Veterinary Gross Anatomy

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

Ox

This vertebra is atypical in form and structure.The body and spinous processes are absent. It has the form of a ring from which two curved plates, the wings project laterally.

Ring

• The ring encloses a large vertebral foramen and consists of two lateral masses connected by dorsal and ventral arches.
• The lateral masses present two deep anterior articular cavities, which receive the occipital condyles; the cavities are partially divided into dorsal and ventral parts by a non- articular area and are separated by a narrow interval below.The posterior articular surfaces are confluent on the ventral arch, flattened behind and are continued into the vertebral canal forming an extensive area for the dens of the axis.
• The dorsal arch presents a median dorsal tuberosity and is concave ventrally. It is perforated on either side near its anterior margin by the intervertebral foramen, which is connected by a short groove -the alar groove, with another foramen-alar foramen, which perforates the wing. The anterior border of the dorsal arch is notched. The posterior is thin and shows a central and two lateral notches.
• The ventral arch is thicker and less curved than the dorsal. On its lower surface in the median plane is an obtuse eminence the ventral tubercle for the tendon of longus colli. The dorsal face has posteriorly a transversely flat articular surface the fovea dentis for the dens of the axis. In front of this a transverse rough area for the ligamentum dentis  (odontoid ligament).

Wings

• The wings represent the transverse processes of the other vertebrae.
• Each is a horizontal plate of bone springing from the lateral aspect of the lateral masses and terminating outward in a rough thick edge. This edge furnishes attachment to the muscles of the head and neck.
• The dorsal surface is rough, between the ventral aspect of the wing and the lateral mass is a cavity -fossa atlantis. At the bottom of the fossa are two foramina, the anterior called alar foramen that perforates the wing and is for the passage of the ventral primary branch of the first spinal nerve and branches of vertebral vessels. The posterior foramen pierces the lateral mass and communicates with the spinal canal. The alar foramen is connected on the dorsal aspect by a short alar groove with the intervertebral foramen.

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Sheep and Goat

• The atlas differs chiefly in that the prominence on the dorsal arch is much less developed.

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Horse

• The wing slopes downwards and outwards
• Besides the alar and intervertebral foramina there is foramen transversarium at the posterior part of the wing.
• The posterior edge of the dorsal arch does not present the notches as in the ox.
• The posterior articular surfaces are saddle shaped.  

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Pig

• The atlas has the large tuberosity.
• The wing is flattened and bears a posterior tuberosity.
• Foramen transversarium is small or absent.

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Dog

• Ventral arch is narrower from before backward and bears a small tubercle posteriorly.
• Dorsal surface of dorsal arch is strongly convex and rough centrally.
• Wings are horizontal.
• An alar notch is present on the anterior border instead of alar foramen.
• Foramen transversarium is present.

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Rabbit

• Comparatively short.

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Fowl

• It is ring like.
• The ventral arch presents anteriorly a concave articular area for the single occipital condyle.
• Posteriorly there are three facets for axis - one median on the ventral arch and two on the posterolateral aspects of the dorsal arch.

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