ETHMOID BOXE. 



pterygoid plates is the pterygoid fossa ; and the two plates are separated 

 inferiorly by an angular notch (palatine), which receives the tuberosity, or 

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

 goid 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 commences in the various 

 pieces of the sphenoid in the following order : greater ala?, at about the 

 same time with the other bones of the cranium ; lesser ala3 and posterior 

 body, at the end of the second month ; anterior body at the end of the 

 third ; internal pterygoid plate and 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 be- 

 tween 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 alee ; 

 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 

 oody of the sphenoid with the basilar process of the occipital bone be- 

 tween eighteen and twenty-five. 



Articulations. With twelve bones ; that is, with 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 tym- 

 pani, levator palpebrse, obliquus superior, superior rectus, internal rectus, 

 inferior rectus, and external rectus. 



ETHMOID BONE. The ethmoid (rfyuV, a sieve) is a square-shaped cel- 

 lular bone, situated between the two orbits, at the root of the nose, and 

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

 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 forward two small plates, alar processes, which 

 are received into corresponding depressions in the frontal bone, and often 

 complete posteriorly the foramen caecum. 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 ^vhich supports thr 

 bulb of the olfactory nerve, and gives passage to its filaments, and to the 

 nasal branch of the ophthalmia nerve. In the middle of the groove of 

 this lamella the foramina pierce the bone completely, but at either side 

 they are the apertures of canals, which run for some distance in the 

 substance of the central lamella, inner wall of the lateral mass, and spongy 



