Equine Spine and Head Anatomy

Original Editor - Chelsea Mclene based on the course by

Petra Zikmann

Top Contributors - Chelsea Mclene, Jess Bell and Tarina van der Stockt

Introduction[edit | edit source]

Equine anatomy refers to the gross and microscopic anatomy of horses and other equids (donkeys, and zebras).

This page introduces the Anatomy of Equine Spine and Head.

Horseanatomy.png

Axial Skeleton[edit | edit source]

The axial skeleton consists of the skull, vertebral column, sternum, and ribs. Multiple sternebrae fuse to form one bone, attached to the 8 "true" pairs of ribs, out of a total of 18.[1]

The vertebral column contains 54 bones:

  • 7 cervical vertebrae: includes the atlas (C1) and axis (C2)
  • 18-19 thoracic vertebrae
  • 5-6 lumbar vertebrae
  • 5 sacral vertebrae
  • 15-25 caudal vertebrae[2]

In certain breeds, there may be variations in these numbers.[3]

Skull[edit | edit source]

The skull contains the brain and the most important organs of sense.

Cranium[edit | edit source]

The Roof of the cranium is made up of frontal and parietal bones.

The Floor is made up of sphenoid bone.

The cranium consists of 5 orbital regions:

  • Frontal
  • Lacrimal
  • Palatine
  • Sphenoid
  • Zygomatic

Interparietal bone: only found in horse and cat.

The orbit is complete in horse and ruminants while it is incomplete in carnivores but completed by the orbital ligament.

The lacrimal fossa collects tears and sends them through lacrimal canal into the nasal cavity.

Bones in the Equine Skull[edit | edit source]

Equine skull lateral view.jpeg

There are 34 bones and most of them are flat. During the birth process, these bones overlap and allow the skull to compress as much as possible to allow for parturition.

The 14 major bones are:[1]

  • Incisive bone (premaxillary): part of the upper jaw; where the incisors attach
  • Nasal bone: covers the nasal cavity
  • Maxillary bone: a large bone that contains the roots of the molars
  • Mandible: lower portion of the jaw; largest bone in the skull
  • Lacrimal bone: contains the nasolacrimal duct, which carries fluid from the surface of the eye, to the nose
  • Frontal bone: creates the forehead of the horse
  • Parietal bone: extends from the forehead to the back of the skull
  • Occipital bone: forms the joint between the skull and the first vertebrae of the neck (the atlas)
  • Temporal bone: contains the eternal acoustic meatus, which transmits sound from the ear to the cochlea (eardrum)
  • Zygomatic bone: attaches to the temporal bone to form the zygomatic arch (cheekbone)
  • Palatine bone: forms the back of the hard palate
  • Sphenoid: formed by fusion of the foetal basisphenoid and presphenoid bones, at the base of the skull. Can become fractured in horses that rear over backwards
  • Vomer: forms the top of the inside of the nasal cavity
  • Pterygoid: small bone attached to the sphenoid that extends downward

Cavities[edit | edit source]

The equine skull consists of 4 cavities:

  • The cranial cavity: Protects and encloses the brain, supports sense organs. The cranium consists of a roof made up of the frontal and parietal bones and a floor made up of the sphenoid bone
  • The orbital cavity: Has 5 orbits: frontal, lacrimal, palatine, sphenoid and zygomatic. It protects and surrounds the eye.[4] Horses have both monocular and binocular vision:
    • Monocular vision: The horse can see objects with one eye. This means that the brain receives two images simultaneously
    • Binocular vision: The horse can focus with both eyes just like humans and the brain receives only one signal
  • The oral cavity: A passage into the respiratory and digestive system
  • The nasal cavity: Contains bone that protects the mucous membrane from inspired warm air

Foramina of the Skull and the Structures Passing Through[edit | edit source]

Foramina Structures passing through
Infra-orbital foramen Infra-orbital nerve. CNV
Maxillary foramen
Cribriform foramen Olfactory nerve. CNI
Optic canal Optic nerve. CNII
Orbital fissure CNVII, IV, V and VI (ophthalmic division)
Round foramen CNV (maxillary division)
Oval foramen CNV (mandibular division)
Foramen lacerum Internal carotid artery

CN V3 (horse and pig)

Internal acoustic meatus CNVIII
Jugular foramen CNIX, X, XI
Stylomastoid foramen CNVII
Mandibular foramen CNV (mandibular alveolar nerve)
Mental foramen

Joints and Ligaments[edit | edit source]

Joints and Ligaments of the Skull[edit | edit source]

  • The temporo-mandibular joint - a condylar joint between the mandibular condyles and the mandibular fossae of the temporal bones. It has a loose joint capsule with thickenings that form a lateral ligament, as well as an articular disc[1]
  • Mandibular symphysis
  • Hyoid apparatus - consists of three joints:
    • Tympanohyoid cartilage- skull (syndesmosis)
    • Interhyoid joints (synovial)
    • Thyrohyoid bone- cranial cornu of thyroid cartilage (synovial)[1]

[5]

Cervical Spine[edit | edit source]

Joints[edit | edit source]

Equine cervical spine.jpeg
Atlanto-Occipital Joint[edit | edit source]

A condylar, modified synovial hinge joint. The articulating surfaces are the occipital condyles and the cranial articular surfaces of the atlas (C1). There are three thickenings that strengthen the spacious joint capsule: Dorsal, Ventral, Lateral. The transverse atlantal ligament holds the dens of the axis against the ventral arch of the atlas.[1]

Atlanto-Axial Joint[edit | edit source]

A pivot joint between the atlas and the saddle shaped surface of the axis (C2), which extends upon the dens. It has a loose joint capsule. The apical ligament of dens connects the apex of the dens to the occipital bone. Motion at this joint includes rotation of the atlas and head upon the axis and some accessory lateral flexion.[1] Rotation at this joint makes up 73 percent of cervical rotation.[6]

Cervical Spine C3-C4[edit | edit source]

A planar, extensive, oval shaped joint that is obliquely oriented in transverse plane. The cranial articular processes face dorsomedially and the caudal articular processes face ventrolaterally. Spinous process height increases caudally from C6. Lateral flexion is the primary motion at these joints (25-45 degrees each joint - C1/C2 only has 3.9 degrees of lateral flexion).[6]

Ligaments[edit | edit source]

  • Dorsal longitudinal ligament
  • Ventral longitudinal ligament
  • Ligamentum flavum
  • Nuchal ligament:
    • This ligament connects the thoracic vertebra to the head and assists in supporting its weight. It consists of two paired parts:[1]
      • Funicular (cord) part - extends from the poll to +/- the second to the fourth thoracic spinous process
      • Lamellar part - arises from the second and third thoracic spinous processes and the funicular part, and inserts on the C2-C6 spinous processes. The first digitation going to the axis is very strong, but it decreases in strength caudally

Thoracic Spine (T1-T18)[edit | edit source]

Articular processes[edit | edit source]

Caudal articular processes face ventrally and are positioned at the base of the spinous process. The cranial articular processes are oval facets on the arch of the vertebra and face dorsally. Each thoracic vertebrae has a pair of costal facets on the dorsal body (except the last) forming the costal fovea.[1]

Anticlinal vertebrae: This is the point in the caudal thoracic vertebral column at which the anatomic features of the vertebra start to change.[7] This usually occurs at the 13th vertebra in horses.

Motion:

  • Flexion - most flexion occurs at T17/T18; least flexion occurs at T3-T9
  • Extension - most extension occurs at T14-T18; least extension occurs at T2-9[1]

Rib Neck[edit | edit source]

Has 2 converse facets: Cranial and Caudal

Rib 1 attaches to C7, T1 and the associated IV disc

Motion: rotation of the rib, which is greater caudally

Costovertebral Joint[edit | edit source]

Joints[edit | edit source]

The costovertebral joints have two distinct articulations between most ribs and the vertebral column:[1]

  1. Head of the rib: Cranial and caudal costal facets of adjacent vertebrae; a ball and socket synovial joint
  2. Tubercle of the rib: Transverse process of vertebrae; a plane synovial joint

Ligaments[edit | edit source]

  • Radiate longitudinal ligament
  • Intercapital ligament
  • Costotransverse ligament
  • Ligament of the neck

Lumbar Spine[edit | edit source]

Joints[edit | edit source]

Horses usually have 6 lumbar vertebrae (L1-L6), but some arabian horses only have 5 (L1-L5).[1]

Articular processes[edit | edit source]

Cranial articular processes are fused with mammillary processes. They are concave dorsally and mostly in sagittal alignment. Caudal processes are convex ventrally and correspond with the convexity of the cranial articular processes. They are differentiated from the last thoracic vertebra by the lack of costal facets.[1]

Motion: The lumbar spine and caudal thoracic spine are the least mobile regions of a horse's back.[8] Lateral flexion and rotation is very limited especially at L4-L6 due to intertransverse joints.[1]

Ligaments of Thoraco-Lumbar Spine[edit | edit source]

  • Supraspinous ligament: A heavy band of connective tissue running over the top of spinous processes ( T2/T3 caudally). It prevents abnormal separation of spinous processes during flexion
  • Ventral longitudinal ligament: Marks the ventral surface of vertebrae from the axis to the sacrum. It is strongest and widest caudally. It plays a major role in preventing overextension of the spine
  • Dorsal longitudinal ligament: Extends from the floor of the vertebral canal from the axis to sacrum and helps to prevent spine hyper-flexion
  • Annulus fibrosis of IVD: Thick ventrally
  • Intertransverse ligament
  • Interarcuate ligament/ yellow ligament/ ligamentum flavum: An elastic ligament that fills the dorsal space between the arch of the adjacent vertebra[1]

Lumbosacral Joint[edit | edit source]

The cranial articular process of the first sacral vertebra are concave and face dorsomedially.

Motion: Flexion and Extension - 23.4 degrees[9]

Sacrum[edit | edit source]

The sacrum consists of fused sacral vertebrae and has dorsal and ventral sacral foramina.

[10]

Myology and Neurology[edit | edit source]

Muscles of the Head[edit | edit source]

Muscles of the Face[edit | edit source]

The muscles of facial expressions are innervated by the motor fibers of CNVII (facial nerve).[1]

Muscle Origin Insertion Action
M. Levator labii maxillaris Lacrimal, Zygomatic and Maxillary bones The maxillary lip Elevates the Maxillary lip
M. Levator nasolabialis Nasal and Frontal bones The lateral wing of nostril

The maxillary lip

Elevates and retracts the angle of the mouth
M. Zygomaticus The fascia covering the Masseter The commissure of the lips
M. Buccinator Maxilla and Mandible Flattens the cheeks and thus presses food between the teeth
M. Depressor labii mandibulars The alveolar border of the Mandible The mandibular lip Depresses and retracts the mandibular lip
M. Orbicularis oris The sphincter muscle of the skin and the muscles of the lips

Corner of the mouth

Into the lips as it surrounds the mouth Closes the mouth
M. Risorius Part of M. cutaneous faciei The angle of the mouth Retracts the angle of the mouth
M. Dilator naris Alar cartillage Alar cartillage Dilates the nostril
M. Lateralis nasi Dorsal part

Nasal bone

Parietal cartilage

Dilates the nostril and nasal vestibule
Ventral part Nasal process of Incisive bone Lateral wall of the Nasal vestibule
M. Caninus Maxilla close to the rostral extremity of the facial crest Lateral wing of the nostril Dilates the nostril laterally
M. Levator nasolabialis Frontal and Nasal bones Lateral wing of the nostril Elevates the maxillary lip and the commissure of the mouth

Dilates the nostril

Ear[edit | edit source]

The ear is an organ of hearing and balance. It consists of the outer, middle, and inner ear.

Outer Ear[edit | edit source]

The outer ear includes:[1]

  • Pinna: mobile and can move independently - can hear multiple sounds at the same time[11]
  • Ear canal

Cartilage: Cartilages of the ear collect and transmit sound to the essential organ of hearing within the temporal bone. In order to achieve this, they (especially the concha) need to move.[1]

The muscles of outer ear:[1]

  • Rostral
  • Dorsal
  • Caudal
  • Ventral

There are 3 cartilages:[1]

  • Conchal: Forms the framework of the portion of the ear which stands erect. It has a large vertical opening on one side to receive sound, and is attached below to the annular cartilage
  • Annular: A small ring of gristle connected to the auditory process of the petrous temporal bone
  • Scutiform: A small, flat and somewhat triangular cartilaginous plate situated in front of the base of concha, to which it is attached
Middle Ear[edit | edit source]

The middle ear includes:[1]

  • Eardrum
  • Small, air-filled chamber containing 3 tiny bones: the hammer, anvil, and stirrup. It also includes 2 muscles: the oval window, and the eustachian tube.
Inner Ear[edit | edit source]

The inner ear is a complex structure that includes the cochlea and the vestibular system.[11]

Muscles of Mastication[edit | edit source]

The muscles of mastication are innervated by the mandibular branch of trigeminal nerve CNV.[1]

Muscle Origin Insertion Action
M. Masseter The zygomatic arch and the facial crest The lateral border of the ramus of the mandible Closes the mouth
M. Temporalis The temporal fossa and the temporal crest The coronoid process of the mandible Closes the mouth (to raise the mandible)
M. Pterygoideus medialis The crest formed by the pterygoid processes of the basisphenoid and the palatine bones The medial surface of the ramus of the mandible
M. Pterygoideus lateralis The pterygoid process of the sphenoid bone Rostral border of the condyle of the mandible Draws and moves the mandible rostrally
M. Digastricus The jugular process of occipital bone Medial surface of the ventral border of the molar part of the body of the mandible Opens the mouth
M. Occipitomandibularis The jugular process The caudal border of the ramus of the mandible

Muscles of the Eyes[edit | edit source]

  • M. Orbicularis oculi - innervated by palpebral branch of CN VII
  • M. Levator palpebrae superioris - originates from the posterior orbit and inserts at orbicularis oculi fibers of the lower eyelid. It elevates the upper eyelid and is innervated by CN III (oculomotor nerve)
  • M. Malaris - lowers the ventral eyelid. It is innervated by CN VII (facial nerve)
  • Muller's muscle - innervated by sympathetic nerves
  • Ciliary muscles
  • M. Retractor anguli - retracts and anchors the lateral canthus
  • M. Levator anguli oculi medialis and M. Frontalis - slightly elevates of the upper eyelid[12][13]

Muscles of the Tongue[edit | edit source]

The equine tongue is made up of twelve different muscles[14] including styloglossus, genioglossus and hyoglossus. These muscles are covered by mucosa on the sides and underneath.[1]

Action: prehension, mastication (i.e. chewing)

Innervation: Hypoglossus (CNXII)

Muscles of Pharynx and Soft palate.[edit | edit source]

[15][16]
Muscle Origin Insertion Action Innervation
INTRINSIC MUSCLES
M. Tensor veli palatini Muscular process of the petrous part of the temporal bone, pterygoid bone, and lateral lamina of the auditory tube Palatine aponeurosis Retracts the soft palate away from the dorsal pharyngeal wall, expanding the nasopharynx and slightly depressing it ventrad during inspiration Mandibular branch of the trigeminal nerve
M. Levator veli palatini Muscular process of the petrous part of the temporal bone and the lateral lamina of the Auditory tube and passes along the lateral wall of the nasopharynx Soft palate dorsal to the glandular layer Elevates the soft palate during swallowing Pharyngeal branch of the Vagus nerve
M. Palatinus Caudal aspect of the palatine aponeurosis Caudal free margin of the soft palate Shortens the soft palate and depresses it towards the tongue
M. Palatopharyngeus Palatine aponeurosis and from the palatine and pterygoid bones Upper edge of the thyroid cartilage
M. Stylopharyngeus Rostral Medial surface of the rostral end of the Stylohyoid bone Pharyngeal raphe Pharyngeal constrictor Glossopharyngeal nerve
Caudal Medial aspect of the caudal third of the Stylohyoid bone Dorsolateral wall of the pharynx Pharyngeal dilator
EXTRINSIC MUSCLES
M. Genioglossus Median plane of the Tongue Oral surface of the Mandible Protracts the tongue Hypoglossal nerve
M. Geniohyoideus Medial surface of the Mandible Basihyoid bone Protrudes the tongue
M. Thyrohyoideus Lateral lamina of the Thyroid cartilage Caudal aspect of the thyrohyoid bone Moves the larynx rostrad
M. Hyoglossus Hyoid bones Median plane of the dorsum of the tongue Retracts and depresses the base of the tongue
M. Hyoepiglotticus
M. Styloglossus Lateral aspect of the stylohyoid bone Tip of the tongue Retraction of the tongue
M. Sternohyoideus Sternal manubrium Basihyoid bone and lingual process of the hyoid apparatus Caudal traction Branches of the first and second cervical nerves
M. Sternothyroideus Caudolateral aspect of the thyroid cartilage

Muscles of the Hyoid Apparatus and Larynx[edit | edit source]

Muscles of the hyoid apparatus and larynx are innervated by CNX.

Hyoid Apparatus[edit | edit source]

The hyoid apparatus has muscular connections from the throat to the forelimbs, shoulder, and sternum. Sternohyoid and omohyoid provide a direct connection from the hyoid apparatus to the shoulder of the horse via the ventral neck. The tongue connects to the hyoid apparatus. Small muscles of the hyoid apparatus connect to the TMJ and the poll and the TMJ articulates with the hyoid apparatus.[17][18]

Larynx[edit | edit source]

Intrinsic muscles:

  • Cricoarytenoideus dorsalis - abduction of arytenoids and tensing of vocal cords
  • Thyroarytenoideus - adduction of arytenoids
  • Arytenoideus transversus - adduction of arytenoids
  • Cricoarytenoideus lateralis - adduction of arytenoids[19]

Muscles of Cervical spine[edit | edit source]

[20][21]
Equine Cervical Muscles.jpeg
Equine Cervical Spine.jpeg
Muscle Origin Insertion Action Innervation
M. Omotransversarius
Fascia of shoulder Scapular cartilage and transverse processes of C2-4 Advances limb

Adducts limb

Moves neck laterally

Ventral branch of local cervical spinal nerve
M. Brachiocephalicus
Mastoid process of temporal bone and first cervical vertebra Deltoid tuberosity and crest of the humerus Shoulder extension

Protraction

Flexion of the neck towards the side of the protracting limb

Accessory nerve
M. Cleidobrachialis
Inscription of clavicle Crest of the humerus Advances limb

Adducts limb

Axillary nerve
M. Cleidomastoideus Clavicular intersection Mastoid process of temporal bone Advances limb

Flexes neck

Turns head

Ventral branch of Accessory nerve

(cranial nerve XI)

M. Sternocephalicus (Sternomandibularis)
Manubrium of the sternum Caudal border of mandible Turns head

Opens mouth

M. Omohyoideus
Subscapular fascia Lingual process of basihyoid bone Retracts basihyoid bone and tongue Spinal nerve C1
M. Trapezius
Nuchal ligament and Supraspinous ligaments of C2-10 Cervical part: Entire scapular spine Advances thoracic limb

Abducts thoracic limb

Elevates shoulder

Dorsal branch of Accessory nerve

(cranial nerve XI)

Thoracic part: Dorsal third of Scapular spine
M. Rhomboideus

(cervicis and thoracis)

Nuchal ligament and dorsoscapular ligaments of C2-T8 Scapular cartilage Elevates neck

Draws scapula cranially and dorsally

Local thoracic nerve and Local cervical nerve
M. Serratus ventralis (cervicis)
Transverse processes of C4-7 Scapular cartilage and medial scapula Supports trunk between forelimbs

Raises neck when the limb is fixed

Ventral branch of local cervical nerve
M. Splenius (capitus and cervicis)
Nuchal ligament and spinous processes of T3-T5 Nuchal crest and mastoid process of temporal bone Extends neck

Elevates neck

Bends neck laterally

Dorsal branch of Accessory nerve and dorsal branch of local spinal nerve
M. Longissimus (cervicis, capitus, atlantis)
Transverse processes of cervical and thoracic vertebrae Wing of atlas and mastoid process of temporal bone Elevates head and neck

Bends head and neck laterally

Stabilizes and extends vertebral column

Dorsal branch of local spinal nerve
M. Semispinalis capitis
Articular processes of C2/3-7 and transverse processes of T1-6/7 Occipital bone Elevates head and neck

Bends head and neck laterally

M. Longus capitis
Transverse processes of C3-5 Base of skull Bends head and neck Ventral branch of local spinal nerve
M. Longus colli
Cervical part Transverse processes of C3-7 Ventral tubercle of atlas and bodies of cervical vertebrae Flexes head
Thoracic part Bodies of T1-6 Transverse processes of C6-7 Flexes head

Bends head laterally

M. Obliqus capitis caudalis Spinous process of the axis Wing of the Atlas Rotates atlas and Head Dorsal branch of C2
M. Rectus capitis dorsalis Major Nuchal crest Elevates head Dorsal branch of C1
Minor Dorsal arch of the atlas Occipital bone
M. Scalenes
Transverse processes of the last 4 cervical vertebrae Anterior border and Outer surface of the first rib Assists inspiration by drawing the first rib forward.

With the rib fixed, draws the neck downward and to one side.

Cervical nerves

Muscles of Trunk[edit | edit source]

Equine Trunk muscles.jpeg
Muscles Origin Insertion Action Innervation
M. Latissimus dorsi
Supraspinous ligaments from T3 and thoracolumbar fascia Teres major tuberosity of humerus Flexes shoulder and draws limb caudally.

Draws trunk cranially when the limb is flexed.

Thoracodorsal nerve
M. Serratus ventralis

(thoracis)

Ribs 1-8/9 Scapular cartilage and Medial scapula Supports trunk between forelimbs.

Raises neck when the limb is flexed.

Long thoracic nerve
M. Serratus dorsalis
Cranialis Supraspinous ligament Cranial border of ribs 5-11 Inspiration Intercostal nerve
Caudalis Thoracolumbar fascia Caudal borders of ribs 11-18 Expiration
M. External intercostal
Muscles run caudodorsally in the intercostal spaces Inspiration Intercostal nerve
M. Internal intercostal
Muscles run cranioventrally in the intercostal spaces Expiration
M. External abdominal oblique
Thoracolumbar fascia and lateral aspect of ribs 4-18 Linea alba

Prepubic tendon

Pelvic tendon

Coxal tendon

Inguinal ligament

Flexes the trunk Ventral branch of lumbar nerve and local intercostal nerve
M. Internal abdominal oblique Coxal tuber and inguinal ligament Linea alba

Prepubic tendon

Last rib

Cartilages of ribs 14-18

Flexes the trunk Ventral branches of lumbar nerve and local intercostal nerve
M. Transversus abdominis
Medial surface of Costal cartilage 7-18 and transverse processes of lumbar vertebrae Linea alba
M. Rectus abdominis
Lateral surface of costal cartilages 4-9 Prepubic tendon and the head of the femur Flexes the trunk

Flexes lumbar spine and lumbosacral joint

M. Longissimus thoracis et lumborum
Spinous processes of thoracic, lumbar and sacral vertebrae and wing of ilium Transverse processes of vertebrae and tubercles of ribs Stabilizes and extends vertebral column Dorsal branch of local spinal nerve
M. Semispinalis thoracis and lumborum Sacrum, the articular processes of the lumbar vertebrae and the transverse processes of the dorsa vertebrae Spinous processes of third or fourth vertebra in front of the one from which it arises Fixes the bone during the action of the large spinal muscle, and assists in spine extension
M. Iliocostalis thoracis and lumborum Transverse processes of vertebrae Bodies of the adjacent vertebrae and/or the tuberosities of the ribs Expiration

Thoracolumbar extension

M. Cutaneous trunci Superficial trunk fascia Superficial shoulder fascia and medial surface of humerus Moves the skin of the abdomen Lateral thoracic nerve and intercostobrachial nerve
M. Multifidus lumborum Articular processes of each vertebra from C2 to sacrum Spinous process of the preceding vertebrae Stabilizes and rotates vertebral column Dorsal branches of local spinal nerve
M. Psoas major
Lumbar transverse processes and ventral surface of the last two ribs Lesser trochanter of Femur Rotates pelvic limb outward

Flexed hip

Advances limb

Stabilizes vertebral column when limb is fixed

Ventral branches of lumbar and local intercostal nerve and lumbar plexus

[22]

References[edit | edit source]

  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 Van der Walt A. Equine Spine and Head Presentation. Physioplus, 2021.
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  3. Riegal, Ronald J. DVM, and Susan E. Hakola RN. Illustrated Atlas of Clinical Equine Anatomy and Common Disorders of the Horse Vol. II. Equistar Publication, Limited. Marysville, OH. Copyright 2000.
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  17. Cornelisse CJ, Rosenstein DS, Derksen FJ, Holcombe SJ. Computed tomographic study of the effect of a tongue-tie on hyoid apparatus position and nasopharyngeal dimensions in anaesthetised horses. American journal of veterinary research. 2001 Dec 1;62(12):1865-9.
  18. Chalmers HJ, Cheetham J, Yeager AE, Ducharme NG. Ultrasonography of the equine larynx. Veterinary Radiology & Ultrasound. 2006 Sep;47(5):476-81.
  19. Kelly PG. Studies on serial resting and dynamic endoscopic examination of thoroughbred yearlings (Doctoral dissertation, University of Glasgow).
  20. Payne RC, Veenman P, Wilson AM. The role of the extrinsic thoracic limb muscles in equine locomotion. Journal of Anatomy. 2005 Feb;206(2):193-204.
  21. Wikivet. Online Veterinary Encyclopedia. Thoracic Limb Extrinsic Muscles - Horse Anatomy. Available from https://en.wikivet.net/Thoracic_Limb_Extrinsic_Muscles_-_Horse_Anatomy#:~:text=Innervation%3A%20Dorsal%20and%20ventral%20branches,cervical%20and%207th%20thoracic%20vertebrae [last accessed 01/05/2021]
  22. Superficial muscles. Jessica Blackwell. Available from: https://www.youtube.com/watch?v=2UtMocTNrWQ [last accessed 01/05/2021]