ETHMOID BONE. 39 



process, of the palate bone. The anterior surface of the pterygoid 

 process is broad near its base, and supports Meckel's ganglion. The 

 base of the process is pierced by the Vidian canal. 



Development. By twelve centres ; four for the body, viz. two for 

 its anterior (spheno-orbital), and two for its posterior part (spheno- 

 temporal) ; four for the four wings ; two for the internal pterygoid 

 plates, and two for the sphenoidal spongy bones. Ossification com- 

 mences in the various pieces of the sphenoid in the following order : 

 greater alae, at about the same time with the other bones of the 

 cranium ; lesser alae, posterior body, at the end of the second month ; 

 anterior body at the end of the third ; internal pterygoid plate, spongy 

 bones, between the period of birth and the second year. Osseous 

 union occurs first between the centres for the posterior body, and at 

 about the same time between each centre of the anterior body and its 

 corresponding (lesser) ala ; the third union takes place between the 

 internal pterygoid plate and the greater ala ; the fourth between the 

 two centres of the anterior body, and at the same time between the 

 anterior and posterior body. This is the state of union at birth, the 

 bone consisting of five centres, one being the body and lesser alas ; 

 one on each side, the great ala and internal pterygoid plate ; and the 

 remaining two the sphenoidal spongy bones. The greater alae unite 

 with the body during the first year ; the spongy bones after puberty ; 

 and the body of the sphenoid with the basilar process of the occipital 

 between eighteen and twenty-five. 



Articulations. With twelve bones ; all the bones of the head and 

 five of the face, viz. the two malar, two palate, and the vomer. 



Attachment of Muscles. To twelve pairs ; temporal, external ptery- 

 goid, internal pterygoid, superior constrictor, tensor palati, laxator 

 tympani, levator palpebrse, obliquus superior, superior rectus, internal 

 rectus, inferior rectus, and external rectus. 



ETHMOID BONE. The ethmoid (jj^oj, a sieve) is a square-shaped 

 cellular bone, situated between the two orbits, at the root of the nose, 

 and perforated upon its upper surface by a number of small openings, 

 frdm which peculiarity it has received its name. It consists of a 

 perpendicular lamella and two lateral masses. 



The perpendicular lamella is a thin central plate, which articulates 

 with the vomer and cartilage of the septum, and assists in forming 

 the septum of the nose. It is surmounted superiorly by a thick and 

 strong process, the crista galli, which projects into the cavity of 

 the skull, and gives attachment to the falx cerebri. From the base of 

 the anterior border of this process there project forwards two small 

 plates, alar processes, which are received into corresponding depres- 

 sions in the frontal bone, and often complete posteriorly the foramen 

 coecum. On each side of the crista galli, upon the upper surface of 

 the bone, is a thin and grooved plate perforated by a number of small 

 openings, the cribriform lamella, which supports the bulb of the 

 olfactory nerve, and gives passage to its filaments, and to the nasal 



