The =Eustachian= or =auditory tube= (Tuba auditiva [Eustachii]) extends
from the tympanic cavity to the pharynx; it transmits air to the former
and equalizes the pressure on the two surfaces of the membrana tympani.
It is directed forward, downward, and slightly inward, and is four to
five inches (ca. 10 to 12 cm.) in length. Its posterior extremity lies
at the inner side of the root of the muscular process of the petrous
temporal, and communicates with the anterior part of the tympanic cavity
by the small slit-like =tympanic opening= (Ostium tympanicum tubæ
auditivæ). For a distance of about a quarter of an inch (ca. 6 to 7 mm.)
in front of this opening it is a complete tube, with a curved lumen
which is little more than a capillary space. Further forward it has the
form of a plate which widens anteriorly and is curved to inclose a
narrow groove, that opens ventrally into an extensive diverticulum
termed the guttural pouch. The =pharyngeal opening= (Ostium pharyngeum
tubæ auditivæ) is situated on the postero-superior part of the lateral
wall of the pharynx just below the level of the posterior nares. It is a
slit, an inch or more (ca. 3 cm.) in length, which slopes downward and
backward. It is bounded internally by the thin free edge of the tube,
from the lower part of which a fold of mucous membrane (Plica
salpingo-pharyngea) extends in the same direction on the lateral wall of
the pharynx for a distance of a little more than an inch usually. The
outer boundary of the opening is the lateral wall of the pharynx. The
basis of the tube is a plate of fibro-cartilage (Cartilago tubæ
auditivæ) which is firmly attached dorsally to the fibrous tissue which
closes the foramen lacerum basis cranii, the temporal wing of the
sphenoid, and the pterygoid bone.[222] On cross-section the cartilage
(except at its tympanic end) is seen to consist of two laminæ which are
continuous with each other above. The internal lamina (Lamina medialis)
gradually widens toward the pharyngeal end, where it forms a broad
valvular flap. This is convex internally, and its thin anterior edge
forms the inner margin of the pharyngeal opening. Behind this the lamina
has a thick free edge which projects ventrally from the roof of the
guttural pouch. The external lamina (Lamina lateralis) is narrow and
thin, and is related externally to the levator and tensor palati
muscles, which are in part attached to it; it does not exist at the
pharyngeal end of the tube. The mucous membrane of the tube is
continuous behind with that of the tympanum and in front with that of
the pharynx. On either side it is reflected to form a large
diverticulum, the guttural pouch. It is covered with ciliated
epithelium, and contains mucous glands and lymph nodules.
The pharyngeal opening appears to be closed ordinarily, but the tube
opens during deglutition. This action is apparently produced by the
part of the palato-pharyngeus muscle which is attached to the tube.