Ox
Occipital bone of Ox 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 condylesarticulate 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 basilarparts 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-theinternal 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.
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.
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.
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.