VAN 111: Veterinary Gross Anatomy - I (1+2)

OCCIPITAL BONE
(Ox, Horse, Dog, Fowl)

Ox

It is single bone situated on the lower part of the posterior surface of the skull. It consists of lateral (ex-occipital), squamous (supra-occipital) and basilar (basi-occipital) parts.

Lateral part

• Each consists of a condyle and a paramastoid process.
• The condyles articulate with the atlas. Placed lateral to the condyle is the paramastoid process, which serves for muscular attachment.
• The paramastoid process projects downward and backward and is curved medially. Between the root of the paramastoid process and the condyle is the condyloid fossa, in which a large foramen the hypoglossal foramen is present for the XII cranial nerve. Above this is another (often double) foramen which conducts a vein from the condyloid canal.
• The condyloid canal passes upward from a foramen on the medial side of the condyle and opens into the temporal canal.
• The canal lodges a vein which connects the transverse sinusof the duramater and basilar plexus of veins.

Basilar part 

• It is a wide thick bar of bone which extends forward from the ventral margin of foramen magnum.
• Its ventral surface is rounded. It lodges the pons and medulla oblongata on its canal surface.
• The anterior end is fused to the body of post sphenoid. At its junction with the post-sphenoid it presents two tubercles (basilar tubercles) externally, which serve for muscular attachment.
• The lateral border form the medial margins of the foramen lacerum which is for the passage of IX, X and XI cranial nerves.
• The lateral and basilar parts enclose the large foramen magnum at which the cranial cavity and the vertebral canal join.

Squamous part

• It is quadrilateral plate of bone lying between the lateral parts below, squamous temporal laterally, the parietal and interparietal bones with which it fuse before birth.
• It presents externally, a central external occipital protuberance near its junction with the interparietal for the funicular part of the ligamentum nuchae.
• The mastoid foramen is situated on each side, at the junction of the occipital and squamous temporal bones. It communicates with the temporal and condyloid canals at their junction.
• The cerebral surface of the squamous part presents shallow median fossa for the vermis of the cerebellum and above this is a small eminence-the internal occipital protruberance.
• A groove on either side (lodging in life the transverse sinus) leads into the temporal canal.
• In the adult is excavated to form a part of the frontal sinus.

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Horse

• The posterior surface of the skull is entirely by the occipital.
• The condyles in the lateral parts are obliquely placed.
• No other foramina besides the hypoglossal are and present in the condyloid fossa.
• The paramastoid process is longer narrower and less curved.
• The basilar part is longer and narrower and the basilar tubercles are smaller.
• The foramen lacerum is wider and appears as a large triangular gap in the dry skull but is covered to most of its extent by fibrous tissue in life, (refer sphenoid for the foramina formed here).
• The squamous part forms the supero-posterior part of the skull and is crossed externally by prominent transverse and horizontal ridge called nuchal crest.
• Its cranial surface shows a deep central and two shallower lateral depressions for the cerebellum.

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Dog

• Its situation is an in the horse.
• The nuchal crest is prominent, angular and directed backward.
• There is only one foramen in the condyloid fossa.
• Mastoid foramen and condyloid canal are as in the ox.
• The paramastoid processes are very short.
• The basilar part is wide and joins the bulla tympanica on either side.
•  The basilar tubercles are at the junction with the bulla.

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Fowl

• The bones of the cranium in the fowl lose their identity early after hatching as the sutures become ossified and the bones fuse together.
• The occipital condyle is in the basilar part and is single, placed below the foramen magnum and articulates with the atlas and axis.

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