Occipital bone

Saucer-shaped membrane bone situated at the back and lower part of the cranium

The number of nuclei for the occipital plane is usually given as four, two appearing near the middle line about the second month, and two some little distance from the middle line about the third month of fetal life.

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.

The occipital plane is said to be ossified from two centers and the basilar portion from one.

About the fourth year the squamous part and the two lateral parts unite, and by about the sixth year the bone consists of a single piece. Between the 18th and 25th years the occipital and sphenoid bone become united, forming a single bone.

In mammals, however, the condyle has divided in two, a pattern otherwise seen only in a few amphibians.

This article incorporates text in the public domain from of the 20th edition of