THE ETHMOID BONE. 



193 



Median plate 



Infundibulum 



Groove for 

 nasal nerve 



Cribriform 

 plate 



Crista ofalli 



Anterior 

 ethmoidal 

 cells 



Orbital 

 plate 



The ethmoid bone from above. 



times they swell out into the cavities of other bones, notably into the frontal sinus. 

 The internal surface of the lateral mass, forming the outer wall of the nasal 

 cavity, cannot be seen on the entire bone. It is best studied on_ the bisected 

 skull ;' but to study the whole bone, further cutting is necessary, since this sur- 

 face is made of a series of con- 

 voluted plates, some of which Fig. 217. 

 conceal others. At least two of 

 these — the superior and the viid- 

 dle turbinate bones ' — are evident. 

 They are curled with their con- 

 vexities towards the median 

 plane, so as to overhang t\yo 

 antero-posterior passages, the 

 superior and the middle meatus 

 of the nasal fossa. According 

 to Zuckerkandl, there are three 

 ethmoidal turbinate bones in 

 more than eighty per cent. , and 

 sometimes four. When only 

 two are seen, it is owing either 

 to the absence of the ^second 

 or to its slight development, so 

 that it is hidden by the upper. 

 It is certain that only two are 

 evident in most cases, and we 

 shall follow the usual method of 

 so describing the bone.'' The 

 inferior ethmoidal (middle) tjir- 



binate is much the larger, very prominent, and joins the ascending process of the 

 superior maxilla at the crista ethmoidalis or superior ttirbinate crest. Its general 

 course is backward and downward, to end in a point at the posterior border of the 



bone. The free edge is so much 

 curled under as to be hidden. 

 The superior turbinate is much 

 smaller, occupying the postero- 

 superior angle. It appears to 

 separate from the turbinate below 

 it at about the middle of this sur- 

 face. The superior ?neatus, which 

 it overhangs, is therefore small. 

 As above implied, an additional 

 ethmoidal turbinate may appear 

 from beneath it, and still another 

 small one may very exceptionally 

 be found above it at the extreme 

 upper posterior angle. At the 

 point at which the middle turbi- 

 nate bone joins the nasal process 

 of the maxilla there is often a 

 slight elevation, the agger 7iasi, 

 which is supposed to be the an- 

 terior end of another turbinate which passes under the preceding. When the mid- 

 dle turbinate is removed, a curved projecting plate, the uncinate process,^ is seen 

 on the lateral mass, curving downward and backward. It. is some two or three 

 millimetres broad and, extending beneath the rest of the bone, joins the inferior 

 turbinate. The uncinate process, together with the agger, is held to represent the 



^ There are practically three turbinate bones, the upper two of which are parts of the 

 ethmoid and the lowest a separate bone. These are called superior, middle, and inferior , 

 hence we speak of the inferior ethmoidal turbinate as the middle one. 



' Concha nasalis superior et media. ^ Processus uncinatus. 



13 



Fig. 218. 



Superior surface 



Alar process 

 of crista galli 



Posterior ethmoidal 

 ce'.ls 



Posterior ethmoidal 

 cells 



Middle turbinate 



Sup. meatus 



Median plate 



The ethmoid bone from behind, showing median plate and lateral 



masses. 



