Equine Spine and Head Anatomy

Original Editor - Chelsea Mclene based on the course by

Petra Zikmann

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

Introduction

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.

Axial Skeleton

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.

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

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

Skull

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

Cranium

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

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:

• 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

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. 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

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

Joints and Ligaments of the Skull

• 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
• Mandibular symphysis

• Hyoid apparatus - consists of three joints:

• • Tympanohyoid cartilage- skull (syndesmosis)
  • Interhyoid joints (synovial)
  • Thyrohyoid bone- cranial cornu of thyroid cartilage (synovial)

Cervical Spine

Joints

Atlanto-Occipital Joint

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.

Atlanto-Axial Joint

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. Rotation at this joint makes up 73 percent of cervical rotation.

Cervical Spine C3-C4

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).

Ligaments

• 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:
    • 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)

Articular processes

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.

Anticlinal vertebrae: This is the point in the caudal thoracic vertebral column at which the anatomic features of the vertebra start to change. 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

Rib Neck

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

Joints

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

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

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

Lumbar Spine

Joints

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

Articular processes

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.

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

Ligaments of Thoraco-Lumbar Spine

• 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

Lumbosacral Joint

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

Motion: Flexion and Extension - 23.4 degrees

Sacrum

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

Myology and Neurology

Muscles of the Head

Muscles of the Face

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

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

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

Outer Ear

The outer ear includes:

• Pinna: mobile and can move independently - can hear multiple sounds at the same time
• 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.

The muscles of outer ear:

• Rostral
• Dorsal
• Caudal
• Ventral

There are 3 cartilages:

• 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

The middle ear includes:

• 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

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

Muscles of Mastication

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

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

• 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

Muscles of the Tongue

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

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

Innervation: Hypoglossus (CNXII)

Muscles of Pharynx and Soft palate.

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

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

Hyoid Apparatus

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.

Larynx

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

Muscles of Cervical spine

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

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