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The eustachian tube lumen is wider at both the proximal (nasopharyngeal) and distal (middle

ear) ends than in the midportion. The isthmus is the most narrow. A recent three-dimensional

study of nine human temporal bone specimens by Sudo and associates (3) demonstrated the

isthmus portion of the lumen to be near the distal end of the cartilaginous portion and not at

the junction of the cartilaginous and osseous portions. They named the segment where the

cartilaginous and osseous portions connect the junctional portion which was determined to be 3 mm in length in the adult (3). !n the lateral wall of the nasopharynx a prominence the

torus tubarius protrudes into the nasopharynx. This prominence is formed by the abundant

soft tissue o"erlying the cartilage of the eustachian tube. Anterior to this is the triangular

nasopharyngeal orifice of the tube (#ig. $%.&A). #rom the torus a raised ridge of mucous

membrane the salpingopharyngeal fold descends "ertically. !n the posterior wall of the

nasopharynx lie the adenoids or pharyngeal tonsil composed of abundant lymphoid tissue.

Abo"e the tonsil is a "ariable depression within the mucous membrane called the pharyngeal

 bursa. 'ehind the torus lies a deep pocet extending to the nasopharynx posteriorly along the

medial border of the eustachian tube. This pocet the fossa of osenm*ller "aries in height

from + to ,% mm and in depth from 3 to ,% mm (). Adenoid tissue usually extends into this

 pocet gi"ing soft-tissue support to the tube.n the adult the eustachian tube is longer than that in the infant and young child. /ost of the

increase in length taes place before age 0 years (1). The eustachian tube has been reported to

 be as short as 3% mm and as long as % mm but the usual range of length reported in the

2.,&1

literature is 3, to 3+ mm (0). t is generally accepted that the posterior third (,, to ,

mm) of the adult tube is osseous and the anterior two-thirds (&% to &1 mm) is composed of

membrane and cartilage (+). n adults the eustachian tube lies at an angle of 1 degrees in

relation to the hori4ontal plane. n infants this inclination is only ,% degrees (). The

anatomy of the cranial base may be related to the length of the eustachian tube which may be

related to a susceptibility for middle-ear disease ($).

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#igure $%., The eustachian tube connects the nose and nasopharynx with the middle ear andmastoid as a system.

The anatomic configuration of the eustachian tube and its relation to other structures are

 presented in #igure $%.&A. The osseous eustachian tube (protympanum) lies completely

within the petrous portion of the temporal bone and is directly continuous with the anterior

wall of the superior portion of the middle ear. The juncture of the osseous tube and the

epitympanum lies mm abo"e the floor of the tympanic ca"ity (+). This relation although

"alid is misrepresented in the more popular descriptions and depictions of the eustachian

tube5middle ear juncture and is of some importance in the functional clearance of middle-ear

li6uids.

The osseous (protympanic or middle-ear) portion of the tube has a course that is linear

anteromedially following the petrous apex and de"iating little from the hori4ontal plane. Thelumen is roughly triangular measuring & to 3 mm "ertically and 3 to mm along the

hori4ontal base. The healthy osseous portion is open at all times in contrast to the

fibrocartilaginous portion which is closed at rest and opens during swallowing or when

forced open such as during the 7alsal"a maneu"er. The osseous and cartilaginous portions of 

the eustachian tube meet at an irregular bony surface and form an angle of about ,0% degrees

with each other. The medial wall of the bony portion of the eustachian tube consists of two

 parts8posterolateral (labyrinthine) and anteromedial (carotid)8whose si4e shape and

relation depend on the position of the internal carotid artery. The a"erage thicness of the

anteromedial portion is ,.1 to 3 mm and in &9 of persons the wall is absent exposing the

carotid artery.

The cartilaginous tube then courses anteromedially and inferiorly angled in most cases 3% to

% degrees to the trans"erse plane and 1 degrees to the sagittal plane ( +). The tube is applied

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closely to the basal aspect of the sull and fitted to the sulcus tubae between the greater wing

of the sphenoid bone and the petrous portion of the temporal bone. The cartilaginous tube is

attached firmly at its posterior end to the osseous orifice by fibrous bands and usually extends

some distance (3 mm) into the osseous portion of the tube. At its inferomedial end it is

attached to a tubercle on the posterior edge of the medial pterygoid lamina (0,%).

The tube in its cartilaginous portion has a croo-shaped mediolateral superior wall (#ig.$%.&'). t is completed

2.,&11

laterally and inferiorly by a "eiled membrane that ser"es as the site of attachment of the fibers

of the dilator tubae or tensor "eli palatini muscle (0,,). The tubal lumen is shaped lie two

cones joined at their apexes. The juncture of the cones is the narrowest point of the lumen and

has been called the isthmus. ts position is usually described as at or near the juncture of the

osseous and cartilaginous portions of the tube. The lumen at this point is about & mm high

and , mm wide (). #rom the isthmus the lumen expands to about + to ,% mm in height and

, to & mm in diameter at the pharyngeal orifice. Tubal cartilage increases in mass from birth

to puberty and this de"elopment has physiologic implications (,&,3).

#igure $%.& A: ;omplete dissection of the eustachian tube and middle ear.

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tympanic membrane.

The cartilaginous eustachian tube does not follow a straight course in the adult but extends

along a cur"e from the junction of the osseous and cartilaginous portions to the medial

 pterygoid plate approximating the cranial base for the greater part of its course. The

eustachian tube crosses the superior border of the superior constrictor muscle immediately

 posterior to its terminus within the nasopharynx. The thicened anterior fibrous in"estment of the medial cartilage of the tube presses against the pharyngeal wall to form a prominent fold

the torus tubarius which measures ,% to ,1 mm thic (). The torus is the site of origin of the

salpingopalatine muscle and is the point of origin of the salpingopharyngeal muscle which

lies within the inferoposteriorly directed salpingopharyngeal fold (,).

The mucosal lining of the eustachian tube is conti-nuous with that of the nasopharynx and

middle ear andis characteri4ed as respiratory epithelium. Structural dif-ferentiation of this

mucosal lining is e"ident: mucousglands predominate at the nasopharyngeal orifice and

graded change occurs to a mixture of goblet columnar and ciliated cells near the tympanum

(,1). The lining isfolded which pro"ides greater surface area (,0). /ucosa-associated

lymphoid tissue also is present (,).

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