CHAP. III.] SKELETON OF THE HEAD AND TBUNK. 11 



tliey unite In a commissure) on what is termed the oUcary eminence. 

 This eminence bounds anteriorly the pituitary fossa, and at each 

 side of its hinder margin a minute process is to be detected. These 

 are the anterior clinoid processes (ac). On each side of the pre- 

 sphenoid there projects upwards and outwards a short triangular 

 plate of bone (os) called the orbital wing of the sphenoid, or orbifo- 

 sphenoid. This is pierced at its root by a considerable opening (7), 

 the optic foramen, which transmits the nerve of sight. The orbito- 

 sphenoids form the anterior lateral part of the floor of the cranium, 

 projecting upwards and outwards between the frontal in front, and 

 the ali-sphenoid behind. 



The optic foramen opens immediately in front of, and above, the 

 sphenoidal fissure and the round and oval foramina, all of which 

 open into the bottom and hinder part of the orbital and temporal 

 fossa) (see Fig. 46, ^, ^, ^, and ^). 



The sphenoid considered as one whole unites with the basi- occipital, 

 temporal, and parietal bones behind ; in front, with the ethmoid, 

 frontals, vomer, and palatines. 



§ 28. The ETHMOID, or sieve-like bone — formed of very delicate 

 lamellae very much contorted — is an exceedingly complex structure 

 which fills up the space {incisura ethmoidalis) left between the con- 

 stricted parts of the two frontals, and thence extends downwards and 

 forwards into the upper parts of the nasal chamber, which it almost 

 entirely occupies. It is thus placed between the cranium and the 

 face, extending forwards between the orbits and forming the hinder 

 wall of the nasal cavity. It consists of a median and two lateral 

 portions. The middle part is a simple vertical lamella of bone — the 

 meset/i moid (Fig. 49, ^6-) .which extends forwards from the middle of the 

 anterior surface of another transversely extended, obliquely ascending 

 plate called, from the number of its foramina, " cribriform.'" From 

 each side of the anterior surface of this cribriform plate (and on each 

 side, therefore, of the mesethmoid), a mass of delicate and excessively 

 plicated osseous tissue extends forward, bearing the name of the 

 lateral ethmoid or ethmo-turbinal (Figs. 49 and 50, et). The meseth- 

 moid has a free margin on every side except where it is anchylosed 

 to the cribriform plate behind. Its superior margin is adjacent to 

 the lower margin of the median deflected portion of the frontal, and 

 to the inferior margin of the nasal. Its inferior margin dips down 

 between the ascending bifurcating lamellse of the vomer. The 

 mesethmoid is much longer from before backwards than it is high. 



The ETHMo-TLRBiNAL riscs (Fig. 50, ef) on each side considerably 

 higher than the upper margin of the mesethmoid, and into the 

 chamber formed by the nasal and nasal process of the frontal on 

 the inner side, and the preorbital process of the frontal on its outer 

 side. It also descends shghtly below the inferior margin of the 

 mesethmoid, and joins the ascending, diverging, superior, and 

 posterior part of the vomer. 



The folds and grooves of the ethmo-turbinal or lateral ethmoid 

 proceed forwards and slightly downwards and outwards. 



