Tongue, hyoid muscles, pharynx and swallowing. Note: A summary description of
swallowing is shown in Figs. 30, 31.
1. Anatomy of the tongue and pharynx;
Hypoglossal paralysis: Tongue protrudes to the paralyzed side and is
atrophic on the paralyzed side;
Surgical repair of two cases, nearly identical but in different horses, in
which the horse had bitten through its tongue, leaving about 2” of the end of the
tongue hanging by a neurovascular cord.
2. Actions of lingual, palatine, pharyngeal, and hyoid muscles in swallowing.
1. Tongue. The tongue is composed of striated muscle held together by a vascular
connective tissue and clothed with a mucosa bearing receptors for taste, for pain,
tactile stimuli, and temperature. It is an active organ and has a good blood supply by
way of the lingual artery and veins. Its striated muscle is innervated by the 12th cranial,
hypoglossal, nerve. Contraction of the tongue muscles is dependent on their
innervation, and, lacking innervation, the activity of the muscles ceases and a rapid
atrophy (usually apparent in 4 – 5 days) commences. Sensory innervation of the
tongue is provided by the lingual branch of the trigeminal nerve (pain, touch,
temperature), the chorda tympani of the facial nerve and the lingual branch of the
glossopharyngeal nerve (taste sensation: bitter, sour, salty, sweet). No
neuromuscular spindles have been found associated with the lingual musculature and
proprioception is not one of its sensory modalities. This is associated with the fact that
its striated muscles are not weight-bearing and its movements do not require the fine
control associated with, for example, the eye muscles in which neuromuscular
spindles are numerous.
The hypoglossal nerve supplies the muscles of the tongue and the geniohyoideus.
The muscles of the tongue are genioglossus, hyoglossus, and styloglossus, which
arise external to the tongue and are designated extrinsic muscles, and longitudinal,
transverse and vertical myofiber bundles, which arise and end in connective tissue
within the tongue and are intrinsic muscles. The hypoglossal nerve is strictly motor
and supplies all of these muscles. The geniohyoideus is not a tongue muscle in the
sense that it does not form a part of the tongue. But, with the genioglossus, it is
responsible for protruding the tongue. This strong muscle extends from its attachment
paramedian to the internal aspect of the junction of right and left halves of the
mandible and, from there, to the basihyoid bone. Its tendon arises from the mandible
just below the genioglossus. Caudally, the muscle attaches to the basihyoideum, and
to its lingual process in ruminants and the Equidae. With lingual movements,
contraction of the geniohyoidei and caudal fibers of the genioglossus muscles draw
the basihyoideum forward and protrude the tongue. With unilateral paralysis of the
tongue, when only the geniohyoideus and genioglossus of the normal side contract,
the tongue is pulled forward, inclined toward the paralyzed side. The tongue muscles
of the affected side are paralyzed and the tongue will appear atrophic on the side
toward which the tongue is protruded.
The tongue muscles are shown in Figs. 1, 2, and, in color, by Figs. 3, 4, and 5. The
tendon of the genioglossus arises from the mandible immediately dorsal to and
sometimes fused with the tendon of the geniohyoideus. It is the most medial of the
extrinsic muscles and radiates flat and fanlike in a vertical plane in paramedian
fashion into the central part of the lingual mucosa. With the geniohyoideus, the caudal
fibers of this muscle are active in drawing on the hyoid apparatus and protruding the
tongue. The hyoglossus arises from the thyro- and basihyoideum of the hyoid
apparatus. This muscle is the largest of the extrinsic muscles, passing from its hyoid
origin rostrodorsally into the body and dorsum of the tongue. The muscle lies between
the genioglossus medially and the styloglossus laterally. The styloglossus arises from
the ventrolateral aspect of the stylohyoideum. It is the smallest and most superficial of
the extrinsic muscles, passing rostrally toward the tip of the tongue lateral to the
hyoglossus.
Fig. 1. Horse, lateral view with the mandible, pterygoid muscles, and zygomatic arch
removed. From Guide for the Laboratory Examination of the Anatomy of the Horse,
Horowitz, A., 1965.
Fig. 2. Dog, lateral view with the mandible and pterygoid muscles removed. From
Fundamental Principles of Anatomy: Dissection of the Dog, Horowitz, A., 1968.
Drawn by David
Stewart Geary
Fig. 3. Modified from Guide for the Laboratory Examination of the Anatomy of
the Horse, Horowitz, A., 1965. Muscles, all innervated by the hypoglossal nerve,
are identified by pastel colors: styloglossus, yellow; hyoglossus, blue;
genioglossus, red; geniohyoideus, green.
Fig. 4. Modified from Guide for the Laboratory Examination of the Anatomy of the Horse,
Horowitz, A., 1965. Deeper dissection. The hyoglossus is cut in its middle and its lower
part is reflected ventrally, exposing intrinsic longitudinal fibers (violet). The upper part of
the stylohyoideum has been removed; only the tendon of origin of the styloglossus
remains on the lower part of the bone. Hyoideus transversus fibers are magenta.
Fig. 5. Modified from Guide for the Laboratory Examination of the Anatomy of the Horse,
Horowitz, A., 1965. The lower, original, part of the hyoglossus has been removed. The
stylohyoideum and ceratohyoideus muscle have been cut away; the pharyngeal mucosa,
with the palatine tonsil, has been cut away, exposing the oral pharynx.
tympanohyoid
cartilages
In the case of protrusion of the tongue (Fig. 6), the
geniohyoidei are critical. In the normal case (A), right and left muscles draw the base
of the tongue forward evenly. Caudal fibers of the
ipsilateral
Fig.
6.
genioglossus assist, exerting some traction at the
base of the tongue
and the attaching parts of the hyoid apparatus. In
unilateral paralysis
A
(B), only the muscles of the normal side contract. This
unequal pull pushes
the tip of the tongue toward the paralyzed, atrophic, side.
The tympanohyoids,
the cartilages that attach the hyoid apparatus on either side to the styloid process of
the petrous temporal bone, are sufficiently flexible to permit the limited distortion of
the lower part of the hyoid apparatus that occurs with the unequal pull.
One nerve, the hypoglossal (Fig. 7), is motor to the tongue. It is the most caudal of
the cranial nerves, large, and it passes from the medulla
oblongata through the
B
hypoglossal canal and lateral to the pharynx and the
external carotid to
reach the base of the tongue. Entering the
tongue from below, the nerve gives off a
branch to the geniohyoideus muscle and
passes rostrodorsally between the hyoand genio-glossus muscles.
Fig. 7. Horse, left view with much of the mandible and its attaching muscles
removed. Styloglossus, , yellow; hyoglossus, blue; genioglossus, red;
geniohyoideus, green. The styloglossus and hyoglossus are partly cut away to
expose the position of the main trunk of the hypoglossal nerve accompanying the
lingual artery between the genioglossus and hyoglossus muscles. Figure is slightly
modified from Topographische Anatomie des Pferdes, W. Ellenberger and H. Baum,
1894; Verlag Paul Parey, Berlin.
mandibular n. (trigeminal)
lingual branch of
mandibular n. (cut)
maxillary a.
stylohyoideum
(stylohyoid bone)
m
chorda tympani
(joins lingual branch
of mand. n.)
lingual branch of
mandibular n. (cut)
ext. carotid a.
hypoglossal n.
styglossus m. (cut)
genioglossus m.
hyoglossus m.
lingual branch of
glossopharyngeal n.
lingual a.
Three nerves are sensory: The lingual branch of the mandibular division of the
trigeminal transmits nerve impulses from lingual receptors for pain, touch and
temperature. The chorda tympani supplies the taste buds of the few fungiform
papillae, the lingual branch of the glossopharyngeal innervates the taste buds of the
foliate and vallate papillae. The blood supply of the tongue is provided by the lingual
artery, a terminal branch of the linguofacial trunk, and the lingual veins. The lingual
vein passes to the linguofacial trunk. In the horse, a prominent dorsal tributary
(designated the dorsal lingual vein by Sisson, passes to the anastomosis of buccal
and maxillary veins medial to the mandible. In all the domestic mammals, the lingual
vessels and the larger trunks of the motor and sensory nerves lie between the
genioglossus and hyoglossus muscles.
In Fig. 7, above, note the following:
- The chorda tympani, shown relatively larger in the figure, passes always on
the caudal aspect of the maxillary artery; the lingual nerve, a branch of the
mandibular division of the trigeminal, passes rostral to the artery;
- The lingual branch of the glossopharyngeal passes only briefly with the
lingual artery, which is a branch of the linguofacial trunk in the deep-jawed
horse and ox, a direct branch of the external carotid in the dog and pig.
Nerve and artery pass forward, the nerve to the area of the foliate and vallate
papillae and the mucosa of the base of the tongue, the artery continuing
forward between the ceratohyoideus and the hyoglossus, then with the mix of
fibers of the lingual branch of mandibular and hypoglossal nerves to the apex
of the tongue. (The term “lingual plexus” is reserved for the plexus of
autonomic and a few sensory fibers, which surrounds the lingual artery; that
is, the mix of fibers of the lingual branch of the mandibular and the
hypoglossal nerve these connections do not constitute the lingual plexus);
- The lingual nerve and the hypoglossal enter the tongue from below, the
hypoglossal being the larger nerve (not obvious in Fig. 7) and the more
caudal.
When in practice, I was twice presented with a horse that had bitten through its
tongue leaving a three inch apical part connected to the body of the tongue by the
neurovascular cord of the lingual artery with end-branches of the lingual and
hypoglossal nerves. That the tip retained its vascular supply was critical. At the time,
another veterinarian, Jack Eichelberger, who was working with me, and I put the
horse down with Rompun-ketamine, sewed the two pieces of tongue together with
interrupted nonabsorbable sutures, and were done with the surgery in less than thirty
minutes. We were able to remove the sutures a little over a week later. Both horses
Fig. 8. Horse, cross-section through the head at the level of the 3rd molar tooth
showing muscle and vessel relations near the base of the tongue. The lower
figures show the level of cross-section. Larger figure is from Sisson. The lower
figures are copies of Figs. 4 and 7.
recovered well.
hyoglossus m.
intrinsic myofibers
lingual a.
genioglossus m.
styloglossus m.
geniohyoideus m.
Pharynx. The pharynx is a chamber common to the respiratory and digestive
systems and extends from the nasal and oral cavities, which are rostral, to the
pharyngeal opening of the larynx (aditus laryngis) and the pharyngeal opening of the
esophagus caudally. The walls of the pharynx are plastic, muscular, clothed by
mucous membrane and, at the small pharyngeal recess of ungulates, are formed by
mucous membrane alone. The arms of the yoke-like hyoid apparatus are immediately
lateral to the rostral pharyngeal wall and the basihyoideum of the apparatus is
immediately ventral at the junction of oral and laryngeal pharynx (laryngopharynx).
The pharynx is divided into three parts, oral pharynx, nasal pharynx, and
laryngopharynx. The oral pharynx extends from the isthmus faucium, the passage
between the tongue and soft palate at the level of the palatoglossal arches, and the
gutter between the base of the tongue and the base of the epiglottis; the isthmus is
ventral to the soft palate. The nasal pharynx extends from the choanae, the passages
leading from right and left nasal cavities to the nasal pharynx, to the intrapharyngeal
ostium, which is the opening formed by the free border of the soft palate in continuity
with the palatopharyngeal arches; the ostium opens to the laryngopharynx. The nasal
pharynx is dorsal to the soft palate. The laryngopharynx is caudal to the
intrapharyngeal ostium. It extends from the gutter between the base of the tongue and
the base of the epiglottis to the pharyngeal opening of the larynx (aditus laryngis) and
the pharyngeal opening of the esophagus. In all species, there is an obvious
narrowing of the esophagus at its pharyngeal opening. In the dog and cat, the
pharyngeal origin of the esophagus is marked by a glandular thickening of the
mucosa, in the cat by a circular constriction, the pharyngoesophageal limen.
Fig. 9. Horse
(probably young as it
shows dorsal
displacement of the
soft palate), median
section of pharynx,
showing nasal
(yellow), oral (blue),
and laryngopharynx
(collapsed, and red)
cavity limits. Figure is
from Sisson.
esophagus
larynx
Fig.10. Horse,
median section of
pharynx, showing
nasal, oral, and
laryngopharynx.
Figure (same as
Fig. 9) is from
Sisson.
Fig. 11. Horse, median section. Similar to Figs. 9 and 10. This is probably from a
mature horse, which normally displays no sign of dorsal displacement of the soft
palate. The larynx usually rests as here, protruding into the nasal pharynx. Figure from
Sisson.
palatopharyngeal arch
Fig. 12. Donkey, paramedian section with the larynx cut away to show how the
free border of the soft palate is continued onto the (right) pharyngeal wall as a
narrow mucosal fold, the palatopharyngeal arch. Photograph is by Mr. Keithroy
Charles.
nasal pharynx
soft palate
palatopharyngeal arch
oral pharynx
soft palate, free border
Fig. 12a. Mucosal
boundaries more
clearly marked. The
double arrow shows
the position of the
intrapharyngeal
ostium.
palatine tonsil
Fig. 13. Endoscopic view of dorsal
displacement
of the soft palate in a
lingual
tonsil
horse. The intrapharyngeal ostium bounded by the free border of the soft
palate and the palatopharyngeal arches is clearly seen. Note how the
palatopharyngeal arch fits behind the arytenoids. Figure is from Anatomy of
the Horse, K.-D. Budras, W.O. Sack, Sabine Röck; transl. by A. Horowitz
and R. Berg, 2009: Schlütersche Verlag, Hannover.
nasal pharynx
palatopharyngeal arch
soft palate
Fig. 14. Equine larynx, lateral view. From Guide for the Laboratory
Examination of the Anatomy of the Horse, Horowitz, A., 1965.
palatopharyngeal arch
nasal
pharynx
In addition to the hyoid apparatus,
significant in pharyngeal and
choanae, are shown in Fig. 15.
cricothyroideus
bone structures that are most
swallowing functions, and the
Fig. 15. Ventral view of the equine skull at the choanae. Figure is from A TextBook of Veterinary Anatomy , Septimus Sisson, 1910: Saunders, Philadelphia.
palatine bone,
horizontal lamina
choanae
hamulus
pterygoid bone
vomer
muscular process of
tympanic temporal bone
styloid process of
Fig.16. Equine skull, left lateral view, cranium and caudal facial parts. The
mastoid temporal bone
zygomatic arch and zygomatic process of the frontal bone are largely removed.
Figure is from A Text-Book of Veterinary Anatomy , Septimus Sisson, 1910:
Saunders, Philadelphia.
styloid process of the
mastoid temporal bone
palatine bone,
perpendicular lamina
palatine bone,
horizontal lamina
hamulus of the
pterygoid bone
muscular process of the
tympanic temporal bone
The soft palate is a musculomembranous flap that attaches rostrally to the free
margin of the horizontal lamina of the palatine bone by a thin aponeurotic sheet, the
palatine aponeurosis. It ends caudally in a free border that is in relation to the
pharyngeal opening of the larynx, the aditus laryngis. Muscles that move the soft
palate are:
1. The tensor veli palatini, which extends from the muscular process of the
tympanic temporal bone and the lateral wall of the auditory tube to the
palatine aponeurosis in which its tendon-fibers expand; this muscle firms up
the aponeurosis, which is the origin of the palatopharyngeus and palatinus
muscles;
2. The levator veli palatini arises from the muscular process of the tympanic
temporal bone medial to the tensor muscle (1., above) and their tendons are
fused where they arise from the bone. The muscle passes rostroventrally and
turns medially into the soft palate, inserting with its fellow on the fibrous
tissue (raphé palati) on the midline of the soft palate. This muscle elevates
the soft palate. The caudal border of the muscle does not quite reach the
caudal free border of the palate. In fluoroscopy of sheep, it can be observed
that the caudalmost part of the soft palate that is unsupported by the levator
hangs down as a narrow flap when the soft palate is raised in eructation. This
diverts much of the eructated gas into the larynx and trachea where it is
inspired in respiration.
3. The palatopharyngeus muscle extends as a sphincter-like mechanism from
the midline of the palatine aponeurosis rostroventrally to the pharyngeal
Fig. 17. Ventral view raphe of the roof of the pharynx caudodorsally. This muscle, in
the horse and dog, passes both medial and lateral to the levator
at the level of the
veli palatini and extends nearly to the caudal border of the soft
choanae with the
palatine aponeurosis palate. In swallowing, contraction of this muscle and the
pterygopharyngeus draws the roof of the pharynx forward while
and the insertiontendons of the tensor at the same time elevating the soft palate. This action of
veli palatine muscles drawing the roof of the pharynx forward places the pharyngeal
tendon of the hyopharyngeus rostral to the bolus. From this
added. The tendons,
point, serial cranial to caudal contraction of the hyo-, thyro-, and
each protected by a
cricopharyngeus muscles moves the bolus into the esophagus.
small bursa (shown
only on the one side
in this figure), pass
lateral to the
hamulus of the
pterygoid bone.
Choanae are
demarcated by
yellow arrows.
Figure is modified
from A Text-Book of
Veterinary Anatomy,
Septimus Sisson,
1910: Saunders,
Philadelphia.
palatine
aponeurosis
hamulus
bursa
tendon of tensor
veli palatini
Fig. 18. Deep dissection with the pterygoid muscles cut away to show the muscles
of the tongue, soft palate, hyoid muscles, and pharynx. The tensor veli palatine is
shown in color, taking origin from the muscular process of the tympanic temporal
bone and with some myofibers arising from the lateral wall of the auditory tube.
Deep to the muscle is the levator veli palatini (colored in Fig. 19) with which its
tendon of origin fuses. Both muscles are lateral to the auditory tube . Inset is for
orientation. Guide for the Laboratory Examination of the Anatomy of the Horse,
Horowitz, A., 1965.
tensor veli palatini
origin of the pterygoid
muscles (muscles largely
cut away)
muscular process of
tympanic temporal bone
m
lateral cartilaginous
wall of auditory tube
levator veli palatini
Fig. 19. Deeper dissection with the belly of the tensor veli palatini muscle and
much of the stylohyoideum removed; the hyoglossus is cut in its middle and
reflected to reveal the ceratohyoideus. Figure is from Guide for the Laboratory
Examination of the Anatomy of the Horse, Horowitz, A., 1965.
origin-tendon of
tensor veli palatini
(cut)
Insertion-tendon of
tensor veli palatini,
reflected forward
levator veli palatini
stylopharyngeus
hamulus
ceratohyoideus
hyoglossus (cut)
4. The palatinus is a muscle that arises from the palatine aponeurosis and ends
in the caudal free border of the soft palate; this muscle shortens the soft
palate, which becomes thicker, converting the soft palate to a thick, cushionlike soft plug. With the lifting action of the levator veli palatine, the soft palate
closes off the nasal pharynx in swallowing;
Fig. 20. Equine, lateral view. Deepest dissection with the auditory tube and
levator veli palatini cut from the muscular process of the tympanic part of the
temporal bone and drawn forward to expose the roof of the pharynx more
completely. The hyo-, thyro-, and part of the cricopharyngeus muscles are
removed. The left lateral wall of the oral pharynx with the left palatine tonsil is
cut away, exposing the lingual tonsil and the right palatine tonsil. Figure is from
Guide for the Laboratory Examination of the Anatomy of the Horse, Horowitz,
A., 1965.
stylopharyngeus m. (cut from the stylohyoid bone)
pterygopharyngeus m. (cut)
palatopharyngeus m.
palatinus m.
mm
right palatine tonsil (you’re looking at the mucosa-covered right
palatine tonsil)
lingual tonsil (lymphatic tissue beneath the mucosa of the base of the
tongue)
Pharyngeal muscles. The muscles of the pharynx are rostral and caudal
constrictors, which serially constrict the pharynx in moving the bolus into the
esophagus, and the stylopharyngeus, which draws the roof of the pharynx dorsally,
returning it to its resting position after swallowing. The constrictors extend from the
hamulus, soft palate, thyrohyoid bone (thyrohyoideum), and thyroid and cricoid
cartilages to the pharyngeal raphe. The raphe is a median seam of connective tissue
that is dorsal, on the roof of the pharynx. In dogs and cats, the raphe extends from the
pharyngeal tubercle, a small roughness on the ventral aspect of the basilar occipital
bone to the level of the pharyngeal opening of the esophagus, which is about at the
level of the cricoid cartilage of the larynx. In ungulates, the raphe is formed just below
the pharyngeal recess. It ends caudally at the same level as in the dog and cat where
it forms the dorsal origin of the esophagus. The rostral constrictors are the pterygoand palatopharyngeus muscles. The caudal constrictors are the hyo-, thyro-, and
cricopharyngeus muscles. Except in ruminants, the stylopharyngeus is a single
muscle, which passes to the dorsolateral roof of the pharynx. In ruminants, there are
rostral and caudal stylopharyngeus muscles.
Fig. 21. Equine, semi-diagrammatic view of muscles of the pharynx, including
palatopharyngeus m.
muscles of the soft palate. The outline of the mucosa is shown and the relations of
muscles to it. From Handbuch der Vergleichenden Anatomie der Haustiere, W.
Ellenberger and H. Baum, Ed. by Zietzschmann, O., Ackerknecht, E., and Grau, H.;
1943: Springer Verlag, Berlin.
levator veli palatini m.
pterygopharyngeus
m.
stylopharyngeus
m.
palatopharyngeus m.
hyo-, thyro-, and cricopharyngeus mm.
Rostral pharyngeal constrictors. The pterygopharyngeus arises from the caudal
margin and distal end of the hamulus and extends caudodorsally to insert in the
pharyngeal raphe. It passes lateral to the levator of the soft palate. The
palatopharyngeus has been described above with muscles of the soft palate.
Fig. 22. Equine. Deep dissection with the tensor levi palatine muscle and the
stylohyoid bone largely cut away to show the rostral (yellow) and caudal (blue)
pharyngeal constrictors and the stylopharyngeus (red). The palatine tonsil conceals
the palatinus muscle. From Guide for the Laboratory Examination of the Anatomy of
the Horse, Horowitz, A., 1965.
pterygopharyngeus m.
pharyngeal
raphe
palatopharyngeus m.
tendon, tensor veli
palatini (cut, reflected)
palatine tonsil
esophagus
cricoaryt.
dors. m.
Caudal pharyngeal constrictors. The hyo-, thyro-, and cricopharyngeus muscles
arise, respectively, from the thyrohyoideum of the hyoid apparatus, the lateral aspect
of the thyroid cartilage and the caudal margin of the more dorsal part of the arch of
the cricoid cartilage of the larynx. All insert in a rostal-caudal sequence on the
pharyngeal raphe. Their insertion on the pharyngeal raphe is superficial to, and
Fig. 22. Equine. Deep dissection with the tensor levi palatine muscle and the
stylohyoid bone largely cut away to show the rostral (yellow) and caudal (blue)
pharyngeal constrictors and the stylopharyngeus (red). The palatine tonsil conceals
the palatinus muscle. From Guide for the Laboratory Examination of the Anatomy of
the Horse, Horowitz, A., 1965.
substantially overlaps, that of the palatopharyngeus.
hyopharyngeus m.
thyropharyngeus m.
cricopharyngeus m.
Stylopharyngeus. The stylopharyngeus arises from the medial aspect
of the stylohyoideum. In the horse, dog, and pig, the stylopharyngeus passes
ventromedially, caudal to the levator veli palatini, then deeply between the pterygoand palatopharyngeus muscles or between bundles of the palatopharyngeus (pig),
where it expands in the submucosal connective tissue of the dorsolateral pharynx. In
ruminants (Fig. 25), the rostral styopharyngeus muscle arises in linear fashion from
the medial aspect of the stylohyoideum. It passes medially to insert in the pharyngeal
raphe between the pterygopharyngeus and the hyopharyngeus. Following constriction
of the pharynx, contraction of the right and left muscles would elevate the pharyngeal
raphe. The stylopharyngeus caudalis arises from a smaller area of the medial aspect
of the stylohyoideum caudal to the dorsal part of the rostral muscle. It passes caudally,
deep to the hyopharyngeus muscle, to insert in the submucosa of the roof of the
laryngopharynx and, by extension of tendinous fibers, to the thyroid cartilage of the
larynx.
Figs. 23 and 24. Equine. Stylopharyngeus muscle, severed from the medial side of
the upper part of the stylohyoideum. It expands in the submucosa of the dorsolateral
roof of the pharynx. From Guide for the Laboratory Examination of the Anatomy of
the Horse, Horowitz, A., 1965.
Fig.
23
stylopharyngeus m.
Fig.
24
stylopharyngeus m.
Fig. 25. Pharyngeal muscles of cattle. Lateral view. Inset, caudodorsal view,
showing arrangement of rostral and caudal stylopharyngeus mm.
pterygoideus med. m. (cut
away, its tendon of origin
extends caudally over a
narrow area concealed here
by the lat. pterygoid)
pterygoideus lat. m.
tensor veli palatini m.
levator veli palatini
stylopharyngeus rostralis m.
pterygopharyngeus m.
stylopharyngeus caudalis m.
palatopharyngeus m.
hyo-, thyro-, and cricopharyngeus mm.
stylopharyngeus rostralis m.
stylopharyngeus caudalis m.
Hyoid muscles. The hyoid apparatus functions in movements of the base of the
tongue and larynx in swallowing. Its extrinsic muscles move its lower, ventral, part
backward and forward in a swing-like mechanism and its intrinsic, intrahyoid, muscles,
flex the lower part in a manner that draws the larynx forward and upward against the
base of the arching tongue. The extrinsic muscles are the geniohyoideus,
mylohyoideus, occipitohyoideus, omohyoideus (ungulates), and sternohyoideus. The
intrinsic muscles are the stylohyoideus, ceratohyoideus, and hyoideus transversus. In
addition to these muscles, the stylo- and hyoglossus muscles are extrinsic muscles of
the tongue, the thyrohyoideus attaches the thyrohyoideum to the lamina of the thyroid
cartilage of the larynx, the stylopharyngeus raises the roof of the constricted pharynx
after swallowing, and the hyoepiglotticus draws the epiglottic cartilage toward the
basihyoideum, which opens the closed aditus laryngis after swallowing.
As its name indicates, the hyoid apparatus is yoke-like, attached dorsally by right and
left tympanohyoid cartilages that in the manner of a synchondrosis attach the
stylohyoideum (stylohyoid bone) of either side to a well defined styloid process of the
petrous temporal bone in horses and cattle, but to a small roughness of the mastoid
temporal in the dog.
Note: In the dog, the “stylomastoid foramen” appears figure-of-eight fashion with its upper part
representing the stylomastoid foramen that is the external opening of the facial canal; the lower opening
admits the tympanohyoid cartilage, which passes deeply and is seated firmly to an internal roughness of the
mastoid temporal.
Figs. 26. Canine skull.
Fig. 27. Relations of facial nerve
and stylohyoideum of hyoid
apparatus at the stylomastoid
foramen.
facial nerve
stylohyoideum
Geniohyoideus, mylohyoideus. The geniohyoideus passes from the internal
aspect of the body of the mandible paramedian to the symphysis and inserts on the
body, and lingual process (equine, ruminants), of the basihyoideum. This is a strong
muscle that draws the basihoideum rostrally in the act of swallowing. The
mylohyoideus arises from the mylohyoid line on the medial aspect of the body of the
mandible. Its fibers meet those of the opposite side at a median seam that attaches
caudally to the basihyoideum. Contraction of this muscle raises the floor of the mouth,
pressing the tongue against the palate. The caudalmost fibers are inclined caudally
and, in contracting, assist the geniohyoid muscles in drawing the basihyoideum
forward in swallowing. The occipitohyoideus passes from the jugular prcess to the
dorsal part of the stylohyoideum: it swings the apparatus caudally after swallowing.
Occipitohyoideus, sternohyoideus, omohyoideus (ungulates; also in humans).
The occipitohyoideus extends from the jugular process of the occipital bone to the
dorsal part of the stylohyoideum; the omohyoid and sternohyoid muscles have their
attachment to the basihyoideum and scapula (omohyoid) or sternal manubrium
(sternohyoid)> These three muscles draw the basihyoideum caudally following
swallowing or, if the basihyoideum is held in position by the geniohyoidei, (the
mylohyoid muscles probably have a limited effect in this case), the omohyoidei and
sternohyoidei will draw the scapula and sternum, respectively, forward. The latter
actions take place in running (scapula) and inspiration (sternum). To the writer’s
knowledge, the exact extent of each of these actions has not been determined.
Intrahyoid muscles: stylohyoideus, ceratohyoideus, hyoideus transversus.
The stylohyoideus extends from the angle (or, in the dog, upper part) of the
stylohyoideum to the basihyoideum, the ceratohyoideus passes from the
ceratohyoideum (ceratohyoid bone) to the thyrohyoideum, and the hyoideus
transversus passes between the upper parts of the ceratohyoid bones.
Fig. 28. Equine, extrinsic hyoid
muscles. The stylohyoideus,
an intrinsic muscle, is also
shown.
stylohyoideus m.
mylohyoideus m.
geniohyoideus
m.
omohyoideus m.
sternohyoideus m.
Fig. 29. Equine. Ceratohyoideus and hyoideus transversus muscles. The
thyrohyoideus is also shown.
thyrohyoideus m.
ceratohyoideus m.
hyoideus transversus m.
2. Action of the lingual, pharyngeal, palatine, and hyoid muscles in swallowing.
:
Fig. 30
Fig. 31
Basihyoid brought forward by geniohyoideus chiefly; this
accentuates the curvature of the caudodorsally arched tongue.
As a first
action, lingual movements place the bolus (if solid food) on the dorsum of the base of
the tongue. Rostral myofibers of the hyoglossus and styloglossus draw on the rostral
part of the tongue, which is pushed against the palate by the mylohyoideus. The pull
of the hyo- and styloglossus muscles on the rostral tongue arches the base of the
tongue dorsally, an action that is accentuated by contraction of the hyoideus
transversus and by the rostral movement of the basihyoideum. This excursion of the
basihyoideum is easily followed by palpation and observation in animals and
ourselves. The arching tongue pushes the epiglottic cartilage posteriorly, closing off
the laryngeal opening (aditus laryngis). At the same time, the larynx is drawn forward
and upward against the base of the arching tongue.
The soft palate is made shorter by contraction of the palatinus fibers. Soft tissues are
all essentially incompressible fluids so that shortening the soft palate makes it thicker
and, like a soft plug, it is elevated by the levator muscle, closing off the choanae. In all
species that we study, there is a substantial overlap of the palatopharyngeus beneath
(deep to) the hyopharyngeus at the attachment of these muscles at the pharyngeal
raphe. In swallowing, the pterygopharyngeus and palatopharyngeus muscles draw
the pharyngeal raphe forward and with it the hyo- , thyro-, and cricopharyngeus
attachments and the origin of the esophagus. The hyopharyngeus is placed in front of
the bolus and serial contraction of the three muscles against the firm dorsum of the
larynx puts the bolus in the pharyngeal opening of the esophagus.
During ordinary respiration in the normal, mature horse, the epiglottis (epiglottic
cartilage clothed with mucosa) protrudes through the intrapharyngeal ostium into the
nasal pharynx. In swallowing, the shortening of the soft palate and its elevation by
gcontraction of the levator veli palatini and the palatopharyngeus takes place at the
same time that the epiglottis is pushed backward by the arching base of the tongue.
The placement of the epiglottis forward and facing somewhat upward against the
base of the tongue is brought about by contraction of the ceratohyoideus and
thyrohyoideus, which is chiefly a forward movement, and the upward pull of the
stylohyoid muscle. In normal swallowing, the epiglottis is pushed backward and
completely covered by the base of the tongue. It has been shown that, in persons in
which owing to cancer or other disease the epiglottis has been removed in large part,
swallowing takes place without difficulty (Human Physiology, B. Houssay, et al., 1955:
McGraw-Hill). This is due to the close apposition of the soft tissue of the base of the
tongue against the exposed opening into the larynx. An essential feature of
swallowing is the avoidance of contact with the epiglottis of ingested food or water.
At the conclusion of swallowing, the basihyoideum is brought caudally to its resting
position, the stylopharyngeus “opens” the constricted nasal pharynx and the
hyoepiglotticus (shown in the horse in Fig. 14; in dogs, the muscle splits from its
single attachment to the rostral surface of the epiglottic cartilage to an attachment at
either extremity of the basihyoideum) draws the epiglottis forward, restoring the
normal opening of the aditus laryngis.
Fluids do not pass over the dorsum of the tongue but are shunted to the sides,
passing in the piriform recesses at this level. Fluoroscopic images of suckling babies
show a nice circle where the fluid, complemented with radio-opaque barium sulfate,
then reunites into a stream that enters the esophagus.
Fig. 32. Figure is from Sisson. Horse, dorsal view with roof of the nasal pharynx and
laryngopharynx sectioned on the dorsal midline at the pharyngeal raphe and onto
the esophagus. The right figure shows the course of fluids when, as in swallowing,
the base of the tongue would be apposed to the epiglottis, bending it back against
the arytenoids and closing off the aditus laryngis (laryngeal entrance). You are
looking down on the dorsal, nasal, aspect of the soft palate. In the normal mature
horse, the resting soft palate would be in this position, with the epiglottis protruding
into the nasal pharynx. In figures showing a median section of the head of the dog,
the caudal border of the soft palate rests usually below the epiglottis.
esophagus
trachea
palatopharyngeal
arch
piriform recesses
nasal pharynx
soft palate
Fig. 33. Dog, median section through the head. Note the position of the soft
palate’s free border, which rests here on the base of the tongue, below the
epiglottis. Drawing is by Barbara Bailey at the University of Saskatchewan.
The essential features of swallowing as described here take place in the mammals
that we study. This concludes this presentation.