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.
R and L arytenoid cartilages
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.
body, thyroid cartilage
arch, cricoid cartilage
tracheal cartilage 1
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.
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
arch of cricoid cartilage
tracheal cartilage 1
body of thyroid cartilage
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 two-thirds 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.
cutaneus colli (larger caudal part with sternal origin)
approximate position of fibrous plate from which the sterohyoideus muscles and sternothyroideus muscles arise
mandibular lymph nodes
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.
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)
Fig. 9. Equine (horse), ventral view, showing relations in the mandibular space.
parotid duct (usually more exposed than shown in this specimen, lies ventral to the facial vein upon the medial pterygoid muscle on the medial side of the mandible)
position of basihyoideum
mandibular lymph nodes (shown as if transparent)
medial pterygoid m.
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)
facial artery facial vein
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.
epihyoideum (small, distal tip fused with the stylohyoideum in the mature animal)
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.
tendon of stylohyoideus muscle (reflected to the right)
thyrohyoideum (normally fused with basihyoideum in the horse)
basihyoideum (lingual process)
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 be involved in the condition of dorsal displacement of the soft palate.
Fig. 13. Horse (“Tucker”), face and throat region have been shaved, lateral view.
trachea and sternohyoideus
palpate cricoid arch here
Fig. 13a. Parotid gland and veins.
The parotid gland is bounded ventrally by the linguofacial vein.
external jugular 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.
cricoid cartilage is here
trachea (covered ventrally by sternohyoideus) is here
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.
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.
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)
“Tucker,” ventral view of throat and mandibular space.
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.
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.