Structure and Development of Ethmoid Sinus
- Ethmoid air cells are thin-walled cavities in the ethmoidal labyrinth, representing invaginations of the nasal mucous membrane into the ethmoid bone.
- They are situated between the superior parts of the nasal cavities and the orbits, separated by thin bony lamellae.
- At birth, 3-4 air cells are present, increasing to 5-15 by adulthood.
- There are 5-15 air cells in each ethmoid bone in adults, with a combined volume of 2-3mL.

Drainage of Ethmoid Sinus
- Anterior ethmoidal cells drain into the middle nasal meatus through the ethmoidal infundibulum.
- Middle ethmoidal cells drain directly into the middle nasal meatus.
- Posterior ethmoidal cells drain directly into the superior nasal meatus at the sphenoethmoidal recess.
- Sometimes, one or more posterior ethmoidal cells open into the sphenoidal sinus.

Lamellae of Ethmoid Sinus
- The ethmoidal labyrinth is divided by multiple obliquely oriented, parallel lamellae.
- The first lamella is equivalent to the uncinate process of the ethmoid bone.
- The second lamella corresponds to the ethmoid bulla.
- The third lamella is the basal lamella, separating anterior and posterior ethmoid cells.
- The fourth lamella is equivalent to the superior nasal concha.

Innervation of Ethmoid Sinus
- Ethmoidal air cells receive sensory innervation from the anterior and posterior ethmoidal nerves, derived from the ophthalmic branch of the trigeminal nerve.
- Orbital branches of the pterygopalatine ganglion carry postganglionic parasympathetic nerve fibers for mucous secretion from the facial nerve.

Additional Information
- Ethmoidal cells and maxillary sinuses are present at birth.
- Sometimes, one or more posterior ethmoidal cells open into the sphenoidal sinus.