The ethmoid (french: ethmoide) 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 parts: the horizontal or cribriform plate, two lateral masses, and a perpendicular plate.
The cribriform plate forms part of the anterior cerebral fossa, and is received into the ethmoidal 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 attached; this border divides posteriorly to enclose the ethmoidal spine of the sphenoid. The anterior aspect of the base of the crista constitutes the anterior border of the cribriform plate; it is rough for articulation with the frontal. In the suture between the two bones there is an opening, the foramen caecum, which 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 narrow longitudinal slit for the nasal branch of the fifth nerve.
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 deflected. 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 the vomer. The surfaces of this 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 of two scroll-like pieces of bone, the superior and inferior turbinals (ethmo-turbinals) ; a smooth, quadrilateral 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 portion of the inner wall of the orbit. By the anterior border it articulates with the lachrymal, by the posterior border with the sphenoid and the orbital process of the palate bone; the inferior border articulates with the inner margin of the orbital plate of the maxilla, and by the superior border with the horizontal plate of the frontal. Two notches in the superior border lead into grooves running horizontally across the lateral masses to the cribriform plate. These ethmoidal grooves are converted into canals by the frontal bone. The anterior canal transmits the anterior ethmoidal artery and nasal nerve; the posterior is for the posterior ethmoidal artery.
The turbinals project on the inner aspect of the lateral mass; they coalesce anteriorly, but are separated posteriorly by a space, termed the superior meatus. Each turbinal has an attached upper, and a free, slightly convoluted, lower border. In the recent state they are covered with mucous membrane, and present numerous foramina for blood-vessels, and grooves for twigs of the olfactory nerves.
On the under surface of each lateral mass, near the anterior corner of the os planum, an irregular lamina of bone projects downwards and backwards. This is the unciform process: it articulates with the ethmoidal process of the inferior turbinal, and forms a small part of the inner wall of the antrum.
The ethmoidal cells occupy the space between the os planum and the turbinals; they are divided by a thin septum into an anterior and posterior set. The cells are imperfect in the ethmoid, they require the juxtaposition of other bones to make them complete. Above, they are closed by the horizontal plate of the frontal, posteriorly by the sphenoidal turbinal and the orbital process of the palate, inferiorly by the maxilla, and anteriorly by the lachrymal. The anterior set communicate with the frontal cells above, whilst below they open into the middle meatus of the nose by a sinuous canal, the infundibulum. The posterior cells open into the superior meatus, and occasionally communicate with the sphenoidal cells.
The cells are sometimes divided into groups, according to the bone which lies in immediate juxtaposition. Those along the superior edge are the fronto-ethmoidal; those beneath the lachrymal, lachrymo-ethmoidal, usually two in number. Those along the lower edge are the maxillo-ethmoidal ; and posteriorly there are the spheno-ethmoidal, completed by the sphenoidal-turbinals, and a palato-ethmoidal cell.
The ethmoid articulates with the frontal, the sphenoid, two palate bones, two nasals, vomer, two inferior turbinals, the sphenoidal turbinals, two maxillae, and two lachrymal bones. The hinder surface of each lateral mass comes into relation with the sphenoid on each side of the crest and rostrum, and helps to close in the sphenoidal sinus.
The anterior and posterior ethmoidal, and from the nasal or spheno-palatine branch of the internal maxillary.
The ethmoid has three centers of ossification. Of these, a nucleus appears in the fourth month of intra-uterine life in each lateral cartilage. At birth this bone is represented by two scroll-like bones, very delicate, and covered with irregular depressions, which give it a worm-eaten appearance. Six months afterbirth a nucleus appears in the ethmo-vomerine plate for- the mesethmoid. This gradually extends into the crista galli. During the third year the lateral masses and the mesethmoid (perpendicular plate) ankylose. The cribriform plate is derived from the lateral masses.
The ethmoidal cells do not make their appearance before the third year, and they gradually produce attenuation of the lateral masses. In many places there is so much absorption of bone that the cells perforate the ethmoid in situations where it is overlapped by other bones. Along the lower border of the bone, near its articulation with the maxilla, the absorption leads to the partial detachment of a narrow strip known as the uncinate or unciform process. Sometimes a second but smaller hook-like process is formed, above and anterior to the large one. This process is so very fragile that it is difficult to preserve it in disarticulated bones. The relations of the uncinate process are best studied by removing the outer wall of the antrum.