Equine Spine and Head Anatomy

Original Editor - Chelsea McLene

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).

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.