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General Overview of the Skull

Cranium

For medical students3 min readUpdated 2026-10-10

The skull is a complex bony skeleton of the head. It forms a strong container and protection for the brain, the organs of vision, hearing, balance, smell and taste, and also serves as a sturdy support for the initial segments of the digestive and respiratory systems.

Number of bonesUsually consists of 23 bones (not counting the auditory ossicles).
NeurocraniumIn humans it considerably exceeds the facial part in size.
DevelopmentPasses through three stages: membranous, cartilaginous, and bony.
SymmetryFollows bilateral symmetry overall, but shows individual asymmetry.

Principles of Division and Parts of the Skull

The structural complexity of the skull is directly due to the early formation and development of the brain and sense organs: their anatomy dictates the shape and finer details of the bony skeleton of the head.

Functionally, the skull is divided into two major parts:

Aside from the functional division, there is also a topographic classification that divides the skull along a conventional line into two parts:

  1. Calvaria (Calvaria).
  2. Base of the skull (Basis cranii). The base is further subdivided into the external base (Basis cranii externa) and the internal base (Basis cranii interna).

Classification and List of Bones (*Ossa cranii*)

In the typical anatomical configuration there are 23 bones (the auditory ossicles are not included in this count). All bones are assigned to specific parts and are divided into paired and unpaired bones.

Bones of the neurocranium:

Bones of the facial skull:

A special group is formed by the pneumatized (air-containing) bones. These include the frontal, temporal, sphenoid, and ethmoid bones, as well as the maxilla. Their structure contains special cavities lined internally with mucosa.

Cranial Norms (Views) and Landmarks

To properly describe structures along a given axis and within a single plane, anatomy distinguishes five cranial norms (views):

  1. Facial or frontal norm — Norma facialis (frontalis). Viewed from the front, important structures such as the orbit (Orbita) and the bony nasal cavity (Cavitas nasi osseum) can be identified.
  2. Occipital norm — Norma occipitalis. This is the view of the skull from behind.
  3. Superior or vertical norm — Norma superior (verticalis). This is the view of the skull from above.
  4. Inferior or basal norm — Norma inferior (basalis). Viewed from below, one can identify the hard palate — Palatum durum (osseum), which serves as the upper wall of the oral cavity, as well as part of the external base of the skull (Basis cranii externa).
  5. Lateral norm — Norma lateralis. The lateral view allows identification of the temporal fossa (Fossa temporalis), the infratemporal fossa (Fossa infratemporalis), and the pterygopalatine fossa (Fossa pterygopalatina).

The internal base of the skull is considered separately (usually studied via the vertical norm with the calvaria removed). On it, three fossae are distinguished: the anterior, middle, and posterior cranial fossae — Fossa cranii anterior, media, posterior.

Development and Phylogeny of the Skull

In the course of evolution (phylogenesis), the skull passes through three successive stages of development:

  1. Membranous skull (Desmocranium).
  2. Cartilaginous skull (Chondrocranium).
  3. Bony skull (Osteocranium).

These evolutionary stages explain the emergence of the two distinct parts (neurocranium and viscerocranium), as well as the occurrence of anatomical variation.

Specific features of the human skull:

Mnemonic

To remember the unpaired bones of the neurocranium, recall that they lie strictly along the midline axis (from front to back): frontal, ethmoid, sphenoid, occipital.

Frequently asked questions

Which bones form the anterior cranial fossa?

The anterior cranial fossa is formed by the frontal, sphenoid, and ethmoid bones. The following structures take part in its formation:

  • Frontal bone (Os frontale) — the frontal squama (squama frontalis) bounds the fossa anteriorly, while the orbital part (pars orbitalis) forms its floor.
  • Sphenoid bone (Os sphenoidale) — the lesser wings (Alae minores ossis sphenoidalis) bound the fossa posteriorly.
  • Ethmoid bone (Os ethmoidale) — represented by the cribriform foramina (Foramina cribrosa), through which the fossa communicates with the nasal cavity.
Which bones form the walls of the orbit?

The walls of the orbit are formed by the frontal, sphenoid, zygomatic, ethmoid, lacrimal, and palatine bones, as well as the maxilla.

  • Superior wall (Paries superior) — formed by the orbital part of the frontal bone and the lesser wings of the sphenoid bone.
  • Inferior wall (Paries inferior) — by the orbital surfaces of the body of the maxilla and of the zygomatic bone.
  • Lateral wall (Paries lateralis) — by the zygomatic process of the frontal bone, the orbital surface of the greater wing of the sphenoid bone, and the zygomatic bone.
  • Medial wall (Paries medialis) — by the frontal process of the maxilla, the lacrimal bone, the orbital plate of the ethmoid bone, the lateral surface of the body of the sphenoid bone, and the orbital process of the palatine bone.
Which bones and their parts form the bony hard palate?

The bony hard palate is formed by parts of the maxillae and the palatine bones. The following bony structures take part in its formation:

  • Palatine processes of the maxillae (Processus palatini maxillarum) — form the bony base of the hard palate.
  • Horizontal plates of the palatine bones (Laminae horizontales ossium palatinorum) — complete the hard palate posteriorly.
Through which foramina and canals does the pterygopalatine fossa communicate?

The pterygopalatine fossa communicates with adjacent anatomical structures through the following foramina, fissures, and canals:

  • Foramen rotundum (Foramen rotundum) — communication with the middle cranial fossa.
  • Sphenopalatine foramen (Foramen sphenopalatinum) — communication with the nasal cavity.
  • Pterygoid canal (Canalis pterygoideus) — communication with the region of the foramen lacerum.
  • Greater palatine canal (Canalis palatinus major) — communication with the oral cavity.
  • Inferior orbital fissure (Fissura orbitalis inferior) — communication with the orbit.
  • Pterygomaxillary fissure (Fissura pterygomaxillaris) — communication with the infratemporal fossa.
Which anatomical structures pass through the jugular foramen?

The following structures pass through the jugular foramen (Foramen jugulare):

  • Cranial nerves IX, X, and XI — the glossopharyngeal, vagus, and accessory nerves.
  • Veins.

In the region of the jugular foramen, the intrajugular process (Processus intrajugularis) separates the veins from the nerves.

Is the skull perfectly symmetrical?

There is no absolute symmetry. Although the skull follows the general rules of bilateral symmetry, perfect mirror-image symmetry never occurs — every person shows some degree of normal individual asymmetry.

What are pneumatized bones and what are they for?

These are bones containing special cavities lined with mucosa (for example, the maxilla, frontal, and sphenoid bones). Their development is closely related to the formation of the speech apparatus in humans.

What accounts for the complexity of the shape and structure of the skull bones?

The shape and structural details of the skull are determined by its very early formation during embryogenesis and the subsequent development of the brain and sense organs, for which the skull forms housings and protective capsules.

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