THE ETHMOID BONE. 139 



sphenoids (small wings) and their roots are developed. About the same time another 

 pair of centres, placed medial to the optic foramina, constitute the body of the pre- 

 sphenoid. At first the superior surface of the body of the pre-sphenoid is exposed in the 

 interval between the orbito-sphenoids, but by the ultimate coalescence of the medial borders 

 of the orbito-sphenoids to form the jugum sphenoidale the body of the pre-sphenoid is 

 almost completely covered over superiorly. By the coalescence of these in front, and 

 their ultimate union with the basi-sphenoid behind, a cartilaginous interval is enclosed, 

 of triangular shape, which, however, becomes gradually reduced in size by the ingrowth 

 of its margins so as to form two medially placed foramina, as may be frequently 

 observed in young bones one opening on the surface of the tuberculum sellee, the other 

 being placed anteriorly. (Lawrence, " Proc. Soc. Anat.," Journ. Anat. and Physiol. vol. 

 xxviii. p. 19.) 



As has been seen, the medial pterygoid laminae are developed in membrane and are the 

 first parts of the sphenoid to ossify. (Fawcett, Anat. Anz., vol. xxvi. 1905, p. 280.) Each is 

 derived from a single nucleus which appears about the ninth or tenth week, and fuses with 

 the inferior surface of the great wing, there forming a groove which is converted into the 

 pharyngeal canal when the alisphenoid and medial pterygoid laminae fuse later with the 

 body of the post-sphenoid. The hamulus, however, chondrifies before it ossifies during 

 the third month. Fawcett also regards the lateral pterygoid plate as of membranous 

 origin. 



At birth the sphenoid consists of three parts : one comprising the orbito-sphenoids 

 together with the body of the pre-sphenoid and the basi-sphenoid, the others consisting of 

 the alisphenoids, one on each side. Fusion of the latter with the former occurs near the 

 end of the first year. The dorsum sellse at birth consists of a cartilaginous plate which 

 separates the body of the post-sphenoid from the basi-occipital. This slowly ossifies, but 

 the cartilage does not entirely disappear till the age of twenty-five, by which time bony 

 ankylosis of the basi-cranial axis is complete. For a considerable time the under surface 

 of the body of the pre-sphenoid displays a bullate appearance, with the 'sides of which 

 the sphenoidal conchae articulate. It is only after the seventh or eighth year is reached 

 that the spongy tissue within this part of the bone becomes absorbed to form the 

 sphenoidal sinuses, 



The sphenoidal conchae, or bones of Bertin, best studied in childhood, are formed 

 by the fusion of four distinct ossicles (Cleland), the centres for which appear in the later 

 months of utero-gestation. Each bone consists of a hollow, three -sided pyramid, the 

 apex of which is in contact with the anterior part of the vaginal process of the medial 

 pterygoid lamina, whilst the base fits on to the posterior surface of the labyrinth of the 

 ethmoid. The inferior surface of each forms the roof of the corresponding nasal cavity, 

 and completes the formation of the spheno-palatine foramen, whilst the lateral aspect is 

 united with the palate borte and forms the medial wall of the pterygo-palatine fossa, and 

 occasionally constitutes a part of the orbital wall posterior to the lamina papyracea of 

 the ethmoid. The superior surface of each sphenoidal concha is applied to the anterior 

 and inferior surface of the body of the pre-sphenoid on the corresponding side of the 

 rostrum. It is by the absorption of this surface that the contained sphenoidal sinus is 

 ultimately extended. In the bases of the pyramids are formed the apertures through which 

 the sinuses open in to the nasal cavity in the adult. Up to the age of five these ossicles 

 remain independent, but subsequently, owing to their firm ankylosis with the surrounding 

 bones, they are merely represented in the adult disarticulated skull by the irregular frag- 

 ments adherent to the separated borders of the ethmoid, palate, and sphenoid bones. 



Os Ethmoidale. 



The ethmoid bone lies in front of the sphenoid, and occupies the interval 

 between the orbital parts of the frontal, thus entering into the formation of the 

 anterior cranial fossa as well as the medial walls of the orbits and the roof and medial 

 and lateral walls of the nasal cavities. The bone, which is extremely light, consists 

 of two cellular parts the labyrinth! ethmoidales (ethmoidal labyrinths), which are 

 united superiorly to a median lamina perpendicularis (perpendicular plate) by a thin 

 horizontal lamina which, from its perforated condition, is called the lamina cribrosa 

 (cribriform plate). The general arrangement of the parts of the bone resembles the 

 capital letter T ; the median plate corresponds to the vertical limb, the cribriform 

 plate to the horizontal limb of the T, whilst the ethmoidal labyrinths may be 



