THE ETHxMOID BONE. 



191 



Posterior 

 orbital canal 



Anterior 

 orbital canal 



Crista galli 



sists of the basisphenoid, the presphenold, and the lesser wings in one piece, and a 

 lateral one on each side, — namely, the greater wing and the pterygoids. The dorsum 

 sellae has a separate epiphysis which appears after birth. In the first year the lesser 

 wings spread across the top of the presphenoid, joining the jugum sphenoidale, so 

 that it does not show in the anterior fossa. The external pterygoid plate is an out- 

 growth of the great wing. 



The cor mux sphejioidalia, bones of Berlin, or sphenoidal turbinate bones, are two 

 thin plates which appear before birth at the front of the presphenoid. They cover 

 both the front and its inferior surface at the sides of the rostrum. At five years they 

 are still free, but have approacned their permanent shape of hollow cones. The 

 hollowing out of the body of the sphenoid now begins, and at the same time the 

 upper part of these bones is absorbed, so that the foramina become notches. These 

 bones are ultimately joined to the sphenoid, the ethmoid, and the palate. Though 

 usually reckoned as parts of the sphenoid, there is reason to believe that they 

 are generally fused earlier with the ethmoid. The basisphenoid begins to cobssify 

 with the occipital at about the fifteenth year. The process is first completed above. 



THE ETHMOID BONE. 



The ethmoid ^ consists of a median plate forming a part of the nasal septum, of 

 the cribiform plate joining it at the top on either side and forming the roof of the 

 nasal cavity, and of two lateral masses attached to the lateral border of each cribriform 

 plate, and touching the vertical plate very slightly just below its junction with the 

 front of the cribriform plate. These lateral masses are roughly cubical, interposed 

 between the cavities of the nose 



and of the orbit. They consist of Fig. 215. 



a series of delicate plates forming 

 the walls of air-spaces or cells, 

 which are mostly completed by 

 neighboring bones! 



The vertical or median 

 plate " projects near the front into 

 the cranial cavity as the crista 

 galli, thicker in front than behind, 

 with an oblique upper border run- 

 ning sinuously downward and 

 backward. Its greatest elevation 

 is about one centimetre. The 

 front part is occasionally hollow, 

 forming a part of the frontal sinus. 

 It gives attachment to the falx 

 cerebri, a fold of dura separating 

 the hemispheres of the brain. A 

 little plate, ala^^ facing downward 

 and forward, arises from the front on either side, articulating with the frontal. Just 

 before the crista galli is a pin-hole, the forameji ccsciim, usually formed by both 

 ethmoid and frontal, but which may be in either. It is said to transmit a vein in 

 early life, but is closed later. The part of the vertical plate below the horizontal one 

 is five-sided. The upper border runs along the base of the skull ; one in front of 

 it slants downward and forward under the nasal spine of the frontal, sometimes 

 reaching the nasal bones ;. another descends nearly vertically along the crest of the 

 sphenoid. Of the two inferior borders, the posterior runs downward and forward 

 along the greater part of the vomer, while the anterior, running downward and 

 backward to meet it, is free in the skeleton, but in life is attached to the triangular 

 cartilage which forms a large part of the septum. The sides, covered with mucous 

 membrane, are smooth except at the upper part, where there are vertical grooves 

 for the olfactory nerves. This plate usually slants to one side. 



The horizontal or cribriform plate' forms the floor of a narrow groove on 

 either side of the crista galli and, farther back, in the middle of the anterior fossa of 



^ Os ethmoidale. - Lamina perpendicularis. ^ Processus alaris. * Lamina cribrosa. 



Mid. turbinate 



Uncinate process 

 The ethmoid bone, outer aspect from the right side. 



