Equine larynx, topography, palpation in live animal; summary is at the end.
In lateral view, the hyoid apparatus and cartilages of the larynx look like this:
Fig. 1. Equine hyoid apparatus
and larynx, lateral view. Figure
is from Handbuch der
Vergleichenden Anatomie der
Haustiere by Wilhelm
Ellenberger and Hermann
Baum; revised 1943, by Otto
Zietzschmann, Ernst
Ackerknecht, and Hugo Grau;
Springer Verlag.
Hyoid Apparatus
Stylohyoid bone
Epihyoid bone
Ceratohyoid bone
Thyrohyoid bone
Basihyoid bone
epiglottic
cartilage
R and L
arytenoid
cartilages
cricoid
cartilag
e
thyroid
cartilage
Close to the surface, cartilage and bone are palpable and, with practice, important
parts of the hyoid apparatus, larynx, and associated soft tissues can be distinguished.
The structures that we palpate, and the relations of these structures, is our emphasis
in this presentation. See also the Equine Larynx presentation for a general discussion
of the anatomy of the larynx, its function, and brief references to pathological changes
that may affect it.
The trachea is always of smaller diameter than the larynx. Even with the soft muscle
tissues around it you can follow the horse’s trachea up the ventral neck and the first
bump that you meet ---it will be at the level of the throat--- is the cricoid cartilage of
the larynx. We are palpating the ventral aspect of the trachea and the “bump”
palpated is the ventral part of the arch of the cricoid. The lamina of the cricoid with the
muscles associated with it can also be roughly determined if your fingers follow the
arch dorsally. You can verify that you are palpating the cricoid arch by palpating
ventrally just in front of it. Here, the cricothyroid ligament is present, soft and pliable, a
large, v-shaped, triangular soft spot. You can verify the ligament by feeling to either
side of it the thin, straight edge of the ventral margin of R and L thyroid laminae where
the ligament attaches laterally. The position of the cricothyroid ligament is important
as it is a route of surgical access to the interior of the larynx. With a ventral median
incision, you enter the interior just behind the glottis (glottis is the ring of tissues
surrounding the rima glottidis, which is the name given to the enclosed space). The
interior space behind the glottis is the infraglottic cavity.
Fig. 2. In the larger figure below, arrows point to palpable landmarks. The smaller
inset (next page) identifies palpable ligaments that join the hyoid apparatus to the
thyroid cartilage, the thyroid cartilage to the cricoid cartilage, and the cricoid
cartilage to the first tracheal ring. Of these, the thyrohyoid ligament is felt as a mere
depression; but the cricothyroid and cricotracheal ligaments are distinct.
basihyoid
bone
Inset:
lingual process
body, thyroid
arch, cricoid
cartilage
cartilage
tracheal
cartilage 1
thyrohyoid
ligament
cricothyroid
ligament
cricotracheal
ligament
Fig. 3. Median section, equine
head, oral cavity, pharynx, and
larynx, with arrows indicating areas
of palpation. From Sisson.
Fig. 4. Lateral view showing muscles associated with the hyoid apparatus
and laryngeal cartilages. The position of the deeper lying cricoid cartilage is
shown in green. The muscles are palpable deep to the skin and parotid
gland. The pharyngeal and laryngeal muscles labeled in the figure are
innervated by branches of the vagus nerve.
LONGUS
CAPITIS
OMOHYOIDEUS
STERNOHYOIDEUS
At the cranial end of the soft cricothyroid ligament is a substantial, hard, prominence,
the “body” of the thyroid cartilage. “Body” is not a formal term but is useful in
designating the union of R and L thyroid laminae. It is the body of the thyroid cartilage,
this central joining part, which even in the newborn foal may (key word) exhibit
beginning ossification. The body forms the prominence of the “Adam’s apple” in
human beings. In the horse, the body of the thyroid is not palpated in the throat region
but lies well forward in the mandibular space between R and L halves of the mandible.
Continuing forward from the body of the thyroid is a short, soft, interval and then
another firm structure. This structure is the basihyoid bone (basihyoideum) of the
hyoid apparatus. The basihyoideum is short from side to side but extended cranially
as a long lingual process, which is readily distinguished.
Fig. 5. Interior of the larynx.
median
laryngeal
recess
infraglottic
cavity
body of
thyroid
cartilage
cricothyroid
ligament
arch of
cricoid
cartilage
tracheal
cartilage 1
All of these structures palpated are deep to the skin and, up to the basihyoid bone,
are also deep to the thin, flat sternohyoid muscle, which is medial in position, directly
ventral to the trachea and larynx. The mylohyoideus inserts on the rostral margin of
the body (transverse part) of the basihyoideum. The lingual process of the basihyoid
offers attachment to the strong geniohyoid muscle and, with the body of the
basihyoideum, to the sternohyoid and omohyoid muscles. These muscles are soft and
pliable and offer no impediment to palpation of the process.
The skin of the ventral neck, which is always involved in palpation, is relatively thin,
becomes gradually thicker on the side of the neck, and is thickest dorsally at the crest
of the neck and mane. The increase in thickness is almost entirely due to the dermis
layer; the epidermis is fairly constant in thickness, probably no more than 100 microns.
External to the larynx and trachea, the muscles of chief concern are the
sternohyoideus, omohyoideus, and geniohyoideus. These three muscles arise from
the sternum, medial shoulder, and the area of junction of the right and left halves of
the mandible, respectively. The three insert on the body and lingual process of the
basihyoideum (basihyoid bone) and lie ventrolateral (omohyoideus) and ventral
(sternohyoideus) to the larynx and trachea.
The sternothyrohyoideus muscle arises from the manubrium of the sternum deep to
the sternocephalicus. Right and left muscles are closely bound together for about twothirds of the way up the neck where they display a common fibrous plate (usually
palpable; its position is shown in the figure below). From this plate, four thin muscles
arise, two for each side; the two are the sternothyroideus, which passes
craniodorsally deep to the sternocephalicus and inserts on the caudoventral angle of
the lamina of the thyroid cartilage, and the sternohyoideus, which passes cranially
applied to the ventral aspect of the trachea and larynx and inserts in a paramedian
peosition on the body and lingual process of the basihyoideum. On the ventral midline,
the thin R and L sternohyoideus meet and are the only muscles between the skin and
the cricothyroid ligament.
The omohyoideus muscle arises from the caudal margin of the scapula by way of the
medial scapular fascia. It passes cranioventrally deep to the subclavius and the
omotransversarius and cleidomastoid part of the brachiocephalicus. In front of the
subclavius, the large mass of superficial cervical lymph nodes is interposed between
the omohyoideus deeply and the superficial omotransversarius and cleidomastoid
muscles. The nodes are palpable here in front of the shoulder in association with fat
and abundant loose connective tissue, deep to the omotransversarius and
cleidomastoideus. Cranial to the nodes, the fat and connective tissue are hardly
apparent and the omohyoideus is closely attached to the deep surface of the two
muscles. The omohyoideus continues cranioventrally deep to the sternocephalicus
(easily separated from this muscle by a more abundant loose connective tissue) and
joins the insertion of the sternohyoideus at the basihyoideum. At their insertion, the
mandibular lymph nodes are present, large, superficial, and readily palpable. The
nodes are wedgeshaped, with the open end of the wedge facing caudally. The
mylohyoideus is a thin muscle arising from the medial aspect of the horizontal part
(body) of right and left halves of the mandible. The muscle is composed chiefly of
etransverse fibers that, like a sling, support the tongue; in pressing the tongue against
the roof of the mouth, the muscle contracts firmly. You can easily palpate this action
in your own body. The geniohyoideus muscles lie immediately deep to the
mylohyoideus. Their action is to draw the basihyoideum forward as in swallowing and
in extrusion of the tongue. Caudal fibers of the mylohyoideus are not strictly
transverse but inclined caudally and attach to the body of the basihyoideum.
Fig. 6. Horse, ventral view. Figure is from Handbuch der Vergleichenden
Anatomie der Haustiere by Wilhelm Ellenberger and Hermann Baum; revised
1943, by Otto Zietzschmann, Ernst Ackerknecht, and Hugo Grau; Springer
Verlag.
sternocephalicus muscles
cutaneus colli (larger caudal
part with sternal origin)
approximate position
of fibrous plate from
which the
sterohyoideus
muscles and
sternothyroideus
muscles arise
cleidomastoideus muscles
omohyoideus muscles
sternohyoideus muscles
mandibular lymph nodes
mylohyoideus
Fig. 7. Lateral view. Modified from Handbuch
der Vergleichenden Anatomie der Haustiere by
Wilhelm Ellenberger and Hermann Baum;
revised 1943, by Otto Zietzschmann, Ernst
Ackerknecht, and Hugo Grau; Springer Verlag.
omohyoideus
sternohyoideus
sternocephalicus
omotransversarius
cleidomastoideus
cutaneus colli (larger caudal
part with sternal origin)
Fig. 8. Lateral view, deeper dissection, showing the
omohyoideus muscle’s position. Cutaneus muscles,
trapezius, omotransversarius and
brachiocephalicus (cleidomastoideus and
cleidobrachialis) removed. Modified from Handbuch
der Anatomie der Tiere für Künstler , Wilhelm
Ellenberger, Hermann Baum and Hermann
Ditterich; 1898 - 1911; Dieterich Verlag.
position of the superficial cervical lymph
nodes (deep to the omotransversarius
and cleidomastoideus, superficial to the
omohyoideus)
sternohyoideus
omohyoideus
sternocephalicus
subclavius
Fig. 9. Equine (horse), ventral view, showing
relations in the mandibular space.
sternocephalicus m.
linguofacial vein
parotid gland
sternohyoideus m.
omohyoideus m.
lingual vein
occipitomandibularis m.
facial vein
parotid duct (usually more
exposed than shown in this
specimen, lies ventral to the
position of basihyoideum
mandibular lymph nodes
(shown as if transparent)
masseter m.
medial pterygoid m.
mylohyoideus m.
facial artery
digastricus m.,
rostral belly
Fig. 10. This figure, from Ellenberger – Baum (1894; I think that this is from
Topographische Anatomie des Pferdes, the volume on the head (Kopf)), shows
the usual relationships of the parotid duct, facial vein, and facial artery on the
medial side of the mandible. On the left side. the sternohyoideus and
omohyoideus muscles are removed at their insertions. The duct is ventral and
parallel to the facial vein where they pass on the medial side of the mandible.
mandibular lymph nodes
(largely cut away on the left
side)
mandibular gland
linguofacial vein
facial artery
facial vein
parotid gland
parotid duct
The linguofacial vein forms a ventral boundary for the parotid gland. The tendon of the
sternocephalicus and a thin layer of deep fascia separate the mandibular gland from
the parotid gland, which is the more superficial.
The facial artery and vein and the parotid duct are distinguishable as they lie upon the
medial pterygoid muscle in the mandibular space. The artery is round, smaller,
thicker-walled and will reveal the pulse. It is palpable only rostrally in the space as it
arises from the linguofacial trunk (artery) medial to the medial pterygoid muscle. The
vein and parotid duct are present throughout. The vein is a larger vessel venral to the
artery and easily collapses with digital pressure. The duct is a collapsed, flat tubular
structure, most ventral of the three.
Fig. 11. The hyoid apparatus. From Handbuch der Vergleichenden
Anatomie der Haustiere by Wilhelm Ellenberger and Hermann Baum;
revised 1943, by Otto Zietzschmann, Ernst Ackerknecht, and Hugo
Grau; Springer Verlag.
tympanohyoideum
epihyoideum (small, distal tip fused with
the stylohyoideum in the mature animal)
ceratohyoideum
stylohyoideum
thyrohyoideum
basihyoideum
Save for the tympanohyoideum and the distal cartilaginous tip of the thyrohyoideum,
all of the structures labeled in Fig. 11 are bone with a thin layer of cartilage at their
places of articulation. All develop in endochondral ossification from the 3d and 4th
branchial arches. The small cartilage at the free extremity of the thyrohyoideum forms
a synovial joint with the rostral cornu of the thyroid cartilage of the larynx. The
tympanohyoideum is essentially a synchondrosis uniting the stylohyoideum to the
styloid process of the temporal bone. It has the significance that its ossification and
fusion with the bones that it joins can result in stress fracture of the petrous temporal
bone, giving rise to disturbances of the internal ear and equilibrium, and facial nerve
paralysis (Blythe, LL. 1997. Otitis media and interna and temporohyoid
osteoarthropathy. The Veterinary Clinics of North America. Equine practice. 13(1):21-42).
Fig. 12. Basihyoideum, lateral
view.
ceratohyoideum
basihyoideum
(lingual process)
cartilage
tendon of stylohyoideus
muscle (reflected to the
right)
thyrohyoideum (normally fused
with basihyoideum in the horse)
basihyoideum (body)
Function of the hyoid muscles and the hyoid apparatus. The hyoid apparatus
functions chiefly in swallowing and respiration. Its swallowing functions will be given
most attention in a later presentation. The tympanohyoideum is hyaline cartilage, is
normally entirely cartilaginous, and its limited flexibility is utilized in the rostrocaudal
movement of the hyoid apparatus that occurs with each swallow. In humans, the
hyoid is a single bone with small processes, major and minor cornua, on either side.
The anterior, major, cornua are the larger and are joined to the temporal bone by long
stylohyoid ligaments; the posterior, minor, cornua are joined by short thyrohyoid
ligaments to the thyroid cartilage of the larynx. The hyoid is palpable in ourselves and
its regular excursion can be followed with each swallow.
The respiratory functions of the hyoid apparatus are less obvious. Both the
omohyoideus and sternohyoideus act to pull the hyoid apparatus caudally following its
rostral movement in swallowing. The rostral movement is due to the contraction of the
geniohyoid and mylohyoid muscles. It takes little strength to move the basihyoid back
(caudally) to its resting position. I think (key word) that the connection of the strong
geniohyoid and omohyoid muscles at the basihyoideum functions in a maximum
inspiratory effort in this species. The racing horse has its head extended on the neck
and is making a maximal stride with its limbs. The sternocephalicus muscles are
obviously contracting and I think that electromyography would show that the
geniohyoid-omohyoid are contracting as one and that the sternohyoids are also
contracting with each inspiration. Why is this? The position of the scapula ultimately
determines the length of the stride. The geniohyoid-omohyoid draws the scapula
forward in a maximal stride. Forward movement of the sternum is limited in the horse;
however, forward movement of the sternum in the horse does lower the sternum,
increasing the dorsoventral diameter of the chest as in a maximal inspiration. This
occurs because each sternal rib is longer than the preceding rib and forward
movement, however slight, will increase the volume of the thorax. The analogous
movement takes place in human beings in that a person in respiratory distress will
hold the head and neck rigidly and grasp the head of the bed to take advantage of
muscles attaching to the sternum (sternocephalicus), scapula and ribs (levator
scapulae, serratus anaterior). Animals do the same. For example, the cat with
respiratory edema, holds its head (origin of the sternocephalicus) and its limbs
(scapular attachment of the serratus ventralis) and neck (cervical attachment of the
serratus ventralis) rigidly as far forward as it can reach to take advantage of the
sternal attachment of the former and the scapular and costal attachments of the
cervical and thoracic serratus ventralis, respectively, to effect a maximal inspiration.
The thyrohyoideus, a muscle which extends from the lamina of the thyroid cartilage to
the thyrohyoideum and draws the larynx rostrally toward the base of the tongue, may
Fig. 13. Horse (“Tucker”), face and throat region have been shaved, lateral view.
be involved in the condition of dorsal displacement of the soft palate.
trachea and
sternohyoideus
palpate cricoid arch here
sternocephalicus jugular furrow
Fig. 13a. Parotid gland and veins.
parotid gland
The parotid gland is
bounded ventrally by
the linguofacial vein.
maxillary vein
external jugular vein
linguofacial vein
Fig. 14. Lateral view. Modified from Handbuch
der Vergleichenden Anatomie der Haustiere by
Wilhelm Ellenberger and Hermann Baum;
revised 1943, by Otto Zietzschmann, Ernst
Ackerknecht, and Hugo Grau; Springer Verlag.
maxillary vein
parotid gland
linguofacial vein
cricoid cartilage is here
trachea (covered ventrally
by sternohyoideus) is here
sternocephalicus
In Figs. 13, 13a, 15, 15a, and 15b, the external jugular, maxillary and linguofacial
veins are located in shallow sulci. This is owing to the horse’s head being higher than
its heart and the veins’ lacking significant hydrostatic pressure. In Fig. 14, the artist,
Hermann Dittrich, has shown the veins under higher pressure to illustrate their
position best (the maxillary vein’s passing through the parotid gland is only indicated
by Dittrich, and the vein is often well concealed by the gland). We will see that when
the horse lowers its head to graze, hydrostatic pressure causes the veins of the head
and neck to become more prominent.
Fig. 15. Horse, lateral view. Photo of
horse’s head © Nadezhda Bolotina |
Dreamstime.com.
parotid gland
maxillary vein
(where it
passes
through the
parotid gland)
maxillary vein (where it
passes through the parotid
gland)
linguofacial vein (along the
ventral border of the parotid
gland)
Fig. 15a. From Ellenberger-Baum, 1894,
showing parotid gland and veins. The
thin cutaneous muscles, cutaneus faciei
and colli, are omitted in this figure.
external jugular vein
parotid gland
maxillary vein
linguofacial vein
ext. jugular vein
Fig. 15b. Photo of horse’s
head © Nadezhda Bolotina |
Dreamstime.com.
omohyoideus m.
(passing on lateral
side of larynx)
sternocephalicus m.
sternohyoideus m. (passing
ventral to larynx and trachea)
Fig. 15b.
Palpate the arch of the cricoid
cartilage here.
When the horse lowers its head so that it is below the level of the heart, the veins of
the head and neck are subject to increased hydrostatic pressure and bulge outward,
becoming more prominent:
Fig. 16. Grazing horse. Photo:
Purebred horse grazing ©
Maria Itina | Dreamstime.com.
linguofacial vein
cricoid arch
parotid duct
cricothyroid ligament
facial vein
mandibular lymph nodes
(larger than normal)
Fig. 17. Grazing
horse, “Tucker.”
cricoid arch
area of cricothyroid ligament
Fig. 18. Grazing
horse, “Tucker.”
linguofacial vein
cricoid arch
body of thyroid
cartilage
parotid gland
mandibular
lymph nodes
Summary:
Palpate the trachea, arch of the cricoid cartilage, body of the thyroid cartilage,
basihyoid bone, and its lingual process.
“Tucker,” ventral
view of throat and
mandibular space.
cricoid arch
What’s this?
The first tracheal ring.
Sometimes larger than
the remaining rings but
thin and insubstantial
compared to the firm
cricoid arch. Also
separated slightly more
from the cricoid arch.
See the top 2 figures in
this Summary. Easy to
distinguish the cricoid
arch by palpation.
Palpate the cricothyroid ligament cranial to the cricoid arch:
cricothyroid ligament (cut)
median
recess
“Tucker,” ventral
view of throat and
mandibular space.
cricothyroid ligament
Neck, lateral view:
Find the sternocephalicus, which forms the lower boundary of the jugular
furrow with the external jugular vein;
At the caudal margin of the mandible, this muscle inserts lateral to the
occipitomandibularis;
The parotid gland is large. It extends dorsally to the base of the ear,
caudally to the wing of the atlas, cranially to the caudal margin of the mandible, and
ventrally is bounded by the linguofacial vein; a small projection of this gland passes
onto the masseter and is distinctly observed and palpable a little below the
temporomandibular articulation. The gland covers this entire area behind the
mandible and is itself covered laterally by two thin muscles, the parotidoauricularis
and cutaneus faciei (not much discussed above and the cutaneus faciei has not been
illustrated).
From Handbuch der Vergleichenden Anatomie
der Haustiere by Wilhelm Ellenberger and
Hermann Baum; revised 1943, by Otto
Zietzschmann, Ernst Ackerknecht, and Hugo
Grau; Springer Verlag.
parotid gland
The muscles:
Find the sternocephalicus: manubrium to caudal margin of the mandible.
Find the external jugular vein following the dorsal border of the muscle.
Find the linguofacial vein at the lower margin of the parotid gland. You can see
it when the horse lowers its head or by placing a thumb in the lower part of the jugular
furrow to raise the external jugular vein and its veins of origin, the linguofacial and
maxillary.
Ventral to the parotid gland, the omohyoid and sternohyoid muscles are lateral
and ventral, respectively, to the larynx.
Thin cutaneous muscles cover the ventrolateral neck (cutaneus colli) and the
lateral throat and face (cutaneus faciei). A thin parotidoauricularis muscle passes from
the base of the ear onto the lateral parotid fascia.
This concludes this presentation.