Pharynx
The pharynx is a
musculomembranous tube; much of its anterior wall is absent due to the fact
that the right and left nasal cavities, oral cavity, and larynx open into its
anterior side. It extends from the base of the skull to the inferior border of
the cricoid cartilage at the level of the lower margin of the sixth cervical
vertebra, at which time it becomes continuous with the esophagus. In addition
to the cavities already listed, the pharynx also communicates with the middle
ear on each side by means of the auditory (eustachian) tube; this fact
explains how infections spread from the pharynx to the middle ear, making a
total of seven cavities with which it has communication. The transverse
diameter of the pharynx exceeds the anteroposterior diameter, which is greatest
superiorly and is diminished to nothing inferiorly where the anterior and
posterior walls are in contact unless separated during the act of swallowing.
The transverse diameter does not differ greatly throughout the length of the
pharynx, except where it narrows at the lower end.
The posterior wall of the pharynx is
attached superiorly to the pharyngeal tubercle on the inferior surface
of the basilar part of the occipital bone, its adjacent area, and the
undersurface of the petrous portion of the temporal bone medial to the external
aperture of the carotid canal. The lateral wall has several attachments. It is
attached superiorly to the cartilaginous portion of the auditory tube, which
pierces the wall in this area. Anteriorly it connects to the lower, posterior
border of the medial pterygoid plate and its hamulus, as well as the pterygomandibular
raphe, the inner surface of the mandible (near the posterior end of the
mylohyoid line), the side of the root of the tongue, the hyoid bone, and
finally the thyroid and cricoid cartilages. Inferiorly, the walls of the
pharynx continue as the walls of the esophagus.
` The pharyngeal lining is a mucous
membrane that is continuous with the lining of the cavities communicating with
the pharynx. External to the mucous membrane of the posterior and lateral walls
is a sheet of fibrous tissue, more defined superiorly than inferiorly, known as
the pharyngeal aponeurosis (pharyngobasilar fascia), and external to
this is the muscular layer of the pharynx. On the outer surface of the muscular
layer is another fascial covering, the buccopharyngeal fascia.
The posterior pharyngeal wall is
separated from the prevertebral fascia overlying the anterior arch of atlas and
the bodies of the second to the sixth cervical vertebrae (partially covered by
the longus colli and longus capitis muscles) by a minimal amount of loose
fibrous connective tissue. This allows the pharynx some degree of freedom of
movement and forms a retropharyngeal space. Under anesthesia it is
possible to palpate these bony structures as far caudally as the fourth or
fifth cervical vertebra.
On the basis of openings in its
anterior wall, the pharynx is divided into the nasopharynx, oropharynx, and
laryngopharynx. The nasopharynx typically has a purely respiratory
function (acting as a passageway for air and not for food) and remains patent
because of the bony framework to which its walls are related. The anterior wall
is entirely occupied by the choanae (posterior nares), with the
posterior border of the nasal septum between them. The posterior wall
and roof form a con- tinuous arched wall, with the roof extending from the superior
margin of the choanae (where it is continuous with the roof of the nasal
cavities) to about the midpoint of the basilar portion of the occipital bone;
the posterior wall extends from this point caudally to about the lower border
of the anterior arch of the atlas. In the region where the roof and posterior
wall meet, the mucous membrane is thrown into many variable folds, with an
accumulation of nodular and diffuse lymphoid tissue (extensively developed in
children, atrophied in adults) forming the pharyngeal tonsil (adenoids).
In the midline, near the anterior margin of the pharyngeal tonsil, is a minute
flask-shaped depression of mucous membrane, known as the pharyngeal bursa. Also
in the midline, near the anterior limit of the roof, and submerged in the
mucosa or lying in the periosteum, a microscopic remnant of the Rathke pouch
(pharyngeal hypophysis) can be found, which is grossly visible only when it has
become cystic or has formed a tumor.
The incomplete floor of the
nasopharynx is formed by the posterosuperior surface of the soft palate with
an opening from the nasal to the oral pharynx between the soft palate and the
posterior wall of the pharynx. This opening is closed by bringing these two
structures in contact.
On the lateral wall of the nasopharynx
at the level of the inferior concha is the pharyngeal opening of the
auditory tube, with the pharyngeal recess (fossa of Rosenmüller)
posterior to it. The levator cushion (produced by the levator veli palatini
muscle) bulges into the inferior margin of the triangular opening, and coursing
inferiorly from the posterior lip is the salpingopharyngeal fold produced
by the muscle of the same name. In child-hood, a considerable mass of lymphoid
tissue (tubal tonsil) may be present around the opening of the auditory tube
and may enlarge, blocking drainage from the middle ear.
The oropharynx extends from the
inferior border of the nasopharynx to the level of the pharyngoepiglottic
folds, with the epiglottis protruding into it. This part of the pharynx
carries both air and food. The posterior wall remains in relation to the bodies
of the second to fourth cervical vertebrae, whereas the anterior wall is
deficient superiorly where the oropharynx and oral cavity communicate by means
of the isthmus of the fauces. Inferior to the isthmus, the anterior wall is
formed by the posterior third of the tongue. Between the tongue
and epiglottis are
the valleculae, spaces where
foreign bodies may lodge.
The laryngopharynx lies
posterior to the larynx and anterior to the fifth and sixth cervical vertebrae.
The superior part of the anterior wall contains the roughly triangular laryngeal
inlet (aditus), the borders of which are formed by the margins of the
epiglottis, the aryepiglottic folds, and the interarytenoid incisure.
Inferior to the laryngeal opening, the laryngopharynx only transmits food
and is purely alimentary in function. The mucous membrane of the anterior wall
overlies the posterior surfaces of the arytenoid cartilages and the lamina of
the cricoid cartilage (mostly covered by laryngeal muscles). Inferior to
the laryngoepiglottic fold on each side is the piriform recess, located
between the cricoid and arytenoid cartilages medially and the lamina of the
thyroid cartilage laterally. This is another location where foreign bodies may
lodge.
