THE ETHMOID 



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THE ETHMOID 



The ethmoid is a bone of delicate texture, situated at the anterior part of the 

 skull-base; it is roughly cuboidal in shape, and its delicacy is due to the fact that 

 it is honeycombed by air-cells. The bone consists of four })arts: the horizontal or 

 cril)riforni })late, two lateral masses, and a perpendicular i)late. 



The cribriform plate forms jjart of the anterior cerejjral fossa, and is received 

 into the etlimoidal notch of the frontal bone. Standing vertically upon this plate 

 is the crista galli. To the posterior border of the crista the falx cerebri is 

 attaclied; this border divides posteriorly to enclose the ethmoidal spine of the 

 sphenoid. The anterior aspect of the base of the crista constitutes the anterior 

 bt)rder of the cribriform plate; it is rough for articulation with the frontal. In the 

 suture between the two bones there is usually an opening, the foramen caecum, 

 wliich when present transmits a small vein. On each side of the crista galli the 

 cribriform plate lodges the olfactory bulb, and is perforated for the transmission of 

 the filaments of the olfactory nerves. On each side, near the anterior part of the 

 crista, there is a narroAv longitudinal slit for the nasal branch of the fifth nerve. 



Fio. 59.— Section through the Nasal Fossa to show the Mesethmoid. 



CREST OF SPHENOIl 



GROOVE FOR NASAL NERVE 



CREST OF PALATE BONE 

 SPINE OF PALATE BONE 



NASAL SPINE OF FRONTAL 



CREST OF MAXILLA 



The perpendicular plate (mesethmoid) is directly continuous with the crista 

 on the under aspect of the cribriform plate. It is a lamella of bone, trapezoid in 

 shape, which forms the upper part of the nasal septum; usually it is laterally 

 (lefiected. Its anterior border articulates with the nasal spine of the frontal, and 

 the crest of the nasal bones. The inferior border has the triangular cartilage 

 attached to it. The posterior border is subdivided: the upper half articulates with 

 the crest of the sphenoid, and the lower articulates with tlie vomer. The surfaces 

 of tliis plate present, especially in their upper parts, numerous foramina for vessels, 

 and grooves for filaments of the olfactory nerves. 



The lateral mass, or labyrinth, of the ethmoid consists t)f two scroll-like 

 pieces of bone, the superior and inferior turbinals (ethmo-turl)inals); a smooth, 

 (juadrilateral plate of bone, the os planum, and a number of air-cells. 



The OS planum is on the outer side of the lateral mass, and forms a large jior- 

 tion of the inner wall of the orbit. By the anterior l»order it articulates with the 

 lachrymal, by the posterior border with the sphenoid and the orltital ]»rocess of the 

 ])alate bone; the inferior border articulates with the inner margin of the orlntal 

 Xjlate of the maxilla, and by the sujx'rior l»order with the horizontal plate of the 

 frontal. Two notches in the superior border lead into grooves running liorizontally 

 across the lateral masses to the cribriform plate. These ethmoidal grooves are 

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