Veterinary Notes

Respiratory, Cardiovascular and Lymphatic Systems

General Features of Respiratory and Cardiovascular Systems

• Cells need oxygen for metabolic functions and must eliminate carbon dioxide (waste product of respiration).
• Small organisms obtain oxygen and eliminate waste through simple diffusion.
• Larger organisms require specialized surfaces for gaseous exchange (respiratory system) and internal fluid transport systems (cardiovascular system).
• Vertebrates show adaptations in these systems, with a dichotomy between gill-breathing aquatic organisms and lung-breathing terrestrial vertebrates.
• The cardiovascular system has evolved towards reduced blood vessel complexity and refined pump system (heart), resulting in double circulation (pulmonary and systemic).
• Fish have a primitive single circulatory system, unlike higher vertebrates.

Evolutionary history of gaseous exchange and transport systems

• Fish: Achieve gaseous exchange via gills.
• Cartilaginous fish (sharks) have separate gill slits.
• Bony fish have an operculum (flap) covering the gills.
• Water flows past gills via fast swimming or a pumping mechanism involving buccal musculature and branchial/opercular valves.
• Blood from the ventral aorta enters the afferent branchial capillary bed in gill lamellae (high surface area/volume ratio).
• Gaseous exchange occurs along the short diffusion path of gill membranes.
• Oxygenated blood leaves through efferent branchial vessels to the systemic circulation via the dorsal aorta.
• Lungs appeared early in vertebrate history, possibly more common in ancestral fish.
• Modern lungfish are an offshoot of lobe-finned fish, from which terrestrial air-breathing animals evolved.
• Lungs developed from an outpouching of the gut, resulting in ventral lungs (stability problems for aquatic organisms), overcome by dorsal migration of the lungs in modern lungfish.
• In most modern bony, ray-finned fish (teleosts), a hydrostatic organ called the swim bladder has evolved as the homologue of the lungs of their ancestors.
• Some retain a connection with the gut via a pneumatic duct, but in advanced forms, the direct connection is lost. Gas volume of the swim bladder is progressively isolated.

Transition to Air Breathing

• Air breathing has occurred independently in several amphibian and reptile lines.
• Air-breathing vertebrates needed to support themselves against gravity, leading to increased ossification of the skeleton and development of limbs.
• Changes in the cardiovascular system occurred as veins from the gills became redundant, and a new venous return system developed from the lungs (pulmonary vein).
• The pulmonary circulation was progressively isolated, becoming a completely separate circuit in parallel with systemic circulation.
• The heart has undergone modification.
• Reptiles have a septum dividing the ventricle (incompletely, except in crocodiles).
• Mammals and birds have a four-chambered parallel pump system for proper perfusion of separate systemic and pulmonary circulations.
• Vessels of the aortic arch system (originally associated with blood supply to gills) became redundant and regressed. This culminates in birds and mammals by the retention of only one functional aortic arch in the adult (the right in birds and the left in mammals).

Birds

• Birds have comparatively small lungs relative to their body size but have evolved an efficient means of ventilating their lungs, due to the energetic cost of flight.
• Avian lungs are associated with a system of air sacs (gaseous exchange does not occur here) which convert a tidal ventilatory system into an efficient flow-through system in the lungs.
• Air is drawn into the system by the forward and upward movement of the ribs, but effectively bypasses the lungs via the main bronchus to reach the posterior air sacs.
• The arrangement of the air sacs allows oxygenated air to enter the true lungs both on inspiration and expiration.
• This results in a lower effective dead space and therefore higher partial pressure of oxygen of the air in the lung.
• A counter-current exchange system of blood capillaries and air capillaries of the avian lung leads to a greater extraction of oxygen, allowing birds to cope with the energetic demands of flying, even at high altitudes.

Respiratory System

General Characteristics

• Major anatomical features have evolved to:
  • Exclude particulate material
  • Maximise surface area for gaseous exchange
• The pharynx has a dual role in the digestive and respiratory systems.
• Protective devices have developed which close off the conducting part of the respiratory system during swallowing (skin flap in simpler tetrapods, larynx in higher mammals).
• The larynx prevents aspiration of solid matter and exerts a regulatory influence on the rate and quality of air inflow.
• It is also associated with phonation (sound production).
• The lumen of the larynx is the glottis, and forms the entrance to the trachea, the main air-way.
• The trachea varies in length mainly according to the length of the neck.
• It is stiffened by cartilaginous structures (irregular nodules in amphibians, cartilaginous rings in amniotes).
• The trachea bifurcates terminally into two principal bronchi which in turn divide into further bronchi from which arise bronchioles.
• Structures beyond the level of bronchioles make up the respiratory component.
• Bronchioles branch into smaller respiratory bronchioles (absent in some species), and these in turn branch to become alveolar ducts which end in air sacs or alveolar sacs.
• These sacs have alveoli on their surface, and it is here that gaseous exchange occurs.
• Respiratory bronchioles may also have alveoli on their walls.

The Mammalian Respiratory System

The mammalian respiratory system is divided into:

• The conducting component:

  • nose, nasal cavity, nasopharynx
  • larynx
  • trachea, bronchi and bronchioles

• The respiratory component:

  • respiratory bronchioles
  • alveolar ducts
  • alveoli

• The pumping mechanism:

  • diaphragm
  • rib cage
    The respiratory system is also associated with:

• phonation (voice production)

• olfaction (sense of smell).

A. Conducting Portion

Nose

The wall of the nose is supported by bone or cartilage, and is lined by mucous membrane.

The bony support of the nasal wall is formed by:

• nasal bone
• maxillary bone
• incisive bone
• frontal bone
• lacrimal bone
• zygomatic bone
• ethmoid bone (perpendicular plate)

The floor of the nasal cavity is the roof of the oral cavity. The bones involved are the horizontal parts of:

• incisive bones
• maxillary bone
• palatine bone

The nose consists of

• External nose (apex)
• Paired nasal cavities
• Paranasal sinuses

1. External Nose - Apex

The apex of the nose is supported by the dorsal and ventral lateral nasal cartilages, extensions of the cartilaginous nasal septum. The shape varies considerably in the domestic mammals

• Carnivore, pig and ruminant: the cartilages come together laterally and complete the lateral wall
• Pig: additional medial support is provided by the rostral bone, which is set against the rostral end of the nasal septum, and gives attachment to the nasal cartilages.
• Horse: these cartilages do not project very far laterally and an additional comma-shaped cartilage, the alar cartilage, is attached to the rostral end of the nasal septum. This gives dorsal, medial and ventral support. There is no cartilaginous support laterally, allowing for great distension of the nostrils because the horse is an obligate nose-breather and cannot pant. There is a blind-ending nasal diverticulum dorsally.

The nostrils (external nares) which are at the apex of the nose lead into the nasal cavity. The appearance of the nostrils and surrounding skin varies with species

• Sheep, goat, carnivore: the thick, hairless skin of the nose is the planum nasale. A median groove, the philtrum, divides the upper lip.
• Pig: the thick skin of the snout is the planum rostrale. Philtrum is shallow.
• Ox: the thick skin extends to the upper lip and is called the planum nasolabiale.
• Horse: normal hairy skin around the vastly distensible nostrils.

The planum of the ruminant and pig possesses glands which keep the area moist. These are absent in carnivores, but glands in the nasal septum, lateral nasal gland and lacrimal glands perform the same function.

2. Nasal Cavity

The nasal septum divides the nasal cavity into the right and left halves. The septum consists of the bony perpendicular plate caudally, continuous with the cribriform plate, and the cartilaginous nasal septum rostrally, supported by the vomer.

A large part of each nasal cavity (left and right) is occupied by the dorsal, middle and ventral nasal conchae (turbinate bones). These are thin bony scrolls projecting from the lateral wall almost to the nasal septum

• Dorsal nasal concha - an elongated slightly curled scroll.
• (Middle nasal concha - small concha dorsal to ethmoid concha.)
• Ventral nasal concha - tightly folded series of scrolls.

The caudal part of each cavity is occupied by the ethmoidal conchae (ethmoturbinates), associated with the ethmoid bone.

In the rostral portion of the nasal cavity, the mucosa of the lateral wall forms a number of folds which extend from the nasal conchae to the nostril. The straight fold is continuous with the dorsal concha. Ventral to this is the alar fold continuous with the ventral concha. The alar fold is a bulbous swelling, particularly in the dog, causing air entering the nostrils to be diverted dorsally, medially and ventrally with resultant increase in velocity and evaporative effect. In the horse, the alar fold contains the lamina of the alar cartilage.

The dorsal and ventral conchae divide the cavity into 3 meatuses:

• Dorsal meatus - between roof of the nasal cavity and dorsal concha.
• Middle meatus - between dorsal and ventral conchae.
• Ventral meatus - between the ventral concha and floor of the nasal cavity.

Structures associated with nasal cavity:

a) Incisive ducts

*Paired ducts connecting nasal and oral cavities. The nasal opening is in the ventral meatus at the level of the canine tooth. The oral opening is on the incisive papilla just caudal to the upper incisors.
*In the horse, the duct does not open into the oral cavity, but ends blindly under the oral epithelium.

b) Vomeronasal organ (Organ of Jacobson)

*Paired ducts in the floor of the nasal cavity on each side of the nasal septum, supported by cartilage. They run forward from a blunt end at a level somewhere between the 2nd and 4th cheek teeth to open into the incisive ducts at about the level of the canine teeth
*The epithelium of the ducts has both respiratory and olfactory mucosa. The organs are thought to be concerned with special olfactory functions associated with detection of pheromones, and they are associated with the relationship between taste and smell.

c) Lateral nasal gland

*A serous gland of microscopic proportions, in all domestic species except the ox. Located near the nasomaxillary opening, the duct opens into the middle meatus close to the end of the straight fold
*The secretion moistens the inhaled air, passes through the incisive duct to play a role in the functioning of the vomeronasal organ. In the dog it also moistens the nose and is part of the thermoregulatory system.

d) Naso-lacrimal duct

*Opens into the nasal cavity and is located in the floor of the nostril at the junction of skin and mucosa, where pigmentation stops. There is a second opening of the nasolacrimal duct in the pig - and often in the dog - on the lateral surface at the ventral concha near its caudal end.

3) Paranasal Sinuses

The paranasal sinuses are diverticula of the nasal cavity, into the surrounding bones. They are lined with mucoperiosteum which is continuous with the nasal cavity. They originate in the embryo from sprouts of nasal epithelium that gradually excavate the adjacent cranial bones and into some nasal conchae, and by their enlargement hollow out these bones. The sinuses continue to enlarge as the skull matures.

Function

Not fully understood, but possible functions include:

• Lighten the bones of the skull
• Thermally insulate the nervous centres
• Protect eyes, nasal passages and cranial cavity
• Absorb shock to the head
• Impart resonance to the voice
• Increase the area of olfactory membrane

All species possess maxillary and frontal sinuses, some with additional diverticula into other regions:

i) Maxillary sinus

*Largest, connected to the middle meatus via the nasomaxillary opening. In some species it has diverticula into:

• hard palate: Palatine sinus
• sphenoid bones: Sphenoid sinus
• medial aspect of the orbit: Lacrimal sinus
• nasal conchae: Conchal sinuses

ii) Frontal sinus

*Opens into the ethmoidal meatus of the nasal cavity, except in the horse, where it communicates with the nasal cavity via the caudal maxillary sinus. The ethmoidal meatus and nasomaxillary opening are very narrow and any swelling of the mucous membrane will close them and stop drainage from the sinus.

Horse Paranasal Sinuses

Four pairs:

• Maxillary -rostral and caudal parts.
• Frontal, with dorsal conchal sinuses, sometimes called conchofrontal.
• Sphenopalatine and ethmoidal of lesser importance.

Maxillary - divided by an oblique bony septum into communicating rostral and caudal parts, draining through the slit-like nasomaxillary opening into the middle meatus. The ventral part of both is further divided into connecting medial and lateral compartments, by a longitudinal plate housing the infraorbital canal. These sinuses have important relationships to embedded portions of caudal cheek teeth, and enlarge considerably with age as these teeth are extruded and migrate rostrally.

*Surgical access to this sinus may be required for removal of cheek teeth or to drain infection. Access is restricted by the nasolacrimal duct and the infraorbital nerve. The potential operating area is defined by the following boundaries

• Ventral - facial crest
• Dorsal - line parallel to the facial crest that intersects the infraorbital foramen
• Cranial - an oblique line joining the rostral limit of the crest to the infraorbital foramen
• Caudal - a vertical line tangential to the rostral limit of the orbit.

Frontal (conchofrontal): incompletely divided by bony lamellae. No direct drainage to nasal cavity: instead drains via large oval frontomaxillary opening into caudal maxillary sinus.

Ox Paranasal Sinuses

Six pairs of sinuses: Maxillary with palatine, lacrimal, sphenoidal and conchal connections.

*Frontal - completely surrounds the cranial cavity, and excavates the frontal, parietal, interparietal, part of the temporal, and occipital bones. NB: A cornual diverticulum develops in adult cattle, so dehorning adults exposes the mucoperiosteum of the frontal sinus.

Pig Paranasal Sinuses

Five pairs of sinuses: Maxillary with lacrimal, sphenoidal and conchal connections.

*Frontal: this sinus is large and separates external and internal walls of the frontal and parietal bones by up to 5 cm. As a consequence of the deep location of the brain, the pig cannot be stunned reliably by mechanical means. Electrical stunning is therefore the preferred and more humane method of stunning in pigs.

Canine Paranasal Sinuses

Poorly developed; two pairs of sinuses.

*Maxillary: sometimes called maxillary recess as it communicates freely with the nasal cavity. Important as the upper P4 (carnassial tooth) is embedded in the lateral wall of the sinus. Tooth root infection can be manifest as infection and fistulation of the sinus.
*Frontal: small and insignificant.

Cat Paranasal Sinuses

Maxillary: same as dog, with sphenoidal extension of the maxillary sinus. N.B. Due to the shortening of the nasal passages the roots of upper P3 and P4 are just ventral to the orbit; infections of these teeth may present as ocular dischargeinto the lower conjunctival sac.

Frontal: blockage or infection of the frontal sinus can be alleviated by irrigation of the ethmoidal meatus, which is in the rostral wall about 4 mm from the midline. The sinus can be opened between the midline and the caudal half of the orbit. Note that the olfactory bulb and frontal lobe of the brain lie directly beneath the ventral wall of the sinus, so a flexible catheter should be used, directed rostrally.

Microanatomy of Nasal Cavity

The nasal cavity is divided respiratory and olfactory regions.

a) Respiratory region

Epithelium: pseudostratified columnar, ciliated, with goblet cells, on a thick basement membrane.

The mucoperiosteum of the paranasal sinuses varies from cuboidal or squamous to thin pseudostratified ciliated columnar, with fewer goblet cells and nasal glands than in the nasal cavity proper.

Lamina Propria and Submucosa - loose connective tissue blending with periosteum or perichondrium. Contains numerous leucocytes, especially eosinophils and lymphocytes; and simple branched tubuloacinar mixed nasal glands. Deeper layers contain erectile venous plexuses with smooth muscle sphincters.

b) Olfactory Region

Located on the ethmoturbinates, part of dorsal turbinates, nasal septum and vomeronasal organ. It can sometimes be distinguished grossly from adjacent respiratory mucosa by its yellowish pigmentation. Microscopically, it is thicker than respiratory mucosa.

Epithelium - consists of 3 different cell types:

*Sensory (olfactory) cells - bipolar neurons extending through the entire height of epithelium, with apical dendritic processes.
*Sustentacular (supporting) cells - slender cells with numerous microvilli and six to eight modifies cilia on free apical surface. The basal portions taper to thin processes that surround the basal cells.
*Basal cells - roughly spherical, nuclei close to basement membrane.

Lamina Propria and Submucosa - loose connective tissue containing many leucocytes, plasma cells, and simple branched tubuloacinar olfactory glands, mainly serous (Bowman's glands). These glands continually flush the surface of the epithelium to remove odiferous substances. Deeper layers have erectile plexuses and bundles of non-myelinated olfactory nerve fibres (filum olfactorum) which are the axons of the bipolar olfactory neurons.

Functions of the Nasal Cavity

*Olfaction: Macrosmatic - good sense of smell (most animals). Microsmatic - poor sense of smell (humans). Anosmia - no sense of smell; may follow head trauma, severe rhinitis, meningitis etc.
*Filtration of inspired air - dust and bacteria are trapped in the mucous layer and removed by ciliary action.
*Warming and humidifying air - inspired air must be humidified to prevent drying of the thin film of moisture air which covers the alveolar surfaces in the lung. Warm air has an increased moisture-holding capacity, so the air is warmed as it passes over the complexes of erectile veins in the submucosa. Humidified air is also necessary for olfaction as the odiferous molecules must be in solution to be detected. Nasal and Bowman's gland secretions assist this. Warming the air passing through the nasal cavity also contributes to thermoregulation.

Nasopharynx

Air leaving the nasal cavity passes into the nasopharynx through the choanae (internal nares), separated by the vomer, dorsal to the palatine bone.

The nasopharynx connects with the middle ear via the auditory (Eustachian) tube. The tube is confined by an inverted cartilaginous trough except along its ventral border. In the horse the auditory tube evaginates through this ventral defect in the cartilage support to form the gutteral pouch dorsolateral to the pharynx.

The Gutteral Pouches

The gutteral pouches are large, thin-walled sacs with an average capacity of $300 mis$ each. They are lined by pseudostratified columnar epithelium with goblet cells producing small amounts of mucous secretion.

The air-filled pouches occupy the whole of the space between the base of the cranium, atlas and pharynx. Their floor covers the stylohyoid bone, which impinges on the pouch dividing it into a small lateral and a large medial part. Medially the two sacs back onto one another, their mucous membranes forming a thin median septum.

There are numerous important structures associated with the walls of the gutteral pouches, covered only by thin mucous membrane and vulnerable to damage.

Blood vessels

*External and internal carotid artery
*Maxillary artery and vein
*Ventral cerebral and transverse facial veins

Nervous structures

*Facial VII
*Glossopharyngeal IX
Vagus X
*Accessory XI
*Hypoglossal XII
*Cranial cervical ganglion.

Salivary glands

*Parotid
*Submandibular.

Each pouch communicates with the pharynx through the slit-like pharyngeal orifice of the auditory tube on the lateral wall of the pharynx, just below the level of the posterior nares. The slit is supported by cartilage and opens when the horse is grazing and swallowing. This allows drainage of mucous secretions from the pharyngotubal opening at the rostral end of the pouch, the most dependent part when the head is lowered.

In pathological states, secretion may become excessive, or the pharyngotubal opening may become blocked. Under these circumstances fluid may accumulate, requiring endoscopic or surgical flushing. Surgical access to the pouch is difficult, an the route of choice is through Viborg's Triangle.

This triangle is outlined by

*The linguo facial vein,
*The ramus of the mandible, and
*The tendon of the sternocephalic muscle

Function of gutteral pouches

This has been a source of considerable debate. Two postulated functions are:

*Selective cooling of the brain
*Allowing loose attachment of the pharynx to increase efficiency of swallowing in long-headed aanimal

Larynx

A short musculocartilaginous tube connecting the nasopharynx with the trachea. It is supported by the hyoid apparatus which articulates with theskull at the mastoid process.

Hyoid apparatus

*Bilateral structure joined at the basihyoid. Consists mainly of paired bones:

*Thyrohyoid bone (articulates with the thyroid cartilage)
*Keratohyoid bone
*Basihyoid (unpaired, lies transversely)
*Epihyoid bone
*Stylohyoid bone
Tympanohyoid cartilage (articulates with skull at mastoid process)

The hyoid apparatus suspends the larynx, and also supports the root of the tongue.

Functions of the larynx

i) Assists olfaction - blocks off oral cavity, ensuring air passes through nasal cavity, especially in horses as obligate nose-breathers ii) Deglutition - prevents foreign material passing into trachea iii) Respiration - maintains pathway for air, and controls the amount of air entering or leaving the lungs. iv) Regulation of intrathoracic pressure - capable of acting as an inlet or exit valve. v) Phonation - vocal folds are adapted to produce sound.

Structure

*The larynx consists of several articulating cartilages:

*Epiglottis - elastic cartilage; pointed triangular shape, forming a ventral spout-like entrance to the larynx.
*Thyroid - hyaline cartilage; forms a deep trough open dorsally, formed from 2 lamina (left and right) and body.
*Cricoid - hyaline cartilage; a complete ring, broad dorsally and narrow ventrally.
*Arytenoids - PAIRED, triangular in outline, hyaline cartilage; they lie dorsomedially, and have several processes:

i) muscular process: lateral, crest-shaped, for muscle attachment

ii) cuneiform process: elastic cartilage (part of epiglottis in horse; absent in ruminant and pig)

iii) corniculate process - elastic cartilage

Interarytenoid - hyaline cartilage; small median sesamoid cartilage between the arytenoids in pig and carnivores.

Articulations and ligaments of the larynx

The laryngeal cartilages form three articulations:

*Cricolhyoid: cricoid with thyroid
*Cricoarytenoid: cricoid with arytenoids
*Thyrohyoid: thyroid with hyoid
A number of ligaments and muscles also connect the cartilages to each other, the trachea and the hyoid apparatus.

Ligaments

*Cricothyroid - cricoid to thyroid
*Cricotracheal - cricoid to trachea
*Vocal - thyroid to arytenoid
*Transverse arytenoid - between arytenoids
*Thyroepiglottic - thyroid to epiglottis
*Hyoepiglottic - hyoid to epiglottis
*Vestibular - thyroid to cuneiform process
The cricothyroid ligament is clinically important as the site of entry to the laryngeal cavity, for example in removal of obstructions. It was the historical approach for Lateral Ventriculectomy (Hobday's operation) carried out to resolve the condition of 'roaring' (laryngeal paralysis) in horses. This has been superceded by the laryngeal tie-back operation.

Wall of the larynx

There are a number of paired mucosal folds in the laryngeal wall:

i) Vestibular Fold Runs from the arytenoid to the epiglottis, enclosing the cuneiform process; encloses the vestibular ligament and ventricularis muscle. Absent in ruminants.
ii) Vocal Fold The vocal fold runs from the arytenoid to the body of the thyroid. It projects further medially than the vestibular fold and encloses the vocal ligament. The glottis is the vocal apparatus, consisting of the vocal folds and the space between them.

iii) Aryepiglottic fold: This runs from the lateral margin of the epiglottis to the arytenoid (dog and horse) or to the cricoid in the cat, forming the boundary of the laryngeal entrance. It runs dorsal to the arytenoid and cricoid in the pig and ruminants.

Lateral Ventricle (Saccule) - A deep blind-ending pocket in the lateral wall of the larynx lying against the medial surface of the thyroid cartilage. Present in the dog, pig and horse. In ruminants and cats, it is only a shallow depression. Its entrance lies between the vestibular and vocal folds. In the pig, the vocal ligament is split and the entrance to the ventricle is between the two halves.

The pig and horse also have a median laryngeal ventricle, a mucosal evagination at the base of the epiglottis. Laryngeal mucosa - stratified squamous epithelium rostral to the vocal folds, and pseudostratified columnar ciliated caudally.

The larvngeal cavity

The laryngeal cavity can be divided into four sections:

i) Aditus laryngis: the entrance to the larynx. Bounded by the aryepiglottic fold, the epiglottis and the corniculate processes.

ii) Vestibule: extends from the aditus to the vocal folds. It is bounded laterally by the vestibular folds and the lateral ventricle.

iii) Rima glottidis, the space bounded by the vocal folds and the vocal processes of the arytenoids.

iv) Infraglottic cavity (caudal compartment): continuous with trachea.

Muscles of the larynx

Extrinsic muscles control movement of the whole larynx relative to the body. Intrinsic muscles control movement of the individual cartilages in relation to each other.

Connect the larynx with the hyoid bone, pharynx and sternum and move the entire larynx, expecially during swallowing (deglutition). Indicates those actually attached to the laryngeal cartilages.

i) Those that move the larynx rostrally: - thyrohyoid - hyoepiglottic - geniohyoid (also assisted by stylohyoid, mylohyoid, , stylopharyngeus, palatopharyngeus).

ii) Those that move the larynx caudally: - sternothyroid (also sternohyoid; and omohyoid in horses)

Bilateral, concerned principally with the opening and closing of the glottis by adducting or abducting the vocal folds. All but cricothyroid attach to muscular processes of arytenoid, and rotate these cartilages, altering the tension in the vocal folds.

i) Abductors (open glottis): Dorsal cricoarytenoid - ONLY ABDUCTOR Arises from dorsal part of cricoid and inserts on muscular process of arytenoid. It abducts the ventral edge of the arytenoid, drawing the vocal fold laterally (abducting).

ii) Adductors (close glottis): Cricothyroid Arises from lateral surface of cricoid and inserts on lateral surface of thyroid. Draws the thyroid and cricoid cartilages closer, which tenses the vocal folds.

Lateral Cricoarytenoid Arises from lateral rostral border of cricoid and inserts on muscular process of arytenoid. Draws the ventral edge of arytenoid ventrally and medially, adducting the vocal fold.

Transverse Arytenoid Arises from muscular process of arytenoid, passes dorsally and inserts on the arytenoid. The action of this muscle is in question as it appears to both abduct and adduct the vocal folds. It probably functions to fine tune the actions of the other muscles.

Thyroarytenoid Wide triangular muscle arising from epiglottis and internal midline of the thyroid and inserting on muscular process of arytenoid. In the dog and horse, the muscle is divided into the ventricularis rostrallly, and vocalis caudally. In all species the caudal part of the muscle is associated with the vocal ligament to form the basis of the vocal fold. Adducts the arytenoid.

Larvngeal innervation

The larynx is supplied by two nerves, both branches of the vagus.

Leaves vagus just beyond the distal ganglion and divides into two branches:

i) External - motor to cricothyroid muscle.

ii) Internal - sensory to laryngeal mucosa.

Terminates at the larynx as the caudal laryngeal nerve, and is motor to all the intrinsic muscles of the larynx except the Cricothyroid

The right and left recurrent larygeal nerves arise from the vagus at different sites:

Right recurrent laryngeal - arises from the right vagus nerve at the level of the second rib, turns dorso-laterally around the right subclavian artery, then courses cranially dorso-lateral to the trachea.

Left recurrent laryngeal - arises from the left vagus at the origin of the aorta, turns medially around the aortic arch then runs cranially between oesophagus and trachea. It is subject to degeneration, causing laryngeal paralysis in dogs and horses.

Blood Supply to the Larynx

The larynx is supplied from two arteries which vary in origin and relative importance between species and individuals.

a) Cranial Thyroid Artery A short vessel arising from the common carotid artery opposite the caudal part of the larynx. It divides into 3 or 4 branches, two of which supply the larynx. This artery is the main supply to the larynx for the horse, ruminants and carnivores.

b) Cranial Laryngeal Artery This artery either arises from the common carotid artery at its bifurcation or from the external carotid artery. It is the main supply to the larynx in the pig and an important supply in carnivores.

Trachea

The trachea is a non-collapsible tube which continues the respiratory pathway from the cricoid cartilage of the larynx to the root of the lung where it bifurcates to form the right and left principal bronchi.

Position

In 2 parts, cervical or thoracic, according to the region of the body it is in. The oesophagus is dorsal to the trachea except in the caudal third of the neck where it lies left of the trachea.

*Common carotid arteries
*Vagosympathetic trunks
*Internal jugular veins (except goat, sheep and often horse)
*Recurrent laryngeal nerves
*Tracheal lymphatic trunks

Thoracic Trachea

Continues caudally in the mediastinum dorsal to the cranial vena cava; crosses the aortic arch on the right side, bifurcates dorsal to the base of the heart and at the level of the 4th-6th intercostal spaces.

Structure

The trachea consists of a series of incomplete 'C- shaped cartilaginous rings, trachea! cartilages. The number of cartilages is not constant in all species and varies even in animals of the same species. The rings are connected together by fibro-elastic annular ligaments.

The free ends of the rings are on the dorsal surface of the trachea and the space between is then filled with connective tissue and smooth muscle, the trachealis. (This muscle lies external to the cartilages in the carnivores and internal in the other species).

The cross-sectional appearance of the trachea varies with the species, with the region of the trachea and on the state of contraction of the trachealis muscle

Microanatomv of trachea

4 major layers

*Mucosa
*Submucosa
*Musculocartilaginous
*Adventitia

Lined with respiratory epithelium; pseudostratified columnar ciliated with goblet cells. The sweep of the cilia, as elsewhere in the respiratory passages, is directed towards the larynx. The lamina propria is relatively thin and fibrous. In the deeper layers of the lamina propria and in the submucosa are seromucous trachea! glands. The recurrent laryngeal nerve carries parasympathetic fibres which stimulate secretion.

Submucosa

Rich in elastic fibres, and is generally thin except dorsally where the cartilages are incomplete.

Musculocartilaginous layer

Made up of hyaline cartilage plates, fibroelastic tissue and trachealis muscle, arranged in a circular fashion. In old animals the cartilage may become calcified.

Adventitia

Loose areolar connective tissue with fat cells which blends with the tissue surrounding the trachea.

Role of the trachea

It must function as a rigid tube otherwise a food bolus passing down the oesophagus, or flexion of the neck, could cut off the air supply. Rigidity is provided by the cartilaginous rings.

It must be capable of expansion to accommodate any increase in volume of air passing to and from the lungs. This is achieved by:
inherent flexibility of hyaline cartilage; cartilage rings are incomplete; longitudinally folded mucous membrane; elastic tissue in the submucosa.

It must trap and remove fine particles of foreign material in the inspired air. Particles are trapped in the tracheal mucous and removed by the beating cilia.

It must be flexible and extensible to allow movements of head, neck and larynx. This is achieved by the cartilage providing rigidity in the form of rings held together by fibroelastic ligaments.

(B) RESPIRATORY PORTION

The organs of respiration are the lungs and they are located within the pleural sacs, which come together medially to form the mediastinum.

PLEURA

The pleura is a thin, glistening serous membrane