The auditory tube (also known as the pharyngotympanic tube, Eustachian tube, Latin: tuba auditiva) is a tunnel that connects the tympanic cavity to the nasopharynx and equalizes pressure on both sides of the tympanic membrane.
The auditory tube is 3,5-4 cm long and around 2 mm in diameter. The opening of the auditory tube in the middle ear is located in its anterior wall. From here, the auditory tube extends forward, medially, and downward to enter the nasopharynx, opening in the lateral wall of the nasopharynx just posterior to the inferior meatus of the nasal cavity.
By joining the two structures, the auditory tube acts as a pathway for upper respiratory infection to spread into the middle ear. The tube is shorter and straighter in children than in adults, so middle ear infections are more common in children.
The main functions of the auditory tube are as following:
The auditory tube consists of two parts:
The bony part starts in the anterior wall of the tympanic cavity and is gradually getting narrower. The opening of the bony part of the auditory tube is clearly visible on the inferior surface of the skull at the junction between the squamous and petrous parts of the temporal bone, immediately posterior to the foramen ovale and foramen spinosum.
The cartilaginous part of the auditory tube is formed of a triangular plate of elastic fibrocartilage. Its apex is attached to the superior margin of the medial end of the bony part, but its base lies directly under the mucous membrane of the nasal part of the pharynx. The cartilage lies in a groove between the petrous part of the temporal bone and the great wing of the sphenoid. The groove ends opposite the middle of the medial pterygoid plate.
The auditory tube has two openings - the pharyngeal orifice of the auditory tube in the lateral wall of the pharynx and the tympanic orifice of the auditory tube that opens in the tympanic cavity anterior wall.
On the border between both of the parts of the auditory tube is the narrowest place called isthmus tubae auditivae. The auditory tube is lined with mucosa that is the continuation of the pharynx mucosa.
The air pressure in the external ear and the nasopharynx is equal to open-air pressure. This connection between the middle ear and the nasopharynx serves primarily to equalize the pressure. Equalization occurs while the auditory tube stays closed for most of the time. When we yawn or swallow, the auditory tube opens, and it allows air to flow and pressure to equalize. Having the auditory tube protects the tympanic membrane from rupturing quickly under pressure.
All in all, there are four muscles associated with the auditory tube:
The blood supply to the auditory tube is provided by the following arteries:
The venous blood is drained from the auditory tube via the pterygoid plexus of veins in the infratemporal fossa.
The lymph from the auditory tube is drained into the deep cervical lymph nodes.
The nerve supply to the auditory tube is provided by the tympanic plexus. This is an indirect way for the tympanic nerve (a branch of the glossopharyngeal nerve (CN IX) to supply the auditory tube.