Veterinary Gross Anatomy

AXIS (EPISTROPHEUS)
(Ox, Sheep and Goat, Horse, Pig, Dog, Rabbit, Fowl)

Ox

The axis is the longest of the vertebrae. Like the atlas, it is an atypical vertebra. It is also called vertebra dentata due to a tooth-like process called dens present on the anterior aspect of the centrum. The anterior articular processes are absent.

• The body of the axis is the longest.
• The anterior end of the body presents centrally the dens or odontoid process.
• The dens represent the body of the atlas, which gets fused with the body of the axis during development.
• The process is spout-like, wide and has a convex articular surface below for articulation with the atlas.
• Its dorsal (superior) face is concave and rough for the attachment of the odontoid ligament (ligamentum dentis).
• The two surfaces are separated by a sharp convex edge, which may be notched in its centre. On each side of the root of this process, the anterior end of the body presents a nearly flat articular surface, which is continuous with on the ventral face of the dens and this articular area articulates with the posterior articular surface of the atlas. The posterior extremity the body presents the usual concavity for the anterior extremity of third cervical vertebra.The pedicles present the usual posterior notches but in the place of the anterior ones are large oval inter vertebral foramina for the exit of the second cervical spinal nerves.
• The anterior articular processes are absent and the posterior pair resembles those of the succeeding vertebrae.
• The transverse processes are small, undivided and project backwards.
• The foramen transversarium, when present, is in the form of an osseous canal passing through the root of the transverse process having anterior and posterior openings.
• The spine is a very large and strong process.Its summit is narrow in front but thickens and widens posteriorly and serves for the attachment of the lamellar portion of the ligamentum nuchae and muscles.
• The ventral spine is in the form of a ridge, becoming tuberculate at its posterior end.

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Sheep and Goat

• The spinous process of the axis is not enlarged posteriorly.

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Horse

• The body is larger.
• The dens is longer narrower and more pointer (conical).
• The anterior extremity presents an undulating saddle-shaped articular area.
• The spine is more massive and divides posteriorly into two ridges, which terminate on the posterior articular processes.
• The transverse process is thinner.

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Pig

• The dens is in the form of a thick cylindrical rod.
• The axis has a large spinous process directed upward and backward.

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Dog

• The dens is rounded very long and reaches almost to the occipital bones.
• The anterior articular area, which flanks the dens, are condyloid and very oblique.
• The spine is of great size and it overhangs the dorsal arch of atlas.
• The anterior notches are large and are never converted into foramina.

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Rabbit

• Comparatively short.

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Fowl

• The anterior articular processes are present and they articulate with the facets on the posterior aspects of the dorsal arch of atlas.
• The body presents below the dens a concave articular area for articulation with the ventral arch of atlas.
• The dens articulate with the occipital condyle.
• The dorsal and ventral spines are present.
• The transverse processes are absent.

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