Page 297 - Clinical Anatomy

ECA5 7/18/06 6:50 PM Page 282

282 The head and neck

Nerve supply
The pharyngeal branches of IX and X constitute the principal sensory and
motor supply of the pharynx respectively. The maxillary division of V sup-
plies the sensory innervation of the nasopharynx.

The mechanism of deglutition

The act of swallowing not only conveys food down the oesophagus but also
disposes of mucus loaded with dust and bacteria from the respiratory pas-
sages. Moreover, during deglutition, the Eustachian auditory tube is
opened, thus equalizing the pressure on either side of the ear drum.
Deglutition is a complex, orderly series of reflexes. It is initiated vol-
untarily but is completed by involuntary reflex actions set up by stimu-
lation of the pharynx. If the pharynx is anaesthetized then normal
swallowing cannot take place. The reflexes are coordinated by the degluti-
tion centre in the medulla, which lies near the vagal nucleus and the respi-
ratory centres.
The food is first crushed by mastication and lubricated by saliva. It is a
common experience that it is well nigh impossible to swallow a pill when
the throat is dry. The bolus is then pushed back through the oropharyngeal
isthmus by the pressure of the tongue against the palate, assisted by the
muscles of the mouth floor.
During swallowing, the oral, nasal and laryngeal openings must be
closed off to prevent regurgitation through them of food or fluid; each of
these openings is guarded by a highly effective sphincter mechanism.
The nasopharynx is closed by elevation of the soft palate, which shuts
against a contracted ridge of superior pharyngeal constrictor, the ridge of
Passavant. At the same time, the tensor palati opens the ostium of the
Eustachian tube. The oropharyngeal isthmus is partially blocked by con-
traction of palatoglossus on each side, which narrows the space between
the anterior faucial pillars. The residual gap is closed by the dorsum of the
tongue wedging into it.
The protection of the larynx is a complex affair, brought about not only
by closure of the sphincter mechanism of the larynx but also by tucking the
larynx behind the overhanging mass of the tongue and by utilizing the
epiglottis to guide the bolus away from the laryngeal entrance. The central
nervous component of the swallowing reflex is depressed by narcotics,
anaesthesia and cerebral trauma. In these circumstances aspiration of
foreign material into the pulmonary tree becomes possible, particularly if
the patient is lying on his back or in a head-up position.
The laryngeal sphincters are at three levels:
1◊◊the aryepiglottic folds, defining the laryngeal inlet, which are apposed
by the aryepiglottic and oblique arytenoid muscles;
2◊◊the walls of the vestibule of the larynx, which are approximated by the
thyroepiglottic muscles;
3◊◊the vocal cords, which are closed by the lateral cricoarytenoid and inter-
arytenoid mucles.
The larynx is elevated and pulled forward by the action of the