Skull

Generality

The skull is the set of bones that form the head of the human skeleton.

According to the anatomists, it presents two distinct main regions: the neurocranium and the splanchocranium.

The neurocranium has 8 bones. These bone elements form the so-called cranial vault, in which the brain and some sense organs reside, and the so-called skull base, which acts as a support for the aforementioned cranial vault. Some neurocranium bones are even, others are uneven.

The splanchocranium is home to 14 bones. These make up the face, therefore also the nasal cavities, the orbital cavities and the mouth. Even in the splanchocranium there are the even and uneven bone elements.

The bones of the skull serve mainly to protect the various brain structures (brain, diencephalon, cerebellum and brainstem) and the sense organs included in the cranial vault.

What is the skull

The skull is the bone structure that constitutes the head of the human skeleton.

Thus, it includes both the bones in which the brain is housed and the bones that make up the face.

The components of the brain

The neurology books divide the brain into four main components or regions: the telencephalon (or brain proper), the diencephalon, the cerebellum and the brainstem.

Each of these regions has a very particular anatomy and a specific function.

Together with the spinal cord, the components of the brain constitute the central nervous system, that is the most important part of the entire nervous system.

Anatomy

The anatomists divide the skull into two different parts not only morphologically, but also in terms of embryonic origin. These two parts are called neurocranium and splanchocranium .

Briefly, the neurocranium is the upper portion of the skull, the one that contains the brain and some of the main sense organs. The splanchocranium (or massive facial or viscerocranium ) is the anterior-posterior portion of the skull, a portion that forms the face .

Together, neurocranium and splancnocranium possess 22 bones (NB: this sum considers even bones as two distinct bones; in some texts of anatomy, even bones hold as one bone, which greatly reduces the total number of bone elements).

SKELETON OF NEUROCHRANIO

The neurocranium comprises 8 bones: the frontal bone, the two temporal bones, the two parietal bones, the sphenoid bone, the ethmoid bone and the occipital bone.

Frontal bone : it is the uneven bone of the forehead. Similar to a bowl, it resides above the nasal and maxillary bones and anterior to the two parietal bones.
In the frontal bone, the experts identify three portions: the squamous portion, the orbital portion and the nasal portion. The scaly portion is the largest of the three and corresponds, in fact, to the area commonly called the forehead. The orbital portion is a horizontal area, which contributes to the formation of the orbital cavities (where the eyes reside) and the nasal cavities. Finally, the nasal portion is a small region (the smallest of the three), which articulates with the nasal bone and the frontal jaw process.
Frontal bone belongs to the category of flat bones.
Temporal bone : it is the even bone that constitutes the latero-inferior region of the neurocranium. It borders on: the occipital bone, posteriorly; the parietal bone, superiorly; the sphenoid bone, anteriorly.
The anatomists recognize 5 portions: the squamous portion (or scaly part), the mastoid portion, the porous portion (or petrous part), the tympanic portion and the styloid portion.
Internally, the temporal bone accommodates and protects the hearing organs (the so-called three ossicles: hammer, anvil and stirrup) and one of the two temporal lobes of the brain proper.
Below, it has a particular concavity, called the glenoid fossa, which houses the condyle of the mandible (or mandibular condyle) and forms, together with this element, the so-called temporomandibular joint. The temporomandibular joint is the only articular element of the skull with high mobility.
The temporal bone represents a point of transit for some cranial nerves and for some important arterial and venous blood vessels of the brain.
It is a flat bone.
Parietal bone : it is the even bone that constitutes the latero-superior region of the neurocranium. It borders on: the frontal bone, anteriorly (front edge); part of the sphenoid bone and the temporal bone, inferiorly (scaly edge); the occipital bone, posteriorly (occipital border); the other parietal bone, superiorly (sagittal border).
Belonging to the category of flat bones, the parietal bone has the shape of a quadrilateral, with the concave inner surface and (obviously) the convex outer surface.
On its outer side, the presence of the so-called parietal hole, through which nerves and blood vessels pass, stands out.
On its inner side, the furrow within which the middle meningeal artery takes place deserves a special mention.
Sphenoid bone : it is an uneven bone, which takes place in the middle of the skull (looking at the latter frontally), in front of the temporal bone and the so-called basilar part of the occipital bone.
The sphenoid bone has the shape of a butterfly: it has a central body and two side regions very similar to wings.
It contributes to the formation of orbital cavities and nasal cavities. Furthermore, it has a depression (that is, a concavity), which serves to house and protect the pituitary gland. Morphologically similar to a saddle, this depression is called sella turcica.
The sphenoid bone is articulated with numerous bones of the skull (frontal, parietal, occipital, ethmoid, temporal, zygomatic, maxillary, palatine and vomer) and represents an important point of union between the bony elements of the neurocranium and the bone elements of the splanchocranium .
Ethmoid bone : it is the uneven bone that resides behind the nasal cavities and in front of the brain and that participates in the formation of the orbital cavities (in the middle of which it is located).
The ethmoid bone has a cubic shape, is very light and has several cavities (the so-called ethmoid sinuses).
Furthermore, it is articulated with numerous bones of the neurocranium and the splanchocranium.
Occipital bone : it is the unequal bone, similar to a saucer, which is located in the postero-inferior part of the neurocranium. It borders on: the parietal bones, above; the temporal bones, latero-anterior; the sphenoid bone, anteriorly.
The occipital bone is articulated with the first vertebra of the vertebral column (known as the atlas) and connects the cranial cavity with the vertebral canal, through an opening called the occipital hole (or foramen magnum ).
It falls into the category of flat bones.

According to anatomy experts, the bones of the neurocranium form two distinct sections, identifiable by the name of: cranial vault (or calvarium ) and base of the skull .

At the cranial vault, the upper sections of the frontal bone, parietal bones and occipital bone participate. The cranial vault is the upper bony portion of the skull, used to wrap the lobes of the brain proper. The meninges take place on the inner surfaces of the bones of the cranial vault.

At the base of the neurocranium skull, on the other hand, the lower sections of the frontal bone, the parietal bones and the occipital bone and the entire sphenoid, ethmoid and temporal bones participate.

The base of the skull is the part of the neurocranium that articulates with the mandible and the first cervical vertebra of the vertebral column, that is the atlas.

Figure: bones of the neurocranium and splancnocranium. In this image, due to the impossibility of representing them, the palatine bone and the lower nasal cornet (or tubercle) are missing.

What are the lobes of the brain proper?

The brain itself has two adjacent formations that are very similar (but with enormously different functions), called hemispheres .

Separated from the so-called longitudinal slit, the hemispheres present 4 specific areas, which neurologists call lobes. Each hemisphere has a frontal lobe, a parietal lobe, a temporal lobe and an occipital lobe.

The position of the aforesaid lobes corresponds to that of the cranial bones, called in the same way. Thus, the parietal lobes reside inside the parietal bones, the frontal lobe inside the frontal bone and so on.

SKELETON OF SPLANCNOCRANIO

The splanchocranium comprises 14 bones: the two zygomatic bones, the two lacrimal bones, the two nasal bones, the two lower nasal horns, the two palatine bones, the two maxillary bones, the vomer and the mandible.

These bone elements take part in the constitution of the orbital cavities (latero-inferior part), of the nasal cavities and of the mouth.

Zygomatic bone : it is the even bone that makes up the cheek area. It consists of: the frontal bone, the sphenoid bone, the ipsilateral temporal bone and the homolateral jaw bone (NB: ipsilateral means "relevant to the same side").
Presenting 4 edges (so it is a quadrilateral), the zygomatic bone takes part in the formation of the lateral wall of the orbital cavity.
Lacrimal bone : it is the even bone that forms the medial lateral wall (ie internal) of the orbital cavity. It borders on: the frontal bone, above; the ethmoid bone, posteriorly; the homolateral maxillary bone, on the antero-inferior margin; the inferior nasal cornet, on the postero-inferior margin.
The tear bone is the smallest and most fragile bone in the face; it has 4 edges and forms, together with the homolateral maxillary bone, a small concavity called the lacrimal dimple. The lacrimal dimple accommodates the lacrimal sac, the "container" of tears.
Nasal bone : it is the even bone that forms the so-called "nasal bridge", ie the upper bone region of the nose.
It borders on: the homolateral jaw bone, laterally; the frontal bone, above; the other nasal bone, medially.
Between the two nasal bones, there is a junction area, which is called an internal nasal suture and connects the two bone elements.
The nasal bone has 4 edges and, on its inner surface, runs the so-called nasociliary nerve, a branch of the trigeminal nerve.
Lower nasal cornet (or lower or turbinated nasal conch) : it is an even bone, located inside the nasal cavity, in a lateral position.
Long and narrow, the turbinate runs horizontally and serves to increase the total surface of the nasal mucosa. In this way, there is a greater interaction between the latter and the inspired air.
Palatine bone : situated between the maxillary bone and the sphenoid bone, it is the even bone element that contributes to the formation of: the inferior-inferior margin of the homolateral nasal cavity, the floor of the ipsilateral orbital cavity and the roof of a part of the hard palate.
The palatine bone is articulated with 6 bones of the skull: the sphenoid, the ethmoid, the homolateral maxillary bone, the homolateral turbinate, the vomer and the other palatine.
Seen from the front, it looks like an L.
Maxillary bone : it is the even bone that, joined to the contralateral jaw bone (ie relevant to the opposite side), forms the jaw and completes the upper palate.
It is articulated with as many as nine bones of the skull: the frontal, the ethmoid, the homolateral zygomatic, the homolateral lacrimal, the homolateral turbinate, the homolateral palatine, the vomer and (obviously) the contralateral maxilla.
Seat of the upper dental arch, the so-called jaw is a rather complex structure, which, together with the mandible, guarantees the food and chewing of food.
Vomere : it is the uneven bone that constitutes the lower part of the nasal septum. Shaped similar to the ploughshare used in agriculture, the skull share is articulated with various cranial bones: the sphenoid, superiorly; the ethmoid, anteriorly; the two palatine and the two jaws, below.
Mandible : it is the uneven bone that constitutes the lower part of the mouth and houses the lower dental arch.
With the temporal bone, it forms the already mentioned temporomandibular joint; this is the true architect of chewing and phonation mechanisms.
To know all the peculiarities of the jaw (or lower jaw), the reader can consult the dedicated article on this page.

CRANAL SUTURES: WHAT ARE THEY?

On the skull, precisely between the various bones of the cranial vault, there are immobile joints, similar to hinges, which take the specific name of cranial sutures .

Up to the age of 20, the cranial sutures of each healthy individual are the protagonists of a fusion process, which leads to a consolidation of the cranial vault. This progressive fusion is fundamental to the correct development of the brain, as it guarantees the latter the space necessary for its final growth.

There are some congenital morbid conditions (such as Crouzon syndrome or Apert syndrome ) that determine the premature fusion of cranial sutures: this circumstance - known as craniosynostosis - involves not only craniofacial malformations, but also some cognitive deficits due to a insufficient development of the brain.

The main cranial sutures, present in the human cranial vault, are:

Coronal suture . It is the joint that connects the frontal bone to the two parietal bones.
Premature fusion of the coronal suture (coronal craniosynostosis) is a possible cause of brachycephaly .
Sagittal suture . It is the joint that joins the two parietal bones. It resides on the upper part of the cranial vault.
Premature fusion of the sagittal suture (sagittal synostosis) involves the establishment of a condition known as dolichocephaly (or scaphocephaly ).
Metopic (or frontal ) suture . It is an articular hinge, which, starting from the nose, reaches the frontal bone, dividing it partially into two (NB: despite this, the frontal bone is an uneven bone).
Premature fusion of the metopic suture (metopic synostosis) is responsible for an anomaly of the cranial vault, which is called trigonocephaly .
The lambdoid suture . It is the joint element that joins the parietal bones to the occipital bone.
Premature fusion of the lambdoid suture (lambdoid synostosis) causes a cranial anomaly, called plagiocephaly .

FOUNTAINS

In the cranial vault of newborns, precisely where two cranial sutures meet, there are areas not yet completely ossified, which are called fountains .

The fountains are, in fact, openings between the various bones of the neurocranium, which guarantee a certain malleability to the cranial vault. This malleability has a fundamental importance for the structures of the brain, as it allows them to expand freely, without finding impediments.

In the newborn baby, there are a total of 6 fountains: the anterior fontanel, the posterior fontanel, the two sphenoid fontanelles and the two mastoid fountains.

The anterior fontanelle and the posterior fontanel take the generic name of major fontanelles, as they are decidedly larger than the two sphenoidals and the two mastoid (which for this reason the doctors call minor fountains).
The anterior fontanel localizes at the meeting point between the coronal suture and the sagittal suture, therefore between the frontal bone and the parietal bones. In general, it closes around 12-18 months of life.
The posterior fontanel, instead, is located at the meeting point between the sagittal suture and the lambdoid suture, therefore between the parietal bones and the occipital bone. Usually it closes between the 8th and 12th week of life.
The sphenoid fontanelles are one on each side and localize where the sphenoid, parietal, temporal and frontal bones meet.
Generally, they close at the sixth month of life.
The mastoid fountains are one on each side and are located at the point where the temporal, occipital and parietal bones converge.
Usually, they close after 6-18 months of life.

Figure: the fountains in the newborn. As the reader may note, the sphenoid fontanelles occupy a latero-anterior position, while the mastoid fountains are a latero-posterior position. The image also includes the cranial sutures (NB: the frontal suture is missing).

Functions

The skull covers two important functions: it protects the structures of the encephalon and some sense organs - which are very delicate vital elements - and constitutes the structures of the face, such as the nasal cavities, the orbital cavities and the mouth.

Diseases of the Skull

The bones of the skull are very resistant, however, like all the other bones of the human body, they can suffer a fracture, if subjected to traumas of a certain size.

FRACTURES OF NEUROCRANIOUS BONES

Fractures of the neurocranium bones are often the result of blunt trauma or penetrating trauma.

The weakest cranial areas (therefore also more subject to breakage) are:

The so-called pterion . One on each side, the pterion is the point where the parietal bone, frontal bone and frontal bone meet the ipsilateral wing of the sphenoid bone. It is a particularly delicate area, as it is the thinnest part of the skull.
A fracture of the pterion can tear the blood vessels flowing underneath it. Blood loss, following the rupture of these vessels, can cause the formation of an extradural hematoma.
The anterior cranial fossa . It is a large depression in the skull, which houses part of the frontal lobes of the brain proper.
The frontal bone, the ethmoid bone and the sphenoid bone contribute to its formation.
The average cranial fossa . It is a bilateral depression of the skull, located in the middle position (between the anterior and posterior cranial fossa), in which the temporal lobe of each hemisphere is housed.
In the medial position, the two middle cranial cavities meet: here, the pituitary gland takes place.
The sphenoid bone, the temporal bone and the parietal bone contribute to the formation of the middle cranial fossa.
The posterior cranial fossa . It is a wide depression of the skull, largely enclosed by the occipital bone (so it is in the posterior portion of the skull, in which the brainstem and the cerebellum are lodged).
At its formation, the scaly and mastoid parts of each temporal bone participate and, of course, the occipital bone.

The classic symptoms and signs of a fracture to the bone of the neurocranium are: pain in the head (at the point where the patient has suffered the trauma), loss of blood (always in the area that has suffered the trauma), loss of cerebrospinal fluid (or liquor) from the nose or ears, problems with balance, confusion, language deficiency and neck stiffness.