The ethmoid sinus is each of the two paranasal sinus

Paranasal sinus

Paranasal sinuses are a group of four paired air-filled spaces that surround the nasal cavity , above and between the eyes , and behind the ethmoids...

es within the ethmoid bone

Ethmoid bone

The ethmoid bone is a bone in the skull that separates the nasal cavity from the brain. As such, 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 makes up the orbit of the eye...

, comprising the ethmoidal air cells and filled with air. These are divided into anterior, middle and posterior group. The ethmoidal air cells consist of numerous thin-walled cavities situated in the ethmoidal labyrinth and completed by the frontal

Frontal bone

The frontal bone is a bone in the human skull that resembles a cockleshell in form, and consists of two portions:* a vertical portion, the squama frontalis, corresponding with the region of the forehead....

, maxilla

Maxilla

The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...

, lacrimal

Lacrimal

The term Lacrimal can refer to:*In typography:** A type of Stroke ending *In anatomy** Lacrimal apparatus** Lacrimal artery** Lacrimal bone...

, sphenoidal, and palatine bone

Palatine bone

The palatine bone is a bone in many species of the animal kingdom, commonly termed the palatum .-Human anatomy:...

s. They lie between the upper parts of the nasal cavities and the orbits

Orbit (anatomy)

In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents...

, and are separated from these cavities by thin bony laminae.

Three groups

The groups of the ethmoidal air cells:

• The posterior group (sometimes the posterior ethmoid sinus) drains into the superior meatus
  Superior meatus
  The superior meatus, the smallest of the three meatuses of the nose, occupies the middle third of the lateral wall of the nasal cavity.It lies between the superior nasal conchæ and middle nasal conchæ; the sphenopalatine foramen opens into it behind, and the posterior ethmoidal cells in front.The...
  
   above the middle nasal concha
  Middle nasal concha
  The medial surface of the labyrinth of ethmoid consists of a thin lamella, which descends from the under surface of the cribriform plate, and ends below in a free, convoluted margin, the middle nasal concha....
  
  ; sometimes one or more opens into the sphenoidal sinus.

• The middle group (sometimes the middle ethmoid sinus) drains into the middle meatus
  Middle meatus
  The middle meatus is a nasal opening or canal situated between the middle and inferior conchæ, and extends from the anterior to the posterior end of the latter....
  
   of the nose
  Human nose
  The visible part of the human nose is the protruding part of the face that bears the nostrils. The shape of the nose is determined by the ethmoid bone and the nasal septum, which consists mostly of cartilage and which separates the nostrils...
  
   on or above the bulla ethmoidalis.

• The anterior group (sometimes the anterior ethmoid sinus) drains into the middle meatus
  Middle meatus
  The middle meatus is a nasal opening or canal situated between the middle and inferior conchæ, and extends from the anterior to the posterior end of the latter....
  
   of the nose
  Human nose
  The visible part of the human nose is the protruding part of the face that bears the nostrils. The shape of the nose is determined by the ethmoid bone and the nasal septum, which consists mostly of cartilage and which separates the nostrils...
  
   by way of the infundibulum
  Infundibulum
  An infundibulum is a funnel-shaped cavity or organ.* Lungs: The alveolar sacs of the lungs from which the air chambers open are called infundibula...
  
  .

Innervation

The ethmoidal air cells receive sensory fibers from the anterior and posterior ethmoidal nerves

Ethmoidal nerves

The ethmoidal nerves, which arise from the nasociliary nerve, supply the ethmoidal cells; the posterior branch leaves the orbital cavity through the posterior ethmoidal foramen and gives some filaments to the sphenoidal sinus. There are two ethmoidal nerves on each side of the face:* posterior...

, and the orbital branches of the pterygopalatine ganglion

Pterygopalatine ganglion

The pterygopalatine ganglion is a parasympathetic ganglion found in the pterygopalatine fossa. It is one of four parasympathetic ganglia of the head and neck....

, which carry the postganglionic parasympathetic nerve fibers for mucous

Mucus

In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...

 secretion

Secretion

Secretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...

 from the facial nerve

Facial nerve

The facial nerve is the seventh of twelve paired cranial nerves. It emerges from the brainstem between the pons and the medulla, and controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity...

.