Eustachian (auditory) tube

Bony part

The bony part of the auditory tube comprises the posterolateral third of this structure. It arises is located in an anterior subcompartment of the tympanic cavity, known as the protympanum (the region anterior to the tympanic ring), which communicates with the musculotubal canal of the temporal bone; this canal is further divided into two semicanals by the cochleariform process: one for the auditory tube, the other for the tensor tympani muscle (see below). The former forms the tympanic opening of the auditory tube. 

Within the temporal bone, the bony part of the auditory tube narrows and terminates in the region of the sphenopetrosal suture, a point known as the isthmus of the auditory tube. 

Note: Recent studies involving endoscopy of the middle ear have suggested that the bony part of the auditory tube is better described as an anterior extension of the tympanic cavity rather than part of the auditory tube itself. 

Cartilaginous part

The cartilaginous part of the auditory tube is supported by a curled triangular plate of fibrocartilage located in a groove between the greater wing of the sphenoid bone and petrous portion of the temporal bone. The apex of this triangle is attached to the bony part of the auditory tube while the base projects towards the mucous membrane of the nasopharynx, posterior to the pharyngeal opening of the auditory tube; here it forms an elevation on the nasopharyngeal wall known as the torus tubarius. A collection of a collection of lymphoid nodules, known as tubal tonsils, are located near the pharyngeal opening of the auditory tube.

Mucosa

The lining, or mucosa, of the auditory tube is continuous with that of the tympanic cavity where it arises, as well as with that of the nasopharynx where it terminates. Therefore the mucosa of bony part of the auditory initially begins as a [thin layer of] simple cuboidal/columnar epithelium, which transitions to a [thicker layer of] ciliated pseudostratified epithelium in the cartilaginous part (similar to that of respiratory epithelium found in the nasopharynx). The latter contains mucous glands known as tubal glands.

Muscles

The auditory tube is normally collapsed, i.e. closed, but opens upon positive pressure and swallowing. There are four muscles that are responsible for auditory tube functioning. The tensor and levator veli palatini muscles open the tube, by contracting the muscles of the soft palate.

Levator veli palatini

Levator veli palatini is a thick round muscle that arises from the petrous portion of the temporal bone and auditory tube cartilage. It descends in an oblique fashion, running superior to the superior pharyngeal constrictor, and blends with the contralateral muscle. It is innervated by the pharyngeal branch of the vagus nerve (cranial nerve X), via the pharyngeal plexus. It functions by elevating during swallowing and hence preventing the food entering the nasopharynx.

Tensor veli palatini

Tensor veli palatini is a broad ribbon like muscle that tenses the soft palate which during swallowing, prevents food entry to the nasopharynx. It lies lateral and anterior to the levator veli palatini. It arises from the pterygoid process as well as the spine of the sphenoid bone. As it also takes some of its origin from the auditory tube, it is the main muscle which opens it upon contraction. It descends between the medial and lateral pterygoid plates, and terminates as a tendon that winds around the pterygoid hamulus, and inserts onto the palatine aponeurosis that lies behind the hard palate, i.e. the palatine bone.

Salpingopharyngeus

Salpingopharyngeus is an elongated triangular muscle that originates from the cartilaginous part of the auditory tube. It descends and merges with the palatopharyngeus muscle. It elevates the pharynx during swallowing. It is innervated by the vagus nerve via the pharyngeal plexus.

Tensor tympani 

The tensor tympani is the larger of the two muscles of the middle ear, the other being stapedius. The muscle functions to dampen down sounds such as chewing. It arises from the cartilaginous portion of the auditory tube as well as the adjacent section of the greater wing of the sphenoid bone. It inserts onto the handle of the malleus. The innervation to the muscle arises from the nerve to tensor tympani, which arises from the mandibular (V3) division of the trigeminal nerve (cranial nerve 5).

Ostmann fat pad 

Not exactly a muscle but these fatty bodies, which are found inferomedial to the auditory tube, act to ensure closure of the auditory tube, thereby reducing the transference of pressure from the nasopharynx to the middle ear.

Test your knowledge on the structures of the middle ear with this quiz.

Blood supply

The bony part of the auditory tube is supplied by tubal branches of the middle meningeal artery (branch of the maxillary artery) and/or caroticotympanic arteries (branches of the internal carotid artery), while the cartilaginous part may be supplied by any of the following vessels: deep auricular artery (maxillary artery), pharyngeal branch of artery of pterygoid canal (maxillary artery), ascending palatine artery (facial artery). Venous drainage is carried to the pterygoid venous plexus. 

Innervation

The bony part of the auditory tube is innervated via a tubal branch of the tympanic nerve/plexus (derived from the glossopharyngeal nerve (CN IX). The cartilaginous part, on the other hand, is primarily innervated by branches of the pharyngeal nerve (branch of the maxillary nerve (CNV3) via the pterygopalatine ganglion).