nasal cavities. The concave posterior parts 

 form the ethmoidal fossae, the most anterior recesses of the 

 cranium, which are, in the fresh state, occupied by the olfactory 

 bulbs. The ethmoid articulates with ten bones the sphenoid, 

 vomer, two frontals, two palatines, two superior turbinals, and two 

 superior maxillaries. 



FACIAL BONES. 



The facial skeleton consists of two parts, the superior and 

 inferior maxillary, the regions respectively of the upper and lower 

 jaw, each of which supports the passive organs of mastication, 

 the teeth. The upper portion, which is traversed by the nasal 

 cavities, is formed of nineteen distinct bones, one only of which, 



