142 



OSTEOLOGY. 



Alar process 



Crista galli v 



nects the ethmoidal labyrinths with the perpendicular plate. It occupies the 

 interval between the orbital parts of the frontal bone, roofing in the nasal cavities 

 inferiorly, and superiorly forming, on each side of the crista galli, two shallow 



olfactory grooves, in which, in the recent con- 

 dition, the olfactory bulbs of the cerebrum 

 are lodged. Numerous foramina for the trans- 

 mission of the olfactory nerves pierce this 

 fflitfor Mao-ciliary part o f tne k one . tnose to tne me dial and 



lateral sides of the groove are the largest 

 and most regular in their arrangement. 

 Along the lateral edges of the cribriform plate 

 two notches can usually be distinguished; 

 when articulated with the frontal bone these 

 form the medial openings of the ethmoidal 

 foramina. Leading forwards from the anterior 

 of these there is often a groove which crosses 

 to the side of the crista galli, where it ends 

 in a slit which allows of the transmission of 

 the anterior ethmoidal nerve to the nose. 

 Posteriorly, the cribriform plate articulates 

 with the spine of the sphenoid. 



Connexions. The ethmoid articulates with the 

 sphenoid and sphenoidal conchae, the frontal, the 

 two nasals, two maxillae, two lacrimals, two inferior conchse, two palates, and the vomer. 



Ossification takes place in the cartilage of the nasal capsule. Each labyrinth has 

 one centre, which appears about the fourth or fifth month in the neighbourhood of the 

 lamina papyracea. According to Fawcett ossification first commences in a process which 

 passes outside the naso-lacrimal duct to reach the frontal process of the maxilla. From this 

 the laminae around the ethmoidal air-cells are formed, which are complete at birth, the 

 air-sinuses in this instance not being formed by the absorption of spongy bone. From 

 these centres the conchse are also developed, and these too are ossified at the ninth month. 



At birth the ossified labyrinths are united to the cartilaginous septum by a 

 fibrous layer. Two centres make their appearance in the septal cartilage on either side 

 of the root of the crista galli about the end of the first year ; from these, the crista galli 

 and the perpendicular plate are ossified, as well as the medial part of the cribriform plate, 

 the lateral portions of which are derived from a medial extension of the labyrinths. 



Ossification is usually complete about the fifth or sixth year. About the twenty-fifth 

 year bony union has taken place between the cribriform plate and the sphenoid, but 

 ankylosis between the perpendicular plate and the vomer is not usual till the fortieth or 

 forty-fifth year. 



Conchas Nasales Infcriores. 



The inferior conchae (O.T. inferior turbinated bones) are two shell-like laminae 

 of bone lying along the lower part of the lateral wall of the nasal cavity on either 



Lamina 

 papyracea 



Cribriform plate Ethmoidal labyrinth 



FIG. 151. THE ETHMOID SEEN FROM ABOVE. 



Lacrimal process 



Ethmoidal process 



Ethmoidal process 



Lacrimal process 



Maxillary process 

 B 



FIG. 152. THE EIGHT INFERIOR CONCHA. A, Medial Surface ; B, Lateral Surface. 



side. Of elongated form, the bone displays two curved borders enclosing a medial 

 and lateral surface. 



The superior or attached border is thin and sharp in front and behind, where 



