Adult morphology The otolaryngologic and general anatomic literature focuses on the active, anteroinferior two thirds of the adult eustachian tube. In adults the eustachian tube forms and angle of 45 degrees with respect to the horizontal plane, considerably larger than the 10-degree angle of infants .
Figure 7 : Eustachian tube in adult and infant
It forms a 42 ± 9 degree angle with a parasagittal plane through the medial pterygoid plate. The tube is longer in the adult than in the infant and young child, and its length varies with race; it has been reported to be as short as 30 mm , and as long as 40 mm but the usual range of length reported in the literature is 31 to 38 mm The eustachian tube does not take a straight course from the middle ear to the nasopharynx, but rather a slowly curving inverted S course. Speilberg found that in adults “the tube makes two curves as it leaves the tympanic cavity, arching downward and forward across the space between the anterior canal wall and the bony external auditory meatus in the condyle of the mandible. Before the pharyngeal orifice, it makes another slight curve downward and forward”. Additional observations (Swarts et al, 2005) support Speilberg’s observations, although variability is great. The nasopharyngeal terminus of the eustachian tube lies about 20 mm above the plane of the hard palate (Graves and Edwards, et al 1944). The cartilage protrudes into the nasopharynx; this protrusion is known as the torus tubarius. A thick layer of epithelium continuous with the soft tissue lining of the nasopharynx covers it. An observer viewing the nasopharynx endoscopically cannot see the torus tubarius but can see the mucosa overlying the cartilage. Any inferences drawn from observing motion in this area must consider this point of the anatomy.