The Head

Outline the functions of the nasal cavity

Conduction and preparation of inspired air 

• Moistens air

• Warms air

• FIlters particles 

• Mucus traps particles, which is then swallowed

Heat exchange for cooling the brain

• Airflow cools venous blood

• Subsequent countercurrent flow with arteries will cool arterial blood going to the brain 

Olfaction

• Caudal regions of the turbinates covered by olfactory epithelium

Outline the landmarks of the nasal cavity

• Dorsally: nasal bones

• Ventrally: hard palate 

• Laterally: incisive bone and maxilla 

• Rostrally: nares (nostrils)

• Caudally: nasopharynx 

• Axially: nasal septum

Label the bones on this diagram

Describe the structure of the nares

Surrounded by hairless skin

• Sometimes highly modified, depending on the species

Supported by nasal cartilages which are attached to the nasal septum

Describe the species differences in bovine nares

• Nostrils are surrounded by a smooth, hairless nasolabial plate

• Stratified cornified epithelium

• Serous glands create moisture (nasolabial glands)

Describe the species differences in equine nares

• Horses cannot breathe through their mouth

• No ventral nasal cartilage 

• Incomplete cartilaginous ring: distensible nostrils to improve airflow

• Alar cartilages form comma-shaped nostrils 

  • There is a ventral ‘true’ nostril and a dorsal ‘false’ nostril 

  • False nostril is a skin-lined diverticulum within the nasoincisive notch

Describe the species differences in canine and feline nares

• Divided by medium groove (philtrum)

• Secretions are from the lateral nasal gland

Describe the species differences in porcine nares

• Small nostrils on a flat, mobile snout

• Highly sensitive

• Rostral bone provides the nostril with protection

Describe the species differences in birds

• Slit-like openings (not in diving birds)

• Operculum: overhanging bony flap

Describe the species differences in canine/feline BOAS

• Stenotic nares

• Elongated and thickened soft palate

• Enlarged tongue 

• Narrow trachea

Outline the muscles of the nares

Nostril dilation

• Levator nasolabialis

• Caninus

• Transversus nasi

All are innervated by the facial nerve, and supplied by the facial artery

What is the role of the nasal vestibule?

This is the opening of the nasal cavity

Contains the opening of the nasolacrimal duct

Receives nasal gland secretions in dogs

Describe the borders of the nasal cavity

Dorsal: nasal bones 

Lateral: incisive bones and maxilla

Ventral: palatine bones

Caudal: ethmoidal bone 

Axial: nasal septum

Describe the structure of the nasal cavity

• Extends from the nostrils to the cribiform plate of the ethmoid bone

• Divided into left and right sides by the nasal septum

• Each side is then further divided by nasal conchae

What is the difference between choanae and conchae?

Choanae: paired caudal openings of the nasal cavity

Conchae: scrolls of turbinate bone covered in mucosa

Describe the structure and function of conchae

Structure

• Scrolls of turbinate bone covered in mucosa

• Highly vascular 

Function

• Further increase nasal surface area

What is shown by these arrows?

Which meatus should you intubate?

A tube should always be placed in the ventral meatus to intubate

Describe the species differences in the equine nasal meatus

• 4 nasal meatus

• Dorsal meatus: to olfactory mucosa

• Middle meatus: to paranasal sinuses

• Common meatus

• Ventral meatus: to the pharynx

Describe the species differences in the canine nasal meatus

• Complex turbinates

• More resistance to airflow

Describe the species differences in the bovine nasal meatus

Very small middle meatus

Describe the species differences in the avian nasal meatus

• Rostral concha

• Middle concha

• Caudal concha

What is shown by these numbers?

• Rostral concha

• Middle concha

• Caudal concha

Which epithelium is found in:

• The vestibule

• The nasal cavity

Vestibule: stratified squamous epithelium

Nasal cavity: respiratory epithelium, olfactory epithelium

Describe the structure of the respiratory epithelium in the nasal cavity

• Pseudostratified 

• Ciliated 

• Columnar

• Goblet cells

• Mucous and serous glands in the lamina propria

• Respiratory mucosa covers most parts of the nasal cavity

  • Mucosa: epithelium and lamina propria

Describe the function of the respiratory epithelium in the nasal cavity

• Regulation of air flow by erectile tissue

• Cleaning by cilia

• Humidification

• Warming (variable blood perfusion)

• Protecting reflexes (e.g sneeze reflex)

Describe how olfaction occurs in the domestic animal, including the role of:

• Olfactory cells

• Vomeronasal organ

Ethmoturbinates extend rostrally from the ethmoid bone

These are covered with respiratory epithelium, and also contain olfactory sensory neurons

Sniffing: alters the normal airflow to bring the air into contact with the ethmoturbinates

Olfactory cells

• Non-motile, cilia-like structures

• Olfactory receptors

• Detects chemicals 

• Innervated by cranial nerve I

Vomeronasal organ

• Accessory olfactory sense organ

• Contained within the hard palate

• Has unique chemoreceptors distinct from other olfactory organs

  • May play a role in pheromone detection

Describe how airflow occurs in the domestic animal, and how resistance affects this

More convoluted turbinates: more resistance

The nasal cavity, pharynx and larynx cause over 60% of flow resistance

• Because horses are obligate nasal breathers, they have less complex turbinates to reduce resistance

• Flow is aided by straighter head-neck-thorax alignment 

Resistance = length / radius⁴

Outline the structure of the paranasal sinuses

These are air-filled diverticula of the nasal cavity

• Invaginate skull bones 

Lined by the respiratory epithelium

Innervated from the ophthalmic and maxillary branches of the trigeminal nerve

All species have frontal and maxillary systems, but there are differences between them

These sinuses separate on the left and right side of the head

What is the function of the paranasal sinuses?

Resonating cavities (voice)

Insulation/cooling 

Reduces weight of the skull

Increased insertion surfaces 

Space for teeth

How do the paranasal sinuses of birds differ to other species?

Birds have an infraorbital sinus: a triangular cavity under the skin, rostroventral to the eye

How do the paranasal sinuses of cattle differ to other species?

Frontal sinus extends into the horn

How do the paranasal sinuses of dogs differ to other species?

• Maxillary recess (sinus in other spp.)

• Frontal sinus

How do the paranasal sinuses of horses differ to other species?

• 2 frontal sinuses: frontal, dorsal conchal

• 4 maxillary sinuses: rostral maxillary, caudal maxillary, ventral conchal, sphenopalatine

Which sinus is represented by each colour? This diagram is taken from a horse

• Orange: frontal

• Light green: rostral maxillary

• Dark green: ventral conchal

• Yellow: sphenopalatine

• Dark red: dorsal conchal

• Red: caudal maxillary

Draw a flow chart to show sinus drainage in the horse

What are the tooth routes of:

• 106 and 107

• 108 and 109

• 110 and 111

106 and 107: embedded in the maxilla

108 and 109: in rostral max. sinus

110 and 111: in caudal max. sinus

Outline the structure and function of the nasolacrimal duct

Allows drainage from the medial canthus of the eye to the nasal cavity

This helps to drain excess tears 

Starts at the nasal puncta

Runs within the infraorbital canal, through the maxillary sinus

Continues within the maxilla and exits at 

• Nostril (horses and cattle)

• Nasal cavity (dogs)

What does this diagram show?

The nasolacrimal system

What is shown by these arrows on a CT scan of the horse?

How does sinus disease usually present?

This will usually present as chronic unilateral purulent nasal discharge, and possibly facial swelling

Primary sinusitis: bacterial

Secondary sinusitis: dental, cyst, neoplasia

Describe how you can investigate sinus disease

• Sinus trephination

• Sinus endoscopy

• Radiography can be used to detect fluid lining the sinuses (this may be blood or pus)

• You can also tilt the head and take a second radiograph to see if the fluid has shifted with gravity

What are horns? How do they relate to the sinus system?

Horns are bony structures covered by keratin

In cattle, the frontal sinus goes to the back of the skull (unlike in other spp.), and extends into the base of the horn

Describe how dehorning should be performed across cattle, goats and kids. Include the nerve blocks that should be used

• Disbudding (cauterisation) is preferred in young stock, using a cornual nerve block

• For cattle and goats, cornual and cornual branch of infratrochlear nerve blocks should be used, possibly also the region caudal to the horn

• In goats and kids, the frontal sinus is very small, and excess heat can damage the brain

• During dehorning the frontal sinus will be opened, allowing a route for infection

Describe the innervation of the horn, including a labelled diagram

• Cornual (‘horn’) nerve (in 100% of animals with horns)

• Cornual branch of infratrochlear nerve (in 90% of animals with horns)

• Frontal nerve (in 15% of animals with horns)

• Branches from C1 and C2 (in 5% of animals with horns)

Describe the blood supply of the horn

• Ruminants: branch off maxillary artery (superficial temporal artery and cornual artery)

• The horn is very vascular, therefore dehorning can produce a lot of bleeding if the fetotomy wire is not moved quickly

Describe the general function and structure of the pharynx, including a diagram

Common cavity for food and ingesta

• Oral cavity ventrally (with oesophagus dorsally)

• Nasal cavity dorsally (with larynx and trachea ventrally)

There are three parts of the pharynx:

1. Nasopharynx (red)

2. Oropharynx (green)

3. Laryngopharynx (blue)

Outline the margins of the nasopharynx

Ventral margin: soft palate (soft tissue continuous with palatine bone)

Rostral margin: choanae (opening of nasal cavity into nasopharynx)

Caudal margin: laryngopharynx 

Lateral and dorsal margins: pharyngeal wall

Which type of epithelium lines the nasopharynx?

The nasopharynx is lined with respiratory epithelium

What is the nasopharyngeal recess?

• Caudodorsal extension of the nasopharynx

• Found in most ungulates 

• Blind ending

Outline the borders of the oropharynx

Ventral margin: tongue

Rostral margin: oral cavity 

Caudal margin: laryngopharynx

Dorsal margin: soft palate

Which type of epithelium lines the oropharynx?

The oropharynx is lined with stratified squamous epithelium

Outline the borders of the laryngopharynx

Rostral margin: free tip of the soft palate

Caudal margin: larynx and opening of oesophagus 

Lateral and dorsal margins: pharyngeal wall

Which type of epithelium lines the laryngopharynx?

The laryngopharynx is lined with stratified squamous epithelium

What is critical about the laryngopharynx?

This is where the ‘food’ and ‘wind’ pipes cross over

Describe the structure of the soft palate

• Flexible muscular structure 

• Palatinus muscle: within the main body

• Tensor veli palatini: within the rostral side wall

• Levator veli palatini: within the caudal side wall

• Innervated by the mandibular branch of the trigeminal nerve

Describe the function of the soft palate, and the muscles within it

• Divides the pharynx

• Palatinus muscle: shortens the palate

• Tensor veli palatini: causes lateral traction and thus tension

• Levator veli palatini: raises the palate during swallowing and mouth breathing

Label this diagram

Which muscles make up the pharyngeal wall?

Made up of striated muscle

• This allows for constriction and shortening 

• Rostral: palatopharyngeal

• Middle: hypopharyngeal

• Caudal: thyropharyngeal

All insert onto the roof of the pharynx (dorsal and lateral arches)

Dilation is regulated by one single muscle: stylopharyngeus caudalis

What innervates the pharyngeal wall?

Innervation is by the vagus and hypoglossal nerves

Describe the process of swallowing, using this diagram as an aide

1. Soft palate is elevated to obstruct the nasopharynx and open the oropharynx

2. Palatopharyngeal arch constricts

3. Hyoid apparatus pulls the larynx forward

4. The epiglottis flips back, to cover the tracheal opening 

5. The tongue pushes food into the oesophagus

Describe the structure of the equine guttural pouch

• The auditory tube runs from the nasopharynx to the middle ear, in all species

• The guttural pouch is a large diverticulum of the auditory tube in horses

• The nasopharynx can be entered through slits into the nasopharynx

• Stylohyoid bone: divides into lateral and medial compartments

• Cranial nerves: glossopharyngeal, vagus and hypoglossal

• Blood vessels: internal carotid artery and external carotid artery 

• Longus capitis muscle

• Pharyngeal lymph nodes

Outline the margins of the equine guttural pouch

• Dorsal: base of skull

• Ventral: laryngopharynx and oesophagus 

• Lateral: skin

• Medial: septum

What is the function of the equine guttural pouch?

Function: unknown, but thought to cool the blood moving to the brain

What kind of disease may occur in the equine guttural pouch?

• Bacterial: strangles, strep.equi, sbps.equi

• Fungal: mycosis (signs depend on location of plaques- over the artery may cause epistaxis, and over the nerve may cause neurological signs)

How can you surgically access the equine guttural pouch? Draw a labelled diagram

• Via Vilborg’s triangle, made up of:

  • Sternomandibularis m. (beige)

  • Ramus of mandible 

  • Linguofacial vein (purple)

What is BOAS?

BOAS: brachycephalic obstructive airway syndrome

This is a group of congenital anatomical defects present in brachycephalic breeds, that cause upper airway obstruction- leading to breathing difficulties

Give examples of brachycephalic dog breeds

• Pekingese

• Pug

• Shih Tzu

• French/British Bulldog

• Boston Terrier

Give examples of brachycephalic cat breeds

• Persian

• Scottish Fold

• Himalayan

• British Shorthair/Longhair

Give examples of brachycephalic rabbit breeds

• Netherland Dwarf

• Lionhead

• Miniature Lop

• French/German Lop

How can you diagnose BOAS?

• Examining airways under a GA using an endoscope, usually if surgery is planned 

• CT scanning can also be carried out to further assess

Draw a table to show the anatomical abnormalities associated with BOAS, and their mechanism and relevant clinical sign

Anatomical Abnormality	Mechanism	Clinical Signs
Stenotic nares	Narrowed, pinched or collapsed nostrils	Labored, open-mouth breathing
Elongated, hyperplastic soft palate	The roof of the mouth extends too far back, dangling directly over the larynx and obstructing the rima glottidis	Stertor: heavy snoring/snorting
Aberrant nasal turbinates	The delicate bones inside the nose are crowded together, and extend too far back into the throat	Severe heat intolerance and hyperthermia
Hypoplastic trachea	An abnormally narrow, underdeveloped trachea	Exercise intolerance and syncope
Macroglossia	A disproportionately large, thick tongue relative to the shortened mouth	Exacerbates sleep apnea

Describe the surgical interventions that can help animals with BOAS

• Stenotic nares correction: removing a small wedge of tissue from nostrils to widen the openings and improve airflow

• Soft palate resection: palate is surgically shortened so it no longer blocks the entrance to the larynx

• Laryngeal saccule removal: excess tissue may be trimmed if it contributes further to airway obstruction

• Tonsillectomy: tonsils are removed if they are enlarged and blocking the throat

Describe the conservative management that can help animals with BOAS

• Weight control: reduces respiratory effort and internal airway pressure

• Avoiding strenuous exercise, high humidity and high temperatures 

• Attach leashes to a body harness instead of a neck collar, to eliminate pressure on the trachea

• Anti-inflammatories, corticosteroids or gastroprotectants help to manage airway swelling and secondary acid reflux

What are the possible radiographic changes associated with feline asthma?

• Bronchial thickening: walls of the bronchial tubes appear more pronounced than usual

• Overinflation: the lungs expand beyond their normal resting size

• Air trapping: abnormal retention of air in the lungs during exhalation

Label this diagram

What is feline asthma?

Feline asthma is a chronic respiratory condition, where airborne allergens cause inflammation, swelling and mucus buildup

Give examples of breeds prone to feline asthma

• Siamese

• Himalayan

• Balinese

• Oriental

• Korat 

• Seychellois

Describe how feline asthma can be managed pharmacologically

Glucocorticoids (fluticasone, prednisolone)

• Anti-inflammatory action

• They bind to intracellular glucocorticoid receptors to alter gene transmission

• This decreases the production of pro-inflammatory cytokines, reduces airway eosinophil infiltration and minimises mucosal oedema 

• They also help to prevent or reverse the down-regulation of beta-2 receptors

Bronchodilators (albuterol, salbutamol, terbutaline, theophylline)

• Airway smooth muscle relaxation

• Beta-2 agonists stimulate beta-2 adrenergic receptors

  • This increases intracellular cAMP to relax smooth muscle

• Methylxanthines inhibit phosphodiesterase (PDE) enzymes

  • This prevents the breakdown of cAMP to relax smooth muscle

Describe how feline asthma can be managed non-pharmacologically

Reducing environmental triggers

• Remove smoking, vaping, incense, scented candles, household cleaners etc

• Transition to dust-free litter alternatives 

• Install air purifiers where the cat spends the majority of their time

Weight management

• Keep an optimal body condition score to lower oxygen demand and reduce the effort required to breathe

Diet 

• Omega-3 fatty acid supplementation has strong clinical backing for natural anti-inflammatory effects

• Feeding a moisture-rich diet supports hydration, and keeps the mucosal lining of the respiratory tract moist, helping the airways to better clear mucus

• Probiotics can be used to stabilise immune responses

Why might flare-ups of feline asthma occur?

Missing doses or discontinuing treatment

Obesity

Environmental triggers

• Dusty or clay-based cat litters 

• Tobacco smoke, vaping byproducts, fireplace smoke 

• Aerosol sprays, household cleaning chemicals, perfumes and candles

• Pollen, mold spores, dust mites

Give the clinical signs of lower airway disease

• Coughing 

• Expiratory dyspnea 

• Tachypnea and respiratory distress

• Exercise intolerance

• Abnormal lung sounds (wheezes and crackles)

How can you differentiate between lower and upper airway disease?

Breathing pattern

• Upper airway: inspiratory dyspnea (difficulty breathing in)

• Lower airway: expiratory dyspnea (difficulty breathing out)

Respiratory noises

• Upper airway: loud, audible noises without a stethoscope

• Lower airway: quiet breathing at a distance, audible noises require auscultation (wheezes and crackles)

Coughing

• Upper airway: harsh, loud, honking and dry coughing, often easily induced by gentle tracheal palpation

• Lower airway: soft, deep, productive or wheezy coughing

How can you evaluate lower and upper airway disease

Radiography

• Upper airway: cervical and thoracic radiographs needed to evaluate the nasal passages, pharynx, larynx and extrathoracic trachea

• Lower airway: full thoracic radiographs can show bronchointerstitial patterns, lung lobe consolidation or hyperinflation (air trapping)

Endoscopy

• Upper airway: rhinoscopy and laryngoscopy assesses structural collapse, laryngeal paralysis or elongated soft palates

• Lower airway: bronchoscopy evaluates the lower trachea and principal bronchi for collapse, inflammation and mucus plugs

Airway sampling

• Upper airway: nasal flushes or brushings

• Lower airway: tracheal wash or bronchoalveolar lavage 

Describe the 3-part loop of the cough reflex

Regardless of the disease location, all coughs utilise a three-part reflex loop

1. Afferent pathway: sensory receptors in the respiratory epithelium transmit electrical signals via sensory branches of the vagus nerve up to the brainstem

2. Central pathway: these signals synapse in the nucleus tractus solitarius (nTS), located in the medulla oblongata and pons

3. Efferent pathway: once the threshold is met, the medulla coordinates motor output via the vagus, phrenic and spinal motor nerves to the effector muscles (diaphragm, intercostals, abdominal wall and larynx)

How is a cough initiated during upper airway disease?

• Mechanical stimulus deforms the membranes of rapidly adapting receptors (RARs)

• Myelinated A𝛿 fibres fire rapidly in response to physical changes

• When the upper trachea/larynx is directly stimulated, it triggers a sudden forced expiration against a closed glottis to move the obstruction

How is a cough initiated during lower airway disease?

• Inflammation/chemical stimulus causes the local release of inflammatory mediators along with tachykinins

• These mediators bind to unmyelinated C-fibre receptors, which act locally to release more tachykinins, which further activate neighbouring mechanical RARs

• This mandates an inspiratory phase to expand the lungs fully before an expulsive phase

How should you treat a patient in respiratory distress? Include initial triage, assessment and advanced intervention

Initial triage

1. Provide oxygen as soon as possible

2. Administer sedation if stress is high 

Assessment

1. Observe breathing pattern and effort from a distance

2. Check mucous membrane colour and listen for upper airway noise 

3. Utilise fast, non-stressful tests such as ultrasound rather than radiography

Advanced intervention

• Intubate, or perform a tracheostomy immediately if the patient is fatiguing, cyanotic or suffering from a complete upper airway obstruction 

• Perform thoracentesis if pleural space disease is suspected and contributing to restrictive breathing

• Actively cool hyperthermic patients

Describe the effect of restrictive bronchial disease

Increased airway resistance

Reduced lung compliance → reduced functional residual capacity

Hypoxemia (low blood oxygen)

Hypercapnia (high carbon dioxide)

Increased risk of bronchospasm 

Anatomical dead space decreases due to narrower airways

Alveolar dead space increases due to poorly ventilated alveoli

How can you manage the effects of restrictive bronchial disease in a patient under anaesthesia

• Administer 100% oxygen via mask or flow-by for several minutes before induction to build an oxygen reserve

• Be ready to provide mechanical or positive-pressure ventilation, since anaesthetic agents depress the respiratory centre

• Premedicate with targeted bronchodilators if reactive airway spasms are anticipated 

• Closely monitor oxygenation, breathing patterns and mucous membrane colour

Describe the effects of anaesthetic agents on respiratory function

Alpha-2 agonists

• Can significantly alter pulmonary mechanics and potentially contribute to hypoxemia or pulmonary issues

Opioids

• Cause centrally-mediated respiratory depression

Propofol and thiopental

• Rapid IV administration frequently causes transient apnea or profound hypoventilation

Isoflurane, sevoflurane, desflurane

• Reduce the sensitivity of the respiratory centre to carbon dioxide

• Decreases respiratory rate and/or tidal volume

Ketamine

• Dissociative anaesthetics cause minimal direct respiratory depression, though combinations with sedatives like alpha-2 agonists can still compromise overall ventilation

What are the common causes of coughing in cats?

• Feline asthma

• Chronic bronchitis 

• Respiratory infections (feline herpesvirus, calicivirus)

• Parasites (heartworms or lungworms)

• Foreign objects 

• Chest tumors, lung cancer or severe trauma

What are the common causes of coughing in dogs?

• Kennel cough

• Pneumonia

• Canine influenza

• Chronic bronchitis

• Parasites (heartworms or lungworms)

• Foreign objects

What are the common causes of coughing in horses?

• Equine asthma

• Equine influenza 

• Strangles 

• Pneumonia 

• Environmental irritants

What are the common causes of coughing in cows?

• Infectious pneumonia

• Lungworm

• Environmental irritants

Outline the structure of the larynx

Stratified squamous epithelium at the epiglottis of the larynx, and on the folds

Other areas are pseudostratified ciliated epithelium

Cartilaginous tube

• Made up of elastic and hyaline cartilage

• Hyaline: thyroid (U-shaped), cricoid (signet ring-shaped), paired arytenoids (triangular-shaped)

• Elastic: epiglottis (leaf-shaped)

Suspended by the hyoid bones, which articulate with the temporal bone 

• Linked by synovial or cartilaginous joints

Cavities

• Entrance to the larynx → vestibule (medianus recess in the horse and pig)

• Glottis

• Infraglottal cavity

Label this diagram

Describe the cartilage connections within the larynx

There are connections between the cartilage themselves, and also with the trachea and hyoid bones

This is by:

• Joints (synovial or cartilaginous)

• Collagenous and elastic ligaments

• Striated musculature

Ligaments of arytenoid cartilage

• Rostral: vestibular ligament

• Caudal: vocal ligament

• These are moved when the arytenoids move

Describe the function of the different laryngeal muscles

Glottis: involved in inspiration, swallowing and phonation

Intrinsic muscles

• Between laryngeal cartilages 

• Involved in respiration and phonation

Extrinsic muscles

• Connect the larynx to hyoid bones, pharynx and sternum

• Involved in swallowing

Which intrinsic laryngeal muscles are involved in

• Widening the glottis

• Narrowing the glottis

Widening the glottis

• Cricoarytenoideus dorsalis 

  • Supplied by the recurrent laryngeal nerves

  • Relevant for left laryngeal hemiplegia- left nerve becomes diseased → inspiratory noise, poor performance, fails the slap test

Narrowing the glottis

• Cricothyroideus 

• Cricoarytenoideus lateralis (can close the glottis)