Equine Upper Respiratory Tract - Horse Anatomy
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The nares are supported by nasal cartilages. Unlike other species the ventral and dorsal lateral nasal cartilages, which attach to the rostral end of the nasal septum, do not contact each other. In the hose, the dorsal and ventral cartilages are indistinct or absent. Instead, horses have alar cartilages to support the nostrils, but the lateral walls of the nostrils remain unsupported; allowing greater mobility. The alar cartilages divide the nostril into the dorsal ('false nostril') and ventral ('true nostril'). The dorsal nostril leads to a blind-ending diverticulum in the nasoincisive notch. The ventral nostril leads to the nasal cavity. This is important when placing a nasogastric tube, which must be inserted ventrally. Another species difference is that the nasolacrimal duct opens at the nasal puncta on the ventral floor of the nasal vestibule close to the transition point between the nasal mucosa and the nostril. Horses do not have a nasal philtrum.
The nasal cavity is the area between the nostrils and the cribiform plate, it is divided by a septum into right and left sides. The nasal conchae, also known as ethmoturbinate, are scroll-shaped bones covered with nasal mucosa that project into the nasal cavity. The first endoturbinate is the base of the dorsal nasal conchae. It extends the furthest into the nasal cavity. The second endoturbinate forms the middle nasal concha. Subsequent turbinates are much smaller. The ventral nasal conchus is not formed by turbinates, but by the maxilla. The conchae divide the nasal cavity into three meatuses, which form a common meatus near the nasal septum.
- Dorsal nasal meatus: Between the dorsal nasal meatus and the dorsal concha. It leads directly to the caudal nasal cavity.
- Middle nasal meatus: Between the dorsal and ventral nasal conchae, communicates with paranasal sinuses and leads into the caudal nasal cavity.
- Ventral nasal meatus: Between the floor of the nasal cavity and the ventral nasal concha. This is the largest meatus and is the main passage of airflow to the nasal cavity via the nasopharynx. Nasogastric tubes, passed through the nasal cavity, pharynx and oesophagus to the stomach, must be passed through this meatus to avoid the highly vascularised ethmoturbinates.
- Common nasal meatus: Either side of the nasal septum, communicates with all other meatus.
The paranasal sinuses of the horse are extensive, consisting of six pairs:
- Frontal and dorsal conchal sinuses (known as the conchofrontal sinus)
- Ventral conchal sinus
- Sphenopalatine sinus
- Rostral and caudal maxillary sinuses
The most clinically significant sinuses are the frontal and maxillary. The sinuses all communicate with the nasal cavity to allow drainage. The rostral and caudal maxillary sinuses communicate directly with the nasal cavity. The dorsal, middle & ventral conchal, frontal and sphenopalatine sinuses drain indirectly via the maxillary sinuses. The conchal sinuses lie within the fine, scroll-shaped bones known as conchae or turbinates. These conchae are attached to the lateral wall of the nasal passages. The paranasal sinuses are lined with respiratory epithelium (pseudostratified ciliated columnar) and goblet cells.
The frontal sinus occupies the skull from a point midway between the infraorbital foramen and the medial canthus of the eye to a point midway between the caudal edges of the orbit. The frontal sinus is divided into right and left compartments by a midline septum. The conchofrontal sinus is formed by a communication between the rostromedial frontal sinus and the dorsal conchal sinus. The frontomaxillary aperture is a large area of communication between the frontal sinus and the caudal maxillary sinus, this is important to allow drainage. Blood supply to the frontal sinus is provided by the ethmoidal artery.
The main blood supply is provided by the arterial ethmoid rete, which is an anastamosis between the internal and external ethmoid arteries. A minor suply is provided by the caudal nasal branch of the sphenopalatine artery.
The conchal sinuses include the dorsal, ventral and middle. Each conchus is divided into two compartments, rostral and caudal, by a complete septum.
- Dorsal conchal sinus: This is formed by the caudal compartment of the concha
- Ventral conchal sinus: This is formed by the caudal compartment of the ventral concha
- Middle conchal sinus: Lies within the greater ethmoturbinate, not clinically significant
The blood supply is provided by branches of the sphenopalatine artery. This is the largest sinus and is divided into rostral and caudal compartments by a bony septum. The position of this septum is variable, but it usually lies obliquely across the roots of the 4th and 5th cheeck teeth (Tridan 109, 110, 209, 210). In horses less than 5 years of age, the reserve crown of the 3rd-6th cheek teeth (Tridan 108, 208, 109-111, 209-211) almost fills the maxillary sinus.
The rostral maxillary sinus opens via the nasomaxillary opening into the middle nasal meatus. There is also a communication between the rostral maxillary sinus and the ventral conchal sinus, via the conchomaxillary opening; located just medial to the infraorbital canal. Dorsally, there is communication with the frontal/conchofrontal sinus through the frontomaxillary opening. Between the rostral margin of the frontomaxillary opening and the conchal bulla, there is a passageway which connects the rostral and caudal compartments. This allows the caudal maxillary sinus to drain via the rostral maxillary sinus via the nasomaxillary opening into the middle nasal meatus.
In the horse, the sphenoid and palatine sinus compartments communicate and are hence known as the sphenopalatine sinus. The sphenopalatine sinus drains via the caudal maxillary sinus, with which is communicates freely over the infraorbital canal. This sinus lies under the ethmoidal labrynth.
Also known as: Auditory Tube Diverticulum
The guttural pouches are paired ventral diverticulae of the eustachian (auditory) tubes, formed by escape of mucosal lining of the tube through a relatively long ventral slit in the supporting cartilages. The auditory tube connect the nasal cavity and middle ear and the diverticulum dilates to form pouches which can have a capacity of 300-500ml in the domestic horse. The pouches are normally air filled.
The Guttural Pouch is located below the cranial cavity, towards the caudal end of the skull/wing of atlas. It is covered laterally by the Pterygoid muscles, parotid and mandibular glands. The floor lies mainly on the pharynx and beginning of the Oesophagus. The medial retropharyngeal lymph node lies between the pharynx and ventral wall of the pouches.
Right and left pouches are separated dorsomedially by rectus capitis ventralis and longus capitis muscles. Below this, by fused walls of the two pouches, the median septum is formed.
Each pouch is moulded to the stylohyoid muscle which divides the medial and lateral compartments, the medial compartment being approximately double the size of the lateral one and extends further caudally and ventrally.
The guttural pouch has close association with many major structures including several cranial nerves (glossopharyngeal, vagus, accessory, hypoglossal), the sympathetic trunk and the external and internal carotid arteries. The pouch directly covers the temporohyoid joint. The pouch has an extremely thin wall which is lined by respiratory epithelium which secretes mucus. This normally drains into the pharynx when the horse is grazing.
Several cranial nerves and arteries lie directly against the pouch as they pass to and from foramina in the caudal part of the skull (vessels within mucosal folds that indent the pouches):
- Cranial nerves IX, X, XI, XII.
- Continuation of the sympathetic trunk beyond the cranial cervical ganglion.
- Internal carotid artery.
- Cranial nerve VII - limited contact with the dorsal part of the compartment.
- External carotid artery crosses the lateral wall of the lateral compartment in its approach (as maxillary artery) to the atlas canal. The external maxillary vein is also visible.
Natural drainage of the pouch is throught the slit-like (pharyngeal) openings of the eustachian tube in the lateral wall of the nasopharynx. The connection opens when the horse swallows and grazing normally provides drainage. However, most of the pouch is ventral to his slit, and therefore drainage may be rather ineffective. If blocked, secretions accumulate and the pouch distends producing a palpable swelling.
The function of guttural pouches is largely unknown, however hypotheses have been put forward:
- It may influence internal carotid artery blood pressure. Air pressure varies with phase and forcefulness of respiration and the artery in the mucosal fold is exposed sufficiently to be affected.
- Cerebral blood cooling mechanism. Operates at times of physical stress/exercise, when core body temperature is raised. Vigorous respiration, cool air in guttural pouches, and the exposed artery all lead to cooling of the blood.
The larynx is situated below where the pharynx divides into the trachea and the oesophagus. It is contained partly within the rami of the mandible and extends caudally into the neck. It is important during breathing, vocalisation and deglutition (swallowing). The cartilagenous larynx can be manually palpated in the living animal and is commonly implicated in respiratory conditions such as roaring.
The pharynx is located rostrally to the larynx, whilst the trachea is located caudally. The larynx is suspended from the hyoid apparatus. It is bilaterally symmetrical and 'tube-shaped' and can be described as a musculocartilagenous organ.
Synovial joints can be found between the thyrohyoid bone and the dorsorostral aspect of the thyroid cartilage. Synovial joints include the dorsal joint of thyroid cartilage; between the lateral aspect of the cricoid cartilage and the dorsocaudal aspect of the thyroid cartilage and between the cricoid and arytenoid cartilage. This allows abduction and adduction of the arytenoid cartilages. Movement of the cricoid-arytenoid joint controls the size of the glottic opening, the lumen and the larynx.
Membranes and elastic ligaments attach the laryngeal cartilages, allowing attachment of the epiglottis to the thyroid and cricoid cartilage. The first tracheal ring has attachment with the cricoid cartilage by the cricotracheal ligament.
The thyroid cartilage is a hyaline cartilage and forms most of the floor of the larynx. The fusion of the two lateral plates varies in different species. The rostral part forms the 'Adam's apple'. The thyroid cartilage articulates with the thyrohyoid bone and the cricoid cartilage. It becomes brittle as the animal ages.
The arytenoid cartilages are paired, triangular shaped hyaline cartilages. They lie either side of the cricoid cartilage and articulate with the rostral part of it. A vocal process is present on the caudal surface where the vocal folds attach; a muscular process extends laterally and is the origin of attachment for the cricoarytenoideus dorsalis muscle. A corniculate process extends dorsomedially.
The epiglottic cartilage is an elastic cartilage, which is the most flexible and most rostral type of cartilage. The thinner stalk-like part, is attached to the root of the tongue, the body of the thyroid cartilage and the basihyoid bone. The larger blade-like part lies behind the soft palate and points dorso-rostrally. During deglutition, the large blade part of the epiglottic cartilage partially covers the entrance to the trachea.
The vocal folds are made of (slightly stiffer) elastic ligaments and pass between the arytenoid cartilages and the laryngeal floor. They run caudodorsally, with the ligament positioned medially and the vocalis muscle laterally. Fat surrounds the vocalis muscle. The vocal folds form part of the glottis and secrete mucous. They are used for vocalisation.
The vestibular folds are made of (slightly stiffer) elastic ligaments. The vestibular ligaments are rostral to the vocal ligament. The vestibular folds run caudodorsally, rostral to the vocal folds with the ligament positioned medially and the vocalis muscle laterally.
The vestibule extends from the entrance of the larynx to the arytenoid cartilages and vocal folds. The vestibular folds run parallel, but rostral to, the vocal folds.
The glottic cleft (rima glottidis) is surrounded by the arytenoid cartilages dorsally and vocal cords ventrolaterally. It varies in size and is diamond shaped. The glottic cleft disappears when the glottis is closed. Vocal folds run caudodorsally. The infraglottic cavity extends from the caudal section of the arytenoid cartilages into the lumen of the trachea. It is fixed in size.
The epiglottis is the rostral margin of the larynx. It is a flap of elastic cartilage covered by mucous membrane. It forms the rostral boundary of the larynx and prevents food particles from entering the trachea. The epiglottis can return to its normal size and shape after distortion due to the vast amount of elastic fibres present within.
The intrinsic laryngeal muscles produce changes in caliber of the rima glottis by abducting and adducting the corniculate processes of the arytenoid cartilages and the vocal folds and hence, altering airway resistance. The principle muscle to widen the glottis is the cricoarytenoideus dorsalis. The thyroarytenoideus, arytenoideus transversus, and the cricoarytenoideus lateralis muscles adduct the corniculate processes of the arytenoid cartilages, narrowing the glottis and therefore protecting the lower airway during swallowing.
Cricothyroid muscle (cricothyroideus)
The cricothyroid muscle is innervated by the cranial laryngeal nerve, a branch of the vagus nerve (CN X). It moves the cricoid and arytenoid cartilages caudally to tense the vocal folds.
Dorsal cricoarytenoid muscle (cricoarytenoideus dorsalis)
The dorsal cricoarytenoid muscle is innervated by the caudal laryngeal nerve, a branch of the vagus nerve (CN X). It runs from the dorsal surface of the cricoid cartilage to the arytenoid cartilage. It abducts the vocal process and therefore the vocal fold to widen the glottis and is the principle abductor of the larynx.
Lateral cricoarytenoid muscle (cricoarytenoideus lateralis)
The lateral cricoarytenoid muscle is innervated by the caudal laryngeal nerve, a branch of the vagus nerve (CN X). It adducts the vocal processes and narrows the glottis.
Thyroarytenoid muscle (thyroarytenoideus)
The thyroarytenoid muscle is innervated by the caudal laryngeal nerve, a branch of the vagus nerve (CN X). It runs from the laryngeal floor to the thyroid cartilage and arytenoid cartilage and alters the tension of the vocal and vestibular folds. It forms part of the sphincter muscular arrangement. In the horse, it is divided into 2 parts; the rostral and caudal vocalis, which are situated within the vocal folds and vestibular folds.
Transverse arytenoid muscle (arytenoideus transversus)
The transverse arytenoid muscle is innervated by the caudal laryngeal nerve, a branch of the vagus nerve (CN X). It completes the muscular sphincter arrangment and spans the arytenoid cartilages.
Hyoepiglottic muscle (hyoepiglotticus)
The hyoepiglotticus is a bilobed muscle that originates on the basihyoid bone and inserts ventrally on the epiglottis. In horses, contraction of the hyoepiglotticus muscle pulls the epiglottis toward the basihyoid bone, depressing it against the soft palate. This enlarges the airway during inspiration. Additionally, contraction of the muscle stabilizes the epiglottis during inspiration, preventing its prolapse through the glottis.
The larynx protects the trachea in swallowing, preventing aspiration of foreign material. During swallowing, the larynx is moved rostrally causing the epiglottis to partially cover the laryngeal entrance. Solid foods are carried over the laryngeal entrance by the muscles of the pharynx. Fluids are deflected by the epiglottis. Closure of the glottis also prevents food passing down the larynx. The reflex stimulation of the mucosa promotes the coughing reflex.
The larynx also allows the passage of air to the lungs and increases the intra-abdominal pressure. The glottis can widen by adduction of the vocal folds when breathing is vigorous. In addition to this, the larynx is used for vocalisation.
Laryngeal branch of the superior thyroid artery supplies the rostral larynx and is a branch of the carotid artery.
Laryngeal branch of inferior thyroid artery supplies the caudal larynx and is branching from the subclavian artery from the thyrocervical trunk.
Laryngeal branch of cricothyroid artery branches from the superior thyroid artery.
The larynx is innervated by branches of the vagus nerve (CN X).
Cranial laryngeal nerve has two branches. The internal branch innervates the mucosa and the external branch innervates the cricothyroid muscle and constricts the pharynx.
Caudal (recurrent) laryngeal nerve innervates the intrinsic muscles of the larynx (except the cricothyroid muscle).
The three parts of the pharynx are the oropharynx, laryngopharynx and nasal pharynx. The soft palate separates the rostral part of the pharynx into the nasopharynx (dorsal) and oropharynx (ventral). The laryngopharynx is where these two compartments communicate, located in the caudal part of the pharynx.
- Oropharynx: Extends from the glossopalatine arch in the oral cavity to the base of the epiglottis.
- Nasopharynx: Forms part of the ventral nasal meatus through the choanae. It communicates with the ostia of the auditory tubes, auditory tubes and the guttural pouch.
- Laryngopharynx: Extends from the intrapharangeal opening to the oesophagus and the larynx.
During normal breathing, the soft palate extends to the base of the epiglottis. The free apex of the rostral epiglottis lies dorsal to the soft palate. This means horses are obligate nasal breathers and are unable to vomit. If horses do vomit, it is expelled through the nose.
The muscles are the tensor veli palatine and levator veli palatine. The rostral pharyngeal constrictors are pterygopharyngeus and palatopharyngeus, which arise from the base of the skull. The caudal pharyngeal constrictors are the thyropharyngeus and cricopharyngeaus, which arise from the larynx. The only dilator of the pharynx is the stylopharyngeus caudalis.
The nervous supply comes from the pharyngeal branch of the vagus nerve (CN X) and from the cranial root of the accessory nerve (CN XI). The stylopharyngeous muscle is innervated by the accessory nerve (CN XI). The glossopharyngeal nerve (CN IX) supplies taste to the pharynx.
The trachea begins at the larynx and ends at the hilus, where it divides into the left and right mainstem bronchi. The average length of the trachea in horses is 75-85cm and diameter is 5-6cm. It is comprised of 50-60 cartilagenous rings, which are incomplete dorsally. Between each ring is a tracheal annular ligament. The dorsal tracheal membrane lies on the inner surface of the cartilages and is composed of smooth muscle, mucosa and adventitia. Most of the trachea is cylindrical in shape, but the cervical part is flattened dorsally due to contact with the ‘’longus colli’’ muscle.
Dyce, K.M., Sack, W.O. and Wensing, C.J.G. (2002) Textbook of Veterinary Anatomy. 3rd ed. Philadelphia: Saunders.