Squamous part of occipital bone

The posterior flat part of the occipital bone is known as the squamous part of occipital bone. It connects at the top and back with the two parietal bones at the lambdoid suture and on the sides with the temporal bone’s mastoid part at the occipitomastoid suture.

The external surface of the squamous part of occipital bone has several key features. For example, there’s a bony bump in the middle called the external occipital protuberance, from which curved lines, known as the superior nuchal lines, extend to the sides. The highest point of this bump is termed the inion. Dropping from the external occipital protuberance to the posterior edge of the foramen magnum is the external occipital crest, to which the ligamentum nuchae attaches. Roughly an inch below the superior nuchal lines are two more lines, the inferior nuchal lines, curving to the sides from the middle of the external occipital crest.

The internal surface of the squamous part of occipital bone mainly forms the floor of posterior cranial fossa of the cranial cavity. Significant features here include a central bump called the internal occipital protuberance, marking the point where the confluence of sinuses is located. From this bump, a bony ridge known as the internal occipital crest known runs down to the posterior edge of the foramen magnum. This crest provides a connection point for the falx cerebelli. It also forms a shallow depression called the vermian fossa near the foramen magnum’s posterior edge, and extends outwards as channels for the marginal sinuses. On both sides of the internal occipital crest, there are two large hollows called the cerebellar fossae, which accommodates the cerebellar hemispheres. From the internal occipital protuberance, grooves for transverse sinuses run outward and continue as grooves for the sigmoid sinuses. Each groove then moves down towards the jugular foramen.