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The Eustachian tube connects the middle ear to the nasopharynx, enabling pressure equalization, mucosal clearance and protection from nasopharyngeal contents.

The Eustachian tube, or auditory tube, connects the middle ear with the nasopharynx. Its principal functions are ventilation and pressure equalization, clearance of middle-ear secretions, and protection against inappropriate entry of nasopharyngeal material.

Developmentally, its lumen and epithelial lining arise chiefly from the proximal part of the tubotympanic recess, an extension of the first pharyngeal pouch. The expanded distal part contributes to the tympanic cavity. The neighbouring first pharyngeal cleft is associated with the external ear passage and must not be substituted for the pouch as the origin of the auditory tube. Supporting cartilage and connective tissue develop from surrounding mesenchyme. At birth the tube is shorter and more nearly horizontal; craniofacial growth lengthens it and changes its orientation. The developmental account distinguishes formation of the channel from later growth of its surrounding structures.^[ref]

In an adult the auditory tube is roughly 3.5–4 cm long. From its tympanic opening in the anterior wall of the middle ear, it runs forwards, downwards and medially towards its pharyngeal opening in the lateral wall of the nasopharynx, behind the nasal cavity. Its lateral third is bony, lying in the temporal bone; the medial two thirds are chiefly cartilaginous with a membranous part. The narrow junction is the tubal isthmus. The cartilaginous portion is normally collapsed, so an open passage for sound or reflux does not continuously connect pharynx and middle ear.

The bony part is closely related medially to the canal of the internal carotid artery; the canal for the tensor tympani muscle lies above it. The medial cartilaginous part rests towards the skull base. Its pharyngeal end makes the tubal prominence, or torus tubarius, near the soft palate. The tensor veli palatini muscle acts on the cartilaginous and membranous tube and is the main active dilator. The levator veli palatini muscle passes beneath it and participates in the palatal and tubal functional arrangement. Swallowing and yawning intermittently open the tube.^[ref]

Opening permits gas transfer to compensate for a difference between middle-ear and ambient pressure. If the tube remains obstructed, absorption of gas from the cavity can make middle-ear pressure fall; movement of the tympanic membrane and sound transmission then change. Clearance is helped by the mucociliary lining, which carries material towards the nasopharynx. Closure between openings limits reflux and transmission of nasopharyngeal pressure disturbances. A tube that remains persistently open therefore also functions abnormally; more openness is not invariably better.

The physiological point can be demonstrated by observing the tympanic membrane while swallowing during a change of external pressure, or by recording the membrane's mechanical response over a controlled range of ear-canal pressures using tympanometry. These observations examine pressure communication and membrane mobility, not merely whether air can be forcibly driven through a passage. The relatively immature geometry and opening mechanism in young children helps explain their vulnerability to middle-ear fluid retention, but geometry alone is not an exhaustive account of infection risk. The anatomical structures were recognizable in 1866; the modern measurement method should not be mistaken for apparatus supplied by the examination.