Anatomy of the Equine Head

Chapter 1: Anatomy of the Equine Head

Introduction

  • The equine head is characterized as a heavy and continuously moving structure that necessitates strong and flexible support.

  • Nuchal Ligament: Provides support to the equine head and is composed of:

    • Funiculus Nuchae: A cord-like structure.

    • Lamina Nuchae: A sheet-like structure.

  • Detailed discussions on the nuchal ligament will be covered in the equine neck section.

The Equine Skull

  • The importance of understanding the equine head includes awareness of potential superficial and deep lesions such as wounds, lacerations, and tumors.

  • Key external and internal features of the skull to memorize:

    • Dorsal Surface Features:

    • Incisive Bone

    • Nasal Bone

    • Frontal Bone

    • Parietal Bone

    • Occipital Bone

    • Lateral Surface Features:

    • Incisive Bone

    • Nasal Bone

    • Maxillary Bone

    • Lacrimal Bone

    • Zygomatic Bone

    • Parietal Bone

    • Temporal Bone

    • Ventral Surface Features:

    • Incisive Bone

    • Nasal Bone

    • Maxillary Bone

    • Vomer Bone

Important Features on External Surface of the Skull

  • Includes foramina and landmarks:

    • Infraorbital Foramen:

    • Located below the orbit, positioned 1 cm dorsal to the rostral end of the facial crest.

    • Localized by:

      • Placing the middle finger on the nasoincisive notch and the thumb on the rostral end of the facial crest.

      • Index finger falls upon the area of the infraorbital foramen after elevating the levator labii superioris muscle.

    • Used to block the infraorbital nerve (continuation of the maxillary nerve), desensitizing the upper lip, gums, nose, upper cheek, canine, and incisor teeth.

    • Applications include suturing wounds, removing foreign bodies, cannulating the naso-lacrimal duct, and treating tooth abscesses.

    • Mental Foramen:

    • Located on the lateral surface of the mandible, midway between the first premolar and the canine teeth, covered by the depressor labii inferioris muscle.

    • The mental nerve (branch of the inferior alveolar nerve) passes from the mandibular foramen through the mandibular canal to reach the mental foramen.

    • Blocking the mental nerve desensitizes the lower lip and deeper structures, including canine and incisor teeth.

    • Mandibular Foramen:

    • Located on the medial surface of the mandible.

    • Injected by creating an intersection point between two perpendicular lines:

      • One along the buccal surface of the teeth.

      • Another from the caudal aspect of the orbit.

    • Blocking the mandibular nerve desensitizes the lower cheek teeth and gum, used in tooth abscess treatments.

Nerve Blocks Around the Eye

  • Total of five nerve blocks:

    1. Supraorbital

    2. Auriculopalpebral

    3. Infratrochlear

    4. Zygomaticofacial

    5. Lacrimal

  • The supraorbital, lacrimal, and infratrochlear nerves are branches of the ophthalmic nerve (part of the trigeminal nerve).

  • The zygomaticofacial nerve is a branch of the zygomatic nerve (itself a branch of the trigeminal nerve).

  • The auriculopalpebral nerve is a branch of the palpebral nerve (a branch of the facial nerve).

  • Applications of nerve blocks:

    • Useful for suturing lacerations and performing complete ophthalmic examinations.

Details of Specific Nerve Blocks

  • Supraorbital Block:

    • Located at the supraorbital foramen in the zygomatic process of the frontal bone.

    • Desensitizes the supraorbital branch of the ophthalmic nerve (sensory) and portions of the auriculopalpebral branches of the facial nerve (motor).

    • Prevents movement of the upper eyelid, facilitating eye examination or suturing.

  • Auriculopalpebral Block:

    • Found at the dorsal edge of the zygomatic arch.

    • Prevents blepharospasm (blinking).

  • Zygomaticofacial Block:

    • Located laterally along the ventral margin of the orbit, associated with the zygomatic process of the frontal bone.

  • Lacrimal Block:

    • Located along the dorsolateral edge of the orbit, adjacent to the lateral canthus.

  • Infratrochlear Block:

    • Located along the dorsomedial edge of the orbit at the trochlear notch.

The Nasal Cavity

  • The horse has two nostrils with two distinctive parts:

    1. Dorsal Part:

    • Known as the false nostril or nasal diverticulum, acting as a blind sac.

    1. Ventral Part:

    • Known as the true nostril.

  • Critical for procedures such as nasogastric intubation (for relieving pressure, colic evaluation, or drug administration) and endoscopy (for visual diagnosis and treatment of respiratory system issues).

Nasogastric Intubation Technique

  • One hand (or thumb) closes the false nostril while the tube is placed in the true nostril, guiding it ventrally.

  • Positioning: The examiner should be on the left side of the horse, as horses are typically approached from this side.

  • Recognizing the mucocutaneous junction between skin and mucosa, a crucial area where the nasolacrimal duct (NLD) opens.

Nasolacrimal Duct (NLD)

  • Importance: Can become obstructed due to lesions, leading to lacrimal outflow accumulation and potential infections.

  • Jones Test: Used to test patency; involves dye injection to observe movement into the nasal cavity. If dye is seen in the true nostril, the duct is patent.

  • Saline Test: Another method involves passing a tube and injecting saline, which should return to the eye if the duct is open.

  • To clear a blocked NLD, a small-gauge plastic tube is passed through the opening.

Nasal Septum and Conchae

  • The two nostrils are separated by a naso-septum formed by hyaline cartilage.

  • Deviation of the septum in foals can be surgically corrected.

  • Removal of the nasal septum reveals the conchae or turbinates, with three present in each nostril:

    • Dorsal Concha

    • Ventral Concha

    • Ethmoid Concha

Nasal Meatuses

  • Dorsal Nasal Meatus: Between the dorsal concha and frontal/nasal bones.

  • Middle Nasal Meatus: Between the dorsal and ventral concha.

  • Ventral Nasal Meatus: Between the ventral concha and the floor of the nasal cavity.

  • Common Nasal Meatus: The collective space used in nasogastric intubation and endoscopy.

  • Care is essential when passing equipment to avoid damaging the highly vascularized conchae, which can result in conditions like ethmoid hematoma.

The Paranasal Sinuses

  • Gas-filled cavities located between internal and external plates of skull flat bones, serving to:

    1. Reduce skull weight.

    2. Provide space for teeth development.

  • Key sinus structures include:

    • Conchofrontal Sinus: Communicates with the frontal sinus and the dorsal concha.

    • Maxillary Sinus: Comprising rostral and caudal parts separated by a bony septum; the teeth roots (M1, M2, M3) are partially within this sinus.

    • Sphenopalatine Sinus: Situated cranial to pterygoid and ventral to the ethmoid turbinate, sharing a relationship with the maxillary sinus and can drain infections.

Access and Surgical Considerations

  • Important considerations for surgical drainage include the anatomical borders separating the maxillary sinus:

    • Rostrally: Line from the rostral end of the facial crest to the infraorbital foramen.

    • Caudally: At the bony orbit's rostral border.

    • Ventrally: The facial crest.

    • Dorsally: Line connecting the medial canthus of the eye to the infraorbital foramen.

  • Two techniques for accessing sinuses:

    • Trephination and Bone Flap Techniques.

Guttural Pouches

  • Guttural pouches are air-filled diverticula from the Eustachian tube lined by ciliated, pseudostratified epithelium.

  • Each horse has bilateral guttural pouches, with capacities between 300-500 ml, expandable by exudates such as pus.

  • Divided by the stylohyoid bone into a larger medial compartment and smaller lateral compartment, with additional functions related to:

    1. Blood pressure regulation in the internal carotid artery during respiration.

    2. Cerebral blood-cooling during stress.

    3. Assistance with gait/balance.

  • Natural Openings:

    • Nasopharyngeal openings and openings to the lateral canthus of the eye.

  • Clinical Examination: Typically performed via endoscopy, careful observation of surrounding structures essential.

Clinical Conditions of Guttural Pouches

  • Common diseases:

    1. Mycosis: Fungal growth leading to possible bleeding or neurological issues.

    2. Empyema: Bacterial infection resulting in pus accumulation.

    3. Tympani: Air accumulation due to infection.

  • Treatment Options: Include endoscopic drainage or surgical approaches like:

    1. Viborg Triangle Approach: Bounded by the ramus of the mandible, sternocephalicus tendon, and linguofacial vein.

    2. Whitehouse Approach: A midline approach near the linguofacial vein.

    3. Modified Whitehouse Approach: Medial to the linguofacial vein.

The Larynx

  • The larynx consists of a box of cartilages, connecting the oral cavity and trachea, and has two critical functions:

    1. Protects lower respiratory system from oral material.

    2. Vocalization.

  • Cartilages:

    • Single Cartilages: Epiglottic, Thyroid (largest), and Cricoid cartilage.

    • Paired Cartilage: Arytenoid, involved in laryngeal function and hemiplegia treatment.

  • Intrinsic Muscles: Critical for laryngeal function include:

    • Cricothyroideus

    • Cricoarytenoideus Dorsalis: Used in laryngeal hemiplegia cases.

  • Extrinsic Muscles: Include thyrohyoideus, hyoepiglotticus, and sternothyroideus.

Dysfunction and Surgical Treatments

  • Laryngeal Hemiplegia: Results from recurrent laryngeal nerve injury causing paralysis of cricoarytenoids dorsalis muscle, which impacts breathing.

  • Treatments:

    • Laryngeal Prosthesis (Tie Back): Surgical procedure performed through approaches like Viborg or Whitehouse, aiming to abduct the airway.

    • Laryngeal Ventriclectomy: Removal of mucosal lining to open up airways further.

  • Other Conditions:

    • Dorsal Displacement of Soft Palate (DDSP): Diagnosed via endoscopy; treatment may involve trimming the epiglottis.

    • Epiglottic Entrapment: Excess mucus membrane affects epiglottis mobility. Treatment includes membrane removal or enlarging soft palate openings.

Required Structures for the Laboratory

  1. Incisive bone, nasal bone, frontal bone, parietal bone, occipital bone.

  2. Ventral features: Incisive bone, nasal bone, maxillary bone, vomer bone.

  3. Important foramina: Infraorbital foramen, supraorbital foramen, mental foramen, mandibular foramen.

  4. Facial crest, nasoincisive notch.

  5. Nerve blocks around the eye: auriculopalpebral, infratrochlear, zygomaticofacial, lacrimal.

  6. Two nostrils: false (nasal diverticulum) and true nostril.

  7. Muco-cutaneous junction.

  8. Nasal septum and vomer bone.

  9. Conchae: dorsal, ventral, ethmoid.

  10. Nasal meatuses: dorsal, middle, ventral, and common nasal meatus.

  11. Nasal sinuses: conchal, frontal, maxillary, sphenopalatine.

  12. Bony septum in the maxillary sinus.

  13. Nasolacrimal duct.

  14. Roots of teeth (M1, M2, M3).

  15. Naso-pharynx.

  16. Physis between basisphenoid and basioccipital bones.

  17. Guttural pouches: medial and lateral compartments.

  18. Stylohyoid bone.

  19. Nasopharyngeal opening.

  20. Adjacent structures: Hypoglossal nerve (XII), vagus nerve (X), glossopharyngeal nerve (IX), internal carotid artery.

  21. External carotid artery.

  22. Medial retropharyngeal lymph nodes.

  23. Surgical approaches for guttural pouches: Viborg triangle, Whitehouse approach, modified Whitehouse approach.

  24. Relevant anatomical landmarks: ramus of the mandible, tendon of the sternocephalicus, linguofacial vein.

  25. Cartilages of the larynx: epiglottic, thyroid, cricoid, arytenoid.

  26. Ligaments and structures in the larynx.