The squamous part of occipital bone is situated above and behind the

foramen magnum The foramen magnum ( la, great hole) is a large, oval-shaped opening in the occipital bone of the skull. It is one of the several oval or circular openings (foramina) in the base of the skull. The spinal cord, an extension of the medulla oblo ...

, and is curved from above downward and from side to side.

External surface

The external surface is convex and presents midway between the summit of the bone and the

a prominence, the

external occipital protuberance Near the middle of the squamous part of occipital bone is the external occipital protuberance, the highest point of which is referred to as the inion. The inion is the most prominent projection of the protuberance which is located at the posterioi ...

and inion. Extending lateralward from this on either side are two curved lines, one a little above the other. The upper, often faintly marked, is named the highest nuchal line, and to it the

epicranial aponeurosis The epicranial aponeurosis (aponeurosis epicranialis, galea aponeurotica) is an aponeurosis (a tough layer of dense fibrous tissue). It covers the upper part of the skull in humans and many other animals. Structure In humans, the epicrani ...

is attached. The lower is termed the

superior nuchal line The nuchal lines are four curved lines on the external surface of the occipital bone: * The upper, often faintly marked, is named the highest nuchal line, but is sometimes referred to as the Mempin line or linea suprema, and it attaches to the epi ...

. That area of the squamous part, which lies above the highest nuchal lines is named the occipital plane ''(planum occipitale)'' and is covered by the ''

occipitalis The occipitalis muscle (occipital belly) is a muscle which covers parts of the skull. Some sources consider the occipital muscle to be a distinct muscle. However, Terminologia Anatomica currently classifies it as part of the occipitofrontalis musc ...

muscle''. That below, termed the nuchal plane, is rough and irregular for the attachment of several muscles. From the external occipital protuberance, an often faintly marked ridge or crest, the median nuchal line, descends to the foramen magnum and affords attachment to the

nuchal ligament The nuchal ligament is a ligament at the back of the neck that is continuous with the supraspinous ligament. Structure The nuchal ligament extends from the external occipital protuberance on the skull and median nuchal line to the spinous pr ...

. Running from the middle of this line across either half of the nuchal plane is the inferior nuchal line. Several muscles are attached to the outer surface of the squamous part, thus the superior nuchal line gives origin to the

and

trapezius The trapezius is a large paired trapezoid-shaped surface muscle that extends longitudinally from the occipital bone to the lower thoracic vertebrae of the spine and laterally to the spine of the scapula. It moves the scapula and supports t ...

muscles, and insertion to the

sternocleidomastoid The sternocleidomastoid muscle is one of the largest and most superficial cervical muscles. The primary actions of the muscle are rotation of the head to the opposite side and flexion of the neck. The sternocleidomastoid is innervated by the acces ...

and

splenius capitis The splenius capitis () () is a broad, straplike muscle in the back of the neck. It pulls on the base of the skull from the vertebrae in the neck and upper thorax. It is involved in movements such as shaking the head. Structure It arises from t ...

muscles. Into the surface between the superior and inferior nuchal lines the

semispinalis capitis The semispinalis muscles are a group of three muscles belonging to the transversospinales. These are the semispinalis capitis, the semispinalis cervicis and the semispinalis thoracis. The semispinalis capitis (''complexus'') is situated at the ...

and the obliquus capitis superior are inserted, while the inferior nuchal line and the area below it receive the insertions of the rectus capitis posterior major and minor. The posterior atlantooccipital membrane is attached around the postero-lateral part of the foramen magnum, just outside the margin of the foramen.

Internal surface

The internal surface is deeply concave and divided into four fossae by the

cruciform eminence The cruciform eminence (or cruciate eminence) divides the deeply concave internal surface of the occipital bone into four fossae: * The upper two fossae are called the cerebral fossae, are triangular and lodge the occipital lobes of the cerebrum. ...

. The upper two fossae are triangular and lodge the

occipital lobe The occipital lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The name derives from its position at the back of the head, from the Latin ''ob'', "behind", and ''caput'', "head". The occipital lobe is the ...

s of the

cerebrum The cerebrum, telencephalon or endbrain is the largest part of the brain containing the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb. In ...

; the lower two are quadrilateral and accommodate the hemispheres of the

cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cer ...

. At the point of intersection of the four divisions of the cruciform eminence is the

internal occipital protuberance Along the internal surface of the occipital bone, at the point of intersection of the four divisions of the cruciform eminence, is the internal occipital protuberance. Running transversely on either side is a groove for the transverse sinus. Add ...

. From this protuberance the upper division of the cruciform eminence runs to the superior angle of the bone, and on one side of it (generally the right) is a deep groove, the sagittal sulcus, which lodges the hinder part of the

superior sagittal sinus The superior sagittal sinus (also known as the superior longitudinal sinus), within the human head, is an unpaired area along the attached margin of the falx cerebri. It allows blood to drain from the lateral aspects of anterior cerebral hemisphe ...

. To the margins of this sulcus the

falx cerebri The falx cerebri (also known as the cerebral falx) is a large, crescent-shaped fold of dura mater that descends vertically into the longitudinal fissure between the cerebral hemispheres of the human brain,Saladin K. "Anatomy & Physiology: The U ...

is attached. The lower division of the cruciform eminence is prominent and is named the internal occipital crest; it bifurcates near the foramen magnum and gives attachment to the

falx cerebelli The falx cerebelli is a small sickle-shaped fold of dura mater projecting forwards into the posterior cerebellar notch as well as projecting into the vallecula of the cerebellum between the two cerebellar hemispheres. The name comes from two L ...

. In the attached margin of this falx is the occipital sinus, which is sometimes duplicated. In the upper part of the internal occipital crest, a small depression is sometimes distinguishable; it is termed the vermian fossa since it is occupied by part of the

vermis The cerebellar vermis (from Latin ''vermis,'' "worm") is located in the medial, cortico-nuclear zone of the cerebellum, which is in the posterior fossa of the cranium. The primary fissure in the vermis curves ventrolaterally to the superior s ...

of the cerebellum. Transverse grooves, one on either side, extend from the internal occipital protuberance to the lateral angles of the bone; those grooves accommodate the

transverse sinuses The transverse sinuses (left and right lateral sinuses), within the human head, are two areas beneath the brain which allow blood to drain from the back of the head. They run laterally in a groove along the interior surface of the occipital bone. ...

, and their prominent margins give attachment to the

tentorium cerebelli The cerebellar tentorium or tentorium cerebelli (Latin for "tent of the cerebellum") is an extension of the dura mater that separates the cerebellum from the inferior portion of the occipital lobes. Structure The cerebellar tentorium is an arche ...

. The groove on the right side is usually larger than that on the left and is continuous with that for the

. Exceptions to this condition are, however, not infrequent: the left may be larger than the right or the two may be almost equal in size. The angle of union of the superior sagittal and transverse sinuses is named the confluence of the sinuses, and its position is indicated by a depression situated on one or other side of the protuberance.

Additional images

File:Squamous part of occipital bone - animation2.gif, Human skull seen from below. Squamous part is shown red. File:Squamous part of occipital bone - animation.gif,

Occipital bone The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobes of the cer ...

, inner surface. Squamous part is shown red. File:Gray130.png,

, inner surface. (Squamous part is top half, portion above

References

{{Authority control Bones of the head and neck