Tongue
The tongue is an
extremely mobile mass of striated muscle, covered by mucous membrane. Arising
from the floor of the mouth, the tongue practically fills the oral cavity when
all its parts are at rest and the individual is in an upright position. The
shape of the tongue may change extensively and rapidly during the various
activities it has to perform.
The areas of the tongue covered by the
mucous membrane are the apex, the dorsum, the right and left
margins, and the inferior surface. These are obvious topographic designations,
except for the dorsum, which needs further description. The dorsal aspect of
the tongue extends from the apex to the reflection of the mucous membrane to
the anterior surface of the epiglottis at the vallecula, forming
an arch that, in its anterior or palatine two thirds, is directed superiorly,
whereas its posterior or pharyngeal one third is directed posteriorly. Several
divisions of the tongue have been proposed. Sometimes the terminal sulcus has
been said to separate the body and the root of the tongue, but in
other instances the portion called the root has been limited to the posterior
and inferior attachment of the tongue or has even been restricted to mean only
the region of attachment through which muscles and other structures enter and
leave the tongue. From the practical point of view, it is rather irrelevant
where one permits the root to start and the body to end, or vice versa, but it
is important to realize that the posterior third of the tongue and its epiglottic
region are not visible by simple inspection even if the tongue is protruded
unless the examiner uses a mirror or presses the tongue down with the aid of a
spatula.
At the posterior end of the body is a
small blind pit, known as the foramen cecum, the remnant of the
thyroglossal duct, from which the thyroid gland developed during the fetal
stage. Angling anterolaterally toward each
side from the
foramen cecum is the terminal sulcus, which is usually referred to as
the dividing line between the anterior and posterior parts of the tongue.
However, the real dividing line may run just anterior to the vallate papillae.
A median sulcus is not always very distinct but is related to the
interior, lingual septum.
The mucous membrane covering the apex
and body of the tongue is moist and pink and is thickly studded with various papillae.
The majority of the papillae are of the filiform type, in which the
epithelium ends in tapered, rough points to provide friction for the handling
of food. Scattered about the field of filiform papillae are the larger,
rounded fungiform papillae. In front of the sulcus terminalis runs a
V-shaped row of 8 to 12 (circum) vallate papillae, which rise far more
prominently over the surface of the mucous membrane than do the two other types
of papillae. The whole mucosa of the anterior two thirds of the tongue is
firmly adherent to the underlying tissue.
The mucous membrane of the posterior
one third of the tongue (the pharyngeal part), though smooth and glistening,
has an uneven or nodular surface owing to the presence of a varying number (35
to 100) of rounded elevations with a crypt in the center. These nodules consist
of lymphoid tissue lying deep to the epithelium.
The lymphoid nodules are grouped
around the epithelium-lined crypt or pit and, taken collectively, are called
the lingual tonsil.
On both margins of the tongue, the
mucous membrane is thinner and, for the most part, devoid of papillae, though a
variable number of vertical folds may be found on the posterior part of each
margin. They are called foliate papillae, and represent rudimentary
structures similar to the well-developed foliate papillae seen in rodents.
The mucous membrane of the inferior
surface of the tongue is thin, smooth, devoid of papillae, and more loosely
attached to the underlying tissue. It exhibits the midline frenulum and some
rather rudimentary fimbriated folds that run posterolaterally from the tip of
the tongue. The frenulum is a duplication of the mucous membrane and connects
the inferior lingual surface with the floor of the mouth. The deep lingual
veins usually shine through the mucosa between the frenulum and the fimbriated
folds on each side.
Many small glands are scattered
beneath the mucous membrane and partly embedded in the muscle. Mucous glands
are located in the posterior third of the dorsum, with their ducts opening
on the surface and into the pits of the lingual tonsil. In the region of the
vallate papillae, the purely serous lingual glands of von Ebner send
numerous ducts (from 4 to 38) into the furrows, or moat, surrounding each of
these papillae. Glands of a mixed type, the lingual glands of Blandin and Nuhn,
are found to each side of the midline inferior and posterior to the apex of the
tongue.
The receptor organs for the sense of taste,
the taste buds, are pale oval bodies (about 70 ยต in their long
axis), seen microscopically in the epithelium of the tongue and to a much
lesser extent in the epithelium of the soft palate, pharynx, and epiglottis.
The taste buds are most prevalent in the epithelial lining of the furrows
surrounding the vallate papillae. A few taste buds are present on the fungiform
papillae and also scattered on the foliate papillae. A taste bud reaches from
the basement membrane to the epithelial surface, where a pore is
situated, into which the microvilli (taste hairs) of the neuroepithelial taste
(gustatory) cells extend. From 4 to 20 taste cells are intermingled with
the more numerous supporting sustentacular cells of the taste buds.
The majority of the tongue is made up
of skeletal (striated) muscles, which are composed of muscular bundles,
interlaced in many directions. An incomplete lingual septum divides the
tongue into symmetric halves. One group of muscles, the extrinsic ones, originates
outside of the tongue, whereas the intrinsic group of lingual muscles
originates and inserts entirely within other muscles of the tongue. The genioglossus
muscle arises from the superior mental spine of the mandible and fans out
along the entire length of the dorsum of the tongue, with the lowest fibers
having some attachment to the hyoid bone. Lateral to this muscle is the hyoglossus
muscle, which arises from the body of the hyoid bone as well as the entire
length of the greater and lesser horns from which it runs vertically upward.
The styloglossus muscle arises from near the tip of the styloid process
and an adjacent part of the stylomandibular ligament. It runs as a band
inferiorly and anteriorly onto the lateral aspect of the tongue. The palatoglossus
muscle descends from the soft palate, forming the framework of the
palatoglossal fold. The intrinsic lingual muscles are named according to the
three spatial dimensions in which their fascicles run. Of the two longitudinal
muscles, the superior longitudinal band extends from anterior to
posterior just deep to the mucous membrane of the dorsum. The inferior
longitudinal band spreads between the genioglossus and hyoglossus muscles
on the undersurface of the tongue. The contraction of both longitudinal muscles
shortens the tongue. The transverse lingual muscle, which is covered by the
superior longitudinal muscle, furnishes nearly all of the transversely running
fibers and is intermingled with fascicles of the extrinsic muscle group.
The vertical lingual muscle is made up
of all the vertical fibers, except those supplied by extrinsic muscles, with
which it forms a closely woven network. By the combined actions of all these
muscles, the shape of the tongue can be extensively altered: lengthened,
shortened, broadened, narrowed, curved in various directions, protruded, and
drawn back into the mouth.
The innervation of the tongue involves
the following nerves: (1) motor from the hypoglossal nerve except for the
palatoglossus muscle, which is innervated by the vagus nerve; (2) general
sensory to the anterior two thirds via the lingual nerve, which is accompanied
by the chorda tympani, a branch of the facial nerve, which is special sensory
(taste) to the same area; and (3) glossopharyngeal nerve, which is general and
special sensory to the posterior one third of the tongue. The vagus nerve is
general and special sensory to the epiglottic region.
