The ethmoid is an unpaired bone that is located in the roof of the nasal cavity and so seperates it from the cranial cavity. It also acts as part of the medial border of the orbit.
The ethmoid bone helps make up a small portion of the anterior cranial fossa.
Specifically, it's the cribriform plate of the ethmoid bone that is helping form the floor of the anterior cranial fossa. If you removed the cribriform plate the top of the nasal cavity and a small section of the anterior cranial fossa would be joined.
Its also what seperates the nasal cavity and the eye, through it's lateral mass, and it helps form the septum of the nose.
The ethmoid Bone in isolation
When viewed from the front, as if you are looking at a persons face (and as if magically looking through a transparent nose and midfiace), the ethmoid bone in its simplest conceptualisation is of 2 lateral masses connected by a cross of bone...
This very simple overview is useful in that it immediately conveys several important structures of the ethmoid bone.
The Crista Galli, latin for the rooster's comb which is an adept description, projects upwards from the cribriform plate in the anterior cranial fossa. It is the anterior attachment for the falx cerebri.
The cribriform plate, latin for sieve, is the horizontal section of bone in the anterior cranial fossa (connecting the two lateral masses) that has many small foramina for the olfactory nerve fibres.
The Perpendicular plate, appears like an inferior extension of the crista galli, it extends downwards until another bone, the vomer, rises up to meet it.
The lateral Mass
The lateral mass - which are the two symmetrical bony complexes on either side of the perpendicular plate in the image above - is composed mainly of air cells - but it has two projections from its medial side - the superior and middle concha.
Its lateral wall, the orbital plate of the lateral mass, forms a medial boundary of the orbit.
Full credit to http://allinnursing.blogspot.com.au/2012/07/nasal-cavity.html
As you can see in the image from the all-in-nursing blog the ethmoidal air cells of the lateral mass are actually open superiorly and inferiorly. It is the frontal bone that provides a roof to the superior ethmoidal air cells of the lateral mass, termed, the orbital plate of the frontal bone. It's a portion of the maxilla that rises up to close the floor of the lateral masses ethmoidal air cells.
The Ethmoid in relation to other bones
The ethmoid forms part of the neurocranium (bones surrounding the brain) and the vescicranium (face)
In the neurocranium, it's related to the frontal bone (anterior) and the sphenoid bone (posterior)
under a creative commons license from openstax - http://cnx.org/contents/1w-m01MB@4/The-Skull
In the vescicranium, it's related to...
How's this for a mnemonic.
lacrimal bones. (anterior to lamina papyracea)
the palatine bone
and the inferior concha.
Under a creative commons license from Open Stax - http://cnx.org/contents/1w-m01MB@4/The-Skull
The Ethmoid Bone and it's role in the osteomeatal complex
To understand the osteomeatal complex, it's useful to understand a little on the embryology of the ethmoid bone.
As the lateral mass develops, structures start to project from the laterally wall, medially, into the nasal space.
The attachments of these medial structures to their origin at the lateral wall, are the basel lamella, of which there are five (pictured above). You can see that each one assumes a curvilinear course from high anteriorly, to swoop downwards posteriorly.
The 1st lamella is the structure for the uncinate process of the ethmoid.
The 2nd lamella is the support strut for the bullosa ethmoidalis
The 3rd lamella is the attachment of the middle terbinate
The 4th lamella is the attachment of the superior terbinate
The 5th is the attachmentn of the supreme terbinate (doesn't fully develop in most people).
These structures explain the anatomical orientation in adults.