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Occipital


Type

flat

Description

This bone forms most of the back part of the skull. When being viewed from behind it has an oval to round shape. It can be divided into four regions. The greater part of the bone is formed by an internally concave posterosuperior plate, the squama. The base of the bone has a central square portion of bone the basi-occipital or basilar part. Lateral to this and converging with the squama are the two condylar parts or exoccipitals. Together the four regions of the bone form the borders to the large circular opening, the foramen magnum, for passage from the cranial vault to the spinal canal. The cerebellum and posterior lobes of the cerebral hemispheres are nestled on the internal concavity of the bone. Portions of the occipital bone are the cranial most serial homologue of the vertebral skeleton. The bone is typical of the flat cranial bones, consisting of inner and outer laminae of compact bone sandwiching spongy bone in between.

Etymology

Occiput is Latin for the back portion of the head. It is derived from the prefix OB- meaning on, against, or opposite, combined with the Latin caput meaning head.

Articulations

The occipital bone articulates with six other bones: the right and left parietal bones, right and left temporal bones, sphenoid bone, and atlas. The parietal articular surfaces form a denticulate suture on the anterosuperior border of the bone. The anterior border of the lateral portions of the bone form a dentate suture with the posterior squamous and mastoid parts of the temporal bones. The anterior surface of the basi-occipital bone presents a rough quadrilateral surface for articulation with basisphenoid. Finally, the prominent occipital condyles on the inferior aspect of the bone present a convex oval surface that is situated in the shallow concavity of the superior articular facets of the atlas.

Ossification

The occipital bone has an interesting developmental history. Phylogenetically, the base of the bone is the cranial most serial homologue of the vertebral series of bones. The vertebral-like endochondral portion is joined by a large plate-like lamina that develops intramembranously. This bone can be somewhat plastic in its development, but a general plan is recognized as follows. The flat upper squamous portion of the bone superior to the highest nuchal line ossifies intramembranously in two lateral centers that appear during the second embryonic month. These centers will eventually join, but sometimes remain separate from the rest of the occipital elements becoming the so called interparietal bone. It is common for additional centers to appear in the membrane associated with the squama. These centers sometimes remain unfused to the rest of the squama becoming the sutural bones common to the occipitoparietal suture. The rest of the occipital bone, everything below the highest nuchal line, is preformed in cartilage. Two lateral centers for the lower aspect of the squama appear during seventh embryonic week and unite by the third to fourth postnatal month. Some reports document five centers for this cartilaginous supra-occipital portion of bone, paired lateral centers bounding an intermediate central center. The lateral exoccipitals ossify from a single center arising during the eighth week of embryonic life, while the endochondral basi-occipital center appears during the sixth week. The squama fuses with the exoccipital elements during the third year of life and by the sixth year all elements unite as a single element. A cartilaginous synchondrosis exists between the basi-occipital and basisphenoid until early adulthood, when the two bones fuse into a single element.

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