Anatomy of the Skull

Study Notes

The Skull

The skull is formed of 22 bones.
The mandible is the only movable bone, articulating with a synovial joint.
The remaining 21 bones are immovable and connected by fibrous joints.

Parts of the Skull

The skull is divided into the cranium and the facial skeleton.
The cranium is the upper and posterior part enclosing the brain.
The facial skeleton is the anterior part of the skull, encompassing an upper fixed part and a lower movable part (the mandible).

Norma Verticalis

The upper aspect of the skull is known as the Norma Verticalis.
It is formed by 4 bones:
- Frontal bone anteriorly
- 2 Parietal bones on each side
- Occipital bone posteriorly
Key sutures between the bones:
- Coronal Suture: runs transversely between the frontal bone and the 2 parietal bones
- Sagittal Suture: runs in the median plane, connecting the 2 parietal bones
- Lambdoid Suture: runs between the occipital bone and the 2 parietal bones
Special features:
- Bregma: the point of meeting between the sagittal and coronal sutures
  - At birth, this area is filled with a rhomboidal-shaped membrane called the Anterior Fontanelle, which ossifies by 18 months.
- Vertex: the middle of the sagittal suture and the highest point of the skull.
- Lambda: the point of meeting between the sagittal and lambdoid sutures
  - Occupied by the Posterior Fontanelle at birth, a small triangular membrane that ossifies by 6 months.
- 2 Frontal Eminences: the most convex parts of the frontal bone, indicating the site of its ossification centres.
- 2 Parietal Eminences: the most convex parts of the parietal bones, indicating the site of their ossification centres.
- Parietal Emissary Foramen:
  - One on each side of the sagittal suture, about 3-5 cm in front of the lambdoid suture.
  - It transmits an emissary vein connecting the scalp veins with the superior sagittal sinus.

Norma Occipitalis

The Norma Occipitalis is the posterior aspect of the skull.
Bones forming it:
- 2 Parietal borders on the sides & above
- 2 Mastoid Temporal bones on sides & below
- Squamous part of occipital bone below & behind.
Key sutures:
- Lambdoid suture: between the 2 parietal bones and the occipital bone
- Occipito-mastoid suture: between the occipital and the mastoid temporal bones
Special Features:
- External Occipital Protuberance:
  - Lies midway between the lambda and the foramen magnum.
  - Its centre is called the inion.
  - The upper part gives origin to the trapezius muscle.
  - The lower part gives attachment to the upper part of the nuchal ligament.
- External Occipital Crest:
  - Extends from the external occipital protuberance to the posterior border of the foramen magnum.
  - Provides attachment to the base of the nuchal ligament.
- Superior Nuchal Line:
  - A bony ridge extending from the external occipital protuberance towards the mastoid process
  - Its medial 1/3 gives origin to the trapezius muscle.
  - Its lateral 1/3 gives insertion to the sternocleidomastoid muscle.
- Inferior Nuchal Line:
  - Another bony ridge extending laterally from the middle of the external occipital crest, parallel and below the superior nuchal line.
- Highest Nuchal Line:
  - A faint ridge lying 1 cm above the superior nuchal line.
  - Its medial part gives attachment of the epicranial aponeurosis.
  - Its lateral part gives origin to the occipital belly of the occipitofrontalis muscle.
- Mastoid Emissary Foramen:
  - Located on the back of the mastoid process close to the occipitomastoid suture.
  - Allows passage of an emissary vein, connecting the sigmoid sinus to the occipital veins.