Foramina of the Skull

Gross Anatomy

SKULL DEVELOPMENT

In basic terms, the skull is made up of two sets of bones which begin to develop

early in utero. The neurocranium consists of the bones forming a protective case

around the brain and the viscerocranium consists of the bones of the face. 

Osteogenesis of the skull (bone development) begins in the 7th/8th weeks of fetal life

and continues into adulthood. Osteogenesis occurs by both endochondral and

intramembranous ossification.

NEUROCRANIUM

The neurocranium surrounds the brain creating the cranial vault and consists of 8 flat

bones including the parietal (2), temporal (2), frontal, occipital and ethmoid bones

(Figure 1). However, the bones are in fact curved in shape - convex externally and

concave internally.

Calvaria: 

Superior aspect of the neurocranium – Roof/Skullcap

The flat bones of the roof are formed by intramembranous ossification of neural crest

cells, creating the membranous neurocranium. The membranous portion of the

skull surrounds the brain creating a protective vault.

Ossification of the skull is not complete at the time of birth and the bones of the skull

articulate via sutures or fontanelles (non-ossified articulations). Fontanelles are

spaces between the cranial bones that are filled with fibrous membranes. This is

required during childbirth as the flexibility of the sutures and fontanelles allows the

bones to overlap so the baby's head can pass through the birth canal, without

compressing and damaging the brain. The sutures remain flexible during childhood

Foramina of the Skull

to allow the brain to grow. As brain growth and expansion occurs, the separate

bones of the calvaria are displaced in an outward direction. This creates tension at

the sutures and fontanelles promoting bone deposition, which continues as the brain

continues to develop and grow. This continues until the 4th year of life, when the

bones of the cranium are united by fibrous and hyaline cartilage sutures.

Cranial base: 

Inferior aspect of the neurocranium – Floor

The cranial base consists mainly of the sphenoid, temporal and occipital bones

which are formed by endochondral ossification of cartilage creating cartilaginous neurocranium (chondrocranium).

Figure 1. (a) Superior view of cranial base of the neurocranium. (b) Lateral view of skull. Area superior to the red line shows the calvaria of the neurocranium.

Foramina of the Skull

VISCEROCRANIUM

The viscerocranium is situated anterior to the neurocranium and is comprised of 14

irregular bones including 6 pairs of bones:

1. Zygomatic (x2)

2. Palatine (x2)

3. Nasal (x2)

4. Lacrimal (x2)

5. Maxilla (x2)

6. Inferior nasal concha (x2)

7. Mandible

8. Ethmoid

The viscerocranium develops in the mesenchyme (neural crest cells from which the

calvaria of the neurocranium is also derived) or sclerotome (derived from mesoderm)

of the 1st and 2nd embryonic pharyngeal arches. The bones of the viscerocranium

create what is commonly referred to as the facial skeleton (Figure 2).

Within commonly used texts there is debate as to which bones are included in the

facial skeleton (viscerocranium). Some make a distinction between the bones of the

skull based on the embryological origins. Other texts include all the bones that can

be seen on an anterior view of the skull as part of the . As a result, it is not

uncommon to see the frontal, hyoid, parietal and sphenoid bones described as part

of the facial skeleton.

Foramina of the Skull

Figure 2. (a) Anterior view of the facial bones of the skull (viscerocranium). (b) Inferior view of the skull base with mandible removed showing the relations of

the bones of the viscerocranium and neurocranium.

CRANIAL FOSSAE

The internal surface of the neurocranium base has 3 depressions which create the

bowl shape of the cranial cavity that accommodate the brain. Figure 2 displays the 3

depressions/fossae. The fossae increase in depth from anterior to posterior and are

termed the:

1. Anterior cranial fossa

2. Middle cranial fossa

3. Posterior cranial fossa

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Figure 2. (a) Lateral view of cranial fossae of the adult skull with brain in situ. (b) Superior view of cranial fossae of adult skull with brain removed.

ANTERIOR CRANIAL FOSSA

The anterior fossa is formed from the frontal bone anteriorly, the ethmoid bone in the

midline and the body and the lesser wings of the sphenoid bone posteriorly (Figure

3). Two skull foramina located in the anterior fossa:

1. Foramen caecum

2. Foramen of the cribriform plate 

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Figure 3. (a) Superior view of the skull base with the anterior cranial fossa highlighted in green. (b) Bones that form the anterior cranial fossa.

MIDDLE CRANIAL FOSSA

The middle cranial fossa is butterfly shaped and is located posteroinferior to the

anterior fossa (Figure 4). Both the greater wings of the sphenoid and temporal bone

create the lateral sections of the fossa. The middle cranial fossa contains 6 foramina:

1. Optic canal

2. Superior orbital fissure

3. Foramen rotundum

4. Foramen ovale

5. Foramen spinosum 

6. Foramen lacerum 

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Figure 4. (a) Superior view of the skull base with the middle cranial fossa highlighted in orange. (b) Bones that form the middle cranial fossa.

POSTERIOR CRANIAL FOSSA

The posterior fossa is the largest and deepest of the 3 fossae. The occipital bone is

the main contributor to the fossa and the temporal bone forms the antero-lateral

boundaries (Figure 5). There are 4 foramina found in the posterior cranial fossa:

1. Internal acoustic meatus

2. Jugular foramen 

3. Hypoglossal canal

4. Foramen magnum

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Figure 5. (a) Superior view of the skull base with the posterior cranial fossa highlighted in red. (b) Bones that form the posterior cranial fossa.

IMAGES AND TABLES OF NEUROCRANIUM FORAMINA AND CONTENTS

Figure 6. Superior view of the cranial cavity with the brain removed and the foramina of the neurocranium highlighted.

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Figure 7. Inferior view of the skull base with the mandible removed and neurocranium foramina highlighted.

Table 1. Overview of the foramina found in the anterior cranial fossa and the foramina contents.

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Table 2. Overview of the foramina found in the middle cranial fossa and the foramina contents.

Table 3. Overview of the foramina found in the posterior cranial fossa and the foramina contents.

Foramina of the Skull

VISCEROCRANIUM FORAMINA

Forming the anterior part of the cranium, the viscerocranium region has 7 foramina

that allow structures to enter and exit the cranial cavity. The bones of the

viscerocranium surround the mouth, nasal cavity and orbit. A branch of the trigeminal

nerve is found in each foramina - highlighting the importance of the trigeminal (CN V)

nerve in facial innervation.

Read through the Table 4, which summarises the foramina and the key structures.

Use Figures 8 and 9 to aid your understanding of the foramina location.

Table 4. Overview of the foramina found in the facial bones of the skull and the foramina contents.

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Figure 8. Foramina of the viscerocranium region of the skull. (Not depicted is the mandibular foramen as it is located on the posterior surface of the

mandible)

Figure 9. Lateral view of adult skull showing foramina and nerve contents of viscerocranium.

Foramina of the Skull