Principles of division and sections of the skull
The complexity of the skull structure is directly due to the early formation and development of the brain and sense organs: their anatomy dictates the shape and fine details of the bony skeleton of the head.
From a functional perspective, the skull is divided into two major parts:
- Cranial vault / Neurocranium (Neurocranium) — forms the cranial cavity (Cavitas cranii), which is the receptacle for the brain.
- Facial or visceral skull (Viscerocranium) — forms the strong bony framework of the face.
In addition to functional division, there is a topographical classification that divides the skull into two parts using an imaginary line:
- Skull roof (Calvaria).
- Skull base (Basis cranii). The base itself is further subdivided into external (Basis cranii externa) and internal (Basis cranii interna).
Classification and list of bones (Ossa cranii)
In the standard anatomical structure, there are 23 bones (auditory ossicles are not included in this number). All bones are strictly distributed by sections and divided into paired and unpaired.
Bones of the neurocranium:
- Unpaired bones (4 total): frontal bone (Os frontale), occipital bone (Os occipitale), sphenoid bone (Os sphenoidale), and ethmoid bone (Os ethmoidale).
- Paired bones (two pairs): temporal bone (Os temporale) and parietal bone (Os parietale) — left and right.
Bones of the facial skull:
- Paired bones: maxilla (Maxilla), palatine bone (Os palatinum), nasal bone (Os nasale), lacrimal bone (Os lacrimale), zygomatic bone (Os zygomaticum), and inferior nasal concha (Concha nasalis inferior).
- Unpaired bones: mandible (Mandibula), vomer (Vomer), and hyoid bone (Os hyoideum).
A special group consists of pneumatic bones. These include the frontal, temporal, sphenoid, ethmoid bones, and the maxilla. Their structure contains special cavities lined on the inside with a mucous membrane.
Cranial norms (views) and landmarks
To accurately describe structures along a specific axis and in a single plane, anatomy distinguishes five cranial norms:
- Facial or frontal norm — Norma facialis (frontalis). When viewed from the front, important formations such as the orbit (Orbita) and the bony nasal cavity (Cavitas nasi osseum) are defined.
- Occipital norm — Norma occipitalis. Represents the posterior view of the skull.
- Superior or vertical norm — Norma superior (verticalis). This is the top view of the skull.
- Inferior or basal norm — Norma inferior (basalis). When viewed from below, one can distinguish the hard palate — Palatum durum (osseum), which serves as the upper wall of the oral cavity, and examine part of the external skull base (Basis cranii externa).
- Lateral norm — Norma lateralis. The side view allows identification of the temporal fossa (Fossa temporalis), infratemporal fossa (Fossa infratemporalis), and pterygopalatine fossa (Fossa pterygopalatina).
The internal skull base is considered separately (usually studied via the vertical norm with the roof removed). Three fossae are distinguished on it: the anterior, middle, and posterior cranial fossae — Fossa cranii anterior, media, posterior.
Development and phylogeny of the skull
In the process of evolution (phylogeny), the skull goes through three successive stages of development:
- Membranous skull (Desmocranium).
- Cartilaginous skull (Chondrocranium).
- Bony skull (Osteocranium).
This is precisely what explains the emergence of two distinct divisions (neurocranium and viscerocranium), as well as the occurrence of variant anatomy.
Specific characteristics of the human skull:
- The total number of bones has decreased compared to ancestors.
- The neurocranium is located strictly above the facial part and significantly exceeds it in size.
- The dimensions of the facial bones in humans are significantly smaller compared to animals.
- Specific relationships have formed between the mandible and the rest of the skull.
- In connection with the appearance and development of speech, pneumatic bones have formed.
Briefly: answers to topical 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, and the orbital part (pars orbitalis) forms its base.
- 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, palatine bones, and 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 the zygomatic bone.
- Lateral wall (Paries lateralis) — by the zygomatic process of the frontal bone, the orbital surfaces 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 palatine bones. The following bony structures take part in its formation:
- Palatine processes of the maxillae (Processus palatini maxillarum) — form the bony framework of the hard palate.
- Horizontal plates of the palatine bones (Laminae horizontales ossium palatinorum) — complement the hard palate posteriorly.
Through which openings and canals does the pterygopalatine fossa communicate?
The pterygopalatine fossa communicates with adjacent anatomical formations through the following openings, fissures, and canals:
- Round foramen (Foramen rotundum) — communicates with the middle cranial fossa.
- Sphenopalatine foramen (Foramen sphenopalatinum) — communicates with the nasal cavity.
- Pterygoid canal (Canalis pterygoideus) — communicates with the area of the foramen lacerum.
- Greater palatine canal (Canalis palatinus major) — communicates with the oral cavity.
- Inferior orbital fissure (Fissura orbitalis inferior) — communicates with the orbit.
- Pterygomaxillary fissure (Fissura pterygomaxillaris) — communicates with the infratemporal fossa.
Which anatomical structures pass through the jugular foramen?
The structures passing through the jugular foramen (Foramen jugulare) include:
- CN IX, X, and XI — glossopharyngeal, vagus, and accessory nerves.
- Veins.
In the region of the jugular foramen, the intrajugular process (Processus intrajugularis) separates the veins and nerves.
To memorize the unpaired bones of the neurocranium, remember that they lie strictly along the median axis (from the forehead to the occiput): frontal, ethmoid, sphenoid, occipital.
Class and exam questions
Absolute symmetry does not exist. Although the skull obeys the general laws of bilateral symmetry, perfect mirror-imaging never occurs — every individual always exhibits normal physiological asymmetry.
Absolute symmetry does not exist. Although the skull obeys the general laws of bilateral symmetry, perfect mirror-imaging never occurs — every individual always exhibits normal physiological asymmetry.
What are pneumatic bones and what is their function?
These are bones containing specific mucosa-lined cavities (e.g., maxilla, frontal, and sphenoid bones). Their development is closely linked to the formation of the human speech apparatus.
What determines the complexity of the shape and structure of skull bones?
The shape and structural details of the skull are determined by very early patterning in embryogenesis and subsequent development of the brain and sensory organs, for which the skull forms receptacles and protective capsules.
What else to study on this topic
- Boundaries and communications of the temporal, infratemporal, and pterygopalatine fossae
- Structure of the bony nasal cavity walls (Cavitas nasi osseum)
- Anatomy and foramina of the cranial fossae of the internal skull base
- Formation of the hard palate (Palatum durum) and its anatomical details
- Mechanisms of individual skull asymmetry formation
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