The seven bones which articulate to form the orbit. (Ethmoid is brown, between the red and the purple marked bones)
|Anatomical terms of bone|
The ethmoid bone (//; from Greek ethmos, "sieve") is a bone in the skull that separates the nasal cavity from the brain. It is located at the roof of the nose, between the two orbits. The cubical bone is lightweight due to a spongy construction. The ethmoid bone is one of the bones that make up the orbit of the eye.
The ethmoid has three parts: cribriform plate, ethmoidal labyrinth, and perpendicular plate. Between the orbital plate and the nasal conchae are the ethmoidal sinuses or ethmoidal air cells, which are a variable number of small cavities in the lateral mass of the ethmoid.
The ethmoid articulates with fifteen bones:
- four of the neurocranium—the frontal, and the sphenoid (at the sphenoidal body and at the sphenoidal conchae).
- eleven of the viscerocranium—, two nasal bones, two maxillae, two lacrimals, two palatines, two inferior nasal conchae, and the vomer.
At birth, the bone consists of the two labyrinths, which are small and ill-developed. During the first year after birth, the perpendicular plate and crista galli begin to ossify from a single center, and are joined to the labyrinths about the beginning of the second year.
The cribriform plate is ossified partly from the perpendicular plate and partly from the labyrinths.
The development of the ethmoidal cells begins during fetal life.
Role in magnetoception
Some birds and other migratory animals have deposits of biological magnetite in their ethmoid bones which allow them to sense the direction of the Earth's magnetic field. Humans have a similar magnetite deposit, but it is believed to be vestigial.
Fracture of the lamina papyracea, the lateral plate of the ethmoid labyrinth bone, permits communication between the nasal cavity and the ipsilateral orbit through the inferomedial orbital wall, resulting in orbital emphysema. Increased pressure within the nasal cavity, as seen during sneezing, for example, leads to temporary exophthalmos.
The porous fragile nature of the ethmoid bone makes it particularly susceptible to fractures. The ethmoid is usually fractured from an upward force to the nose. This could occur by hitting the dashboard in a car crash or landing on the ground after a fall. The ethmoid fracture can produce bone fragments that penetrate the cribriform plate. This trauma can lead to a leak of cerebrospinal fluid into the nasal cavity. These openings let opportunistic bacteria in the nasal cavity enter the sterile environment of the central nervous system (CNS). The CNS is usually protected by the blood-brain barrier, but holes in the cribriform plate let bacteria get through the barrier. The blood-brain barrier makes it extremely difficult to treat such infections, because only certain drugs can cross into the CNS.
An ethmoid fracture can also sever the olfactory nerve. This injury results in anosmia (loss of smell). A reduction in the ability to taste is also a side effect because it is based so heavily on smell. This injury is not fatal.
|This section requires expansion. (December 2013)|
Lateral wall of nasal cavity, showing ethmoid bone in position.
- This article uses anatomical terminology; for an overview, see anatomical terminology.
- Saladin, Kenneth S. "Anatomy and Physiology: the Unity of Form and Function" McGraw Hill, 5th ed. New York: 2010
- Banks, Peter; Brown, Andrew E. (2000). Fractures of the facial skeleton. Oxford: Wright. ISBN 0723610347. Retrieved 31 March 2012.
|Wikimedia Commons has media related to Ethmoid bones.|
- Anatomy diagram: 34256.000-1 at Roche Lexicon - illustrated navigator, Elsevier