At the base of the skull in the occipital bone, there is a large oval opening called the foramen magnum, which allows the passage of the spinal cord.

The basilar part is a thick, somewhat quadrilateral piece in front of the foramen magnum and directed towards the pharynx.

The foramen magnum is a large hole situated in the middle, with the clivus, a smooth part of the occipital bone travelling upwards in front of it.

To the sides of the foramen sitting at the junction between the lateral and base of the occipital bone are the hypoglossal canals.

Further out, at each junction between the occipital and petrous portion of the temporal bone lies a jugular foramen.

[2] The inner surface of the occipital bone is marked by dividing lines as shallow ridges, that form four fossae or depressions.

At the midpoint where the lines intersect a raised part is formed called the internal occipital protuberance.

The nuchal plane of the squamous part is ossified from two centers, which appear about the seventh week of fetal life and soon unite to form a single piece.

Union of the upper and lower portions of the squamous part takes place in the third month of fetal life.

An occasional centre (Kerckring) appears in the posterior margin of the foramen magnum during the fifth month; this forms a separate ossicle (sometimes double) which unites with the rest of the squamous part before birth.

Each of the lateral parts begins to ossify from a single center during the eighth week of fetal life.

The basilar portion is ossified from two centers, one in front of the other; these appear about the sixth week of fetal life and rapidly coalesce.

The basion-dens line as seen on a radiograph is the distance between the basion and the top of the dens, used in the diagnosis of dissociation injuries.

In Chondrichthyes and Agnatha, the occipital does not form as a separate element, but remains part of the chondrocranium throughout life.