Equine Spine and Head Anatomy

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Original Editor - Chelsea McLene

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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 consist of 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 difference in the number.

Skull

The skull consists of the brain and the most important organs of sense including bones and cavities.

Bones in the equine skull

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

14 Major bones:

• 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 (cheek bone)
• 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 roof having frontal and parietal bones and floor with sphenoid bone.
• The orbital cavity: It has 5 orbits namely frontal, lacrimal, palatine, sphenoid and zygomatic. It protects and surround the eye. They have unique eyes:
  • 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 is receives only one signal.
• The oral cavity: It is a passage into the respiratory and digestive system.
• The nasal cavity: contains bone that protects 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	Infra-orbital nerve. CNV
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 accoustic meatus	CNVIII
Jugular foramen	CNIX, X, XI
Stylomastoid foramen	CNVII
Mandibular foramen	CNV (mandibular alveolar nerve)
Mental foramen	CNV (mandibular alveolar nerve)

Joints and Ligaments

Joints and Ligaments of Skull

Temporo-mandibular joint is a condylar joint between the mandibular condyles and the mandibular fossae of the temporal bones. It has loose joint capsule with thickening to form lateral ligament, articular disc and mandibular symphysis.

Joints of the hyoid apparatus:

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

Cervical Spine

Joints

Atlanto-occipital joint

It is a condylar modified synovial hinge joint having occipital condyles- cranial articular surface of atlas. There are thickenings that strengthen the spacious joint capsule. They are: Dorsal, Ventral, Lateral. It consists of transverse atlantal ligament that holds the dens of the axis against the ventral arch of the atlas.

Atlanto-axial joint

It is a pivot joint between atlas (C1) and saddle shaped surface of the axis (C2,which extends upon Dens). It has a loose joint capsule and consists of apical ligament of dense that connects apex of dens to the occipital bone. Considering the ROM, ROT of atlas and head upon axis and some accessory lateral flexion. ROT here makes up 73% of cervical ROT.

Cervical spine C3-C4

It is planar, extensive, oval shaped and 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. ROM: Lateral flexion (25-45° each joint; C1/C2- lateral flexion 3,9°)

Ligaments

• Dorsal longitudinal ligament
• Ventral longitudinal ligament
• Ligamentum flavum
• Nuchal ligament:

It connects thoracic vertebra to head and assists in supporting its weight.

It consists of 2 paired parts:

• Funicular (cord) part- poll to +/- 2nd to 4th thoracic spinous process
• Lamellar part- arises from 2nd and 3rd thoracic spinous processes and the funicular part, and inserts on C2-C6 spinous process. 1st digitation going to the axis is very strong and decreases in strength caudally.

Thoracic Spine (T1-T18)

Articular processes

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

Anticlinal vertebrae: It is a point in the caudal thoracic vertebral column at which vertebral anatomic features change. Here its the 13th vertebra.

ROM: Flexion- T17/T18 greatest amount, T3-T9 least amount; Extention- T14-T18 greatest amount, T2-T9 least amount.

Rib neck

It has 2 converse facets: Cranial and Caudal.

Rib 1= C7 and T1 and IV disc

ROM: ROT of rib, greater caudal.

Costovertebral Joint

Joints

It has two distinct articulations between most ribs and the vertebral column:

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

Ligaments

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

Lumbar Spine

Joints

They usually have 6 lumbar vertebrae (L1-L6) while some arabians have only 5 (L1-L5).

Articular processes

Cranial articular processes is fused with mammilary processes. It is concave dorsally and mostly in sagittal alignment.

Caudal processes are convex ventrally, correspond with convexity of cranial articular processes. It is differentiated from last thoracic vertebra by lack of costal facets.

ROM: Lateral flexion and rotation is very limited especially at L4-L6 due to intertransverse joints.

Ligaments of Thoraco-lumbar spine

• Supraspinous ligament: It is 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: It marks the ventral surface of vertebrae from axis to sacrum. It is strongest and is wide caudally. It plays a major role in preventing spine over extention.
• Dorsal longitudinal ligament: It extends from floor of vertebral canal from axis to sacrum and helps to prevent spine hyper-flexion.
• Annulus fibrosis of IVD: thick ventrally.
• Intertransverse ligament
• Interarcuate ligament/ yellow ligament/ ligamentum flavum: It is a elastic ligament that fills dorsal space between arch of adjacent vertebra.

Lumbosacral Joint

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

ROM: Flexion/ Extention 23,4°

Sacrum

It is a fused sacral vertebrae having dorsal and ventral sacral foramina.

Myology and Neurology

Muscles of the Head

Muscles of the Face

Muscles of facial expressions are innervated by 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	To elevate and retract the angle of the mouth
M. Zygomaticus	The fascia covering the masseter	The commissure of the lips	To elevate and retract the angle of the mouth
M. Buccinator	Maxilla and Mandible	The commissure of the lips	To flatten the cheeks and thus pressing the food between the teeth
M. Depressor labii mandibulars	The alveolar border of the mandible	The mandibular lip	To depress and retract the mandibular lip
M. Orbicularis oris	It is the sphincter muscle of the skin and the muscles of the lips Corner of the mouth	Into the lips as it surrounds the mouth	To close the mouth
M. Risorius	Part of M. cutaneous faciei	The angle of the mouth	To retract the angle of the mouth
M. Dilator naris	Alar cartillage	Alar cartillage	Dilate the nostril
M. Lateralis nasi Dorsal part Ventral part	Nasal bone Nasal process of incisive bone	Parietal cartilage Lateral wall of the nasal vestibule	Dilate the nostril and nasal vestibule
M. Caninus	Maxilla close to the rostral extremity of the facial crest	Lateral wing of the nostril	Dilate the nostril laterally
M. Levator nasolabialis	Frontal and nasal bones	Lateral wing of the nostril	Elevate the maxillary lip and the commissure of the mouth Dilate 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: It is mobile and can move independently. It can hear multiple sounds at same time.
• Ear canal

Cartilages: Cartilages of the ear are for the purpose of collecting and transmitting sound to the essential organ of hearing within the temporal bone, and to affect this purpose they, and especially the concha, require to be moved in various directions. This is affected by means of muscles connected to them.

There are 3 cartilages:

• Conchal: It forms framework of all that portion of ear which stands erect. It presents a large vertical opening on one side for the reception of sound, and is attached below to the annular cartilage.
• Annular: It is a small ring of gristle connected with the auditory process of petrous temporal bone.
• Scutiform: It is a small, flat and somewhat triangular cartilaginous plate situated in front of the base of concha, to which it is attached.

The muscles of outer ear:

• Rostral
• Dorsal
• Caudal
• Ventral

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

Muscles of mastication are innervated by Mandibular branch of trigeminal nerve CNV.

Muscle	Origin	Insertion	Action
M. Masseter	The zygomatic arch and the facial crest	The lateral border of ramus of the mandible	Close the mouth
M. Temporalis	The temporal fossa and the temporal crest	The coronoid process of the mandible	Close 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	Close the mouth (to raise the mandible)
M. Pterygoideus lateralis	The pterygoid process of the sphenoid bone	Rostral border of the condyle of the mandible	To draw and move 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	Open the mouth
M. Occipitomandibularis	The jugular process	The caudal border of the ramus of the mandible	Open the mouth

Muscles of Eyes

Muscle	Origin	Insertion	Action	Innervation
M. Orbicularis oculi				Palpebral branch of CN VII
M. Levator palpebrae superioris	Posterior orbit	Fibers of the orbicularis oculi muscle of the upper eyelid	Elevate the upper eyelid	CN III, the oculomotor nerve
M. Malaris		Orbicularis oculi fibers of the lower eyelid	Lower the ventral eyelid	CN VII (facial nerve)
Muller's muscle				Sympathetic nerves
Ciliary muscles
M. Retractor anguli			Retracts and anchors the lateral canthus
M. Levator anguli oculi medialis			Slight elevation of the upper eyelid
M. Frontalis			Slight elevation of the upper eyelid

Muscles of 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.

The tongue’s action: Prehension, Mastication or Chewing.

Innervation: Hypoglossus (CNXII)