Skull Bones

The skull is a vital bony structure of the human body, offering protection for crucial organs like the brain. The cranium, which includes the hyoid and middle ear bones, consists of 29 bones and forms the shape of the head.

The cranium is made up of the neurocranium and the facial skeleton.

Headings, Organization, and Structure of the Bones in the Skull

Neurocranial Area

It is the uppermost section of the skull that encloses and shields the brain, along with the cerebral vasculature and meninges. The cranium comprises 8 (2 paired and 4 unpaired) bones called cranial bones. The cranium is divided into the cranial roof or skullcap and the cranial base.

The cranial bones consist of:

1. Frontal bone (1)
2. Occipital bone (1)
3. Parietal bones (2)
4. Sphenoid bone (1)
5. Ethmoid bone (1)
6. Temporal bones (2)

The frontal, occipital, and parietal bones form the cranial roof, while all six bones contribute to the cranial base.

The cranial bones join together through sutures, with the most notable sutures being formed in this region.

The cranial bones, especially those forming the cranial base, link the lower part of the skull to the rest of the body via connections with the facial bones, the mandible (temporomandibular joint or TMJ), and 1st cervical vertebra. The frontal bone forms the upper segment of the bony orbits or eye sockets.

Internally, there are three major subdivisions or fossae – the anterior, middle, and posterior cranial fossae.

View from Anterior and Lateral angles

Facial Skeleton

The facial skeleton, also known as the viscerocranium, comprises 14 facial bones (2 unpaired and 6 paired) that form the structure of our face, cheeks, nose, mouth, and jaws. The bones in the facial skeleton include:

1. Maxillae/upper jaw bones (2)
2. Lacrimal bone (2)
3. Zygomatic bone/cheek bones (2)
4. Palatine bone (2)
5. Nasal bones (2)
6. Inferior nasal concha (2)
7. Vomer (1)
8. Mandible (1)

Additionally, the hyoid bone, and the ear ossicles (middle ear bones) are categorized as facial bones. The three paired middle ear bones are malleus, incus, and stapes. Besides forming the face, these bones offer protection and support for the soft tissues in the area. The maxilla occupies most of the front part of the facial skeleton and forms the boundaries for the nasal aperture with the two nasal bones. The zygomatic, lacrimal, palatine, vomer, nasal bones, and the inferior nasal concha constitute the rest of the orbits.

Synostoses of the Skull

Synostoses are a unique type of fibrous joint that connect the skull bones. These joints allow for movement during infancy, facilitating brain and skull growth. These joints fuse and become immovable around the age of 22-24.

In newborns, multiple soft spots or gaps exist between the skull bones due to the partially joined synostoses, termed as fontanelles, with the most crucial ones being the frontal and occipital fontanelles. These soft spots gradually solidify and disappear as the synostoses fuse in adults.

• Coronal synostosis – Between the frontal and the two parietal bones
• Sagittal synostosis – Between the two parietal bones
• Lambdoid synostosis – Between the occipital and the two parietal bones

View from the Inferior angle of the Skull

Apertures

The cranial base contains multiple apertures and foramina to allow passage to the spinal cord, blood vessels, and cranial nerves. The names of some important foramina, along with the nerves they allow passage to, are:

• Foramen magnum in the occipital bone – the spinal cord
• Optic nerve canal in the sphenoid bone (the optic foramen is the opening to this canal) – Optic nerve and ophthalmic artery
• Foramen ovale in the sphenoid bone – Mandibular nerve (trigeminal nerve branch)
• Foramen rotundum in the sphenoid bone – Maxillary nerve (trigeminal nerve branch)
• Foramina spinosum in the sphenoid bone – Middle meningeal artery
• Cribriform plate in the ethmoid bone – Olfactory nerve
• Supraorbital and infraorbital foramina in the maxilla – Supraorbital and infraorbital nerves
• Mental foramina in the maxilla – Incisive and mental nerves and mental artery
• Supraorbital foramen in the frontal bone – Supraorbital nerve

Vascularization

The cranium primarily receives blood supply from the common carotid artery, while the vertebral artery also contributes.

Muscles

The scalp and facial muscles are principally innervated by the facial, oculomotor, or trigeminal nerves. The hypoglossal nerve supplies the tongue. These muscles are responsible for various functions like facial expressions, speech, eating, and controlling eye movements.

FAQs

Q. Do skulls contain bone marrow?
Ans. Yes, a skull contains bone marrow. Recent studies have found that the skull bone marrow is directly connected to the brain through special openings.

Q. What is the only movable bone in the skull?
Ans. The mandible or jawbone is the only movable bone in the skull.

Q. What bone protrudes at the base of the skull?
Ans. The occipital bone protrudes at the base of the skull.

Q. How thick and hard is a human skull
Ans. A female skull is slightly thicker than a male skull, with the former being 7.1 mm thick while the latter has a thickness of 6.5 mm. When it comes to the hardness of human skulls, it can withstand a pressure of around 6.5 GPa. To compare this with some of the hardest objects, concrete can withstand 30 GPa, while steel up to 200 GPa.

References

1. Bones of the Skull – Teachmeanatomy.info
2. Skull – Kenhub.com
3. Anatomy, Head and Neck, Skull – Ncbi.nlm.nih.gov
4. Bones of the Skull – Joionline.net
5. The Skull – Oregonstate.education
6. Bones of the Skull – Libretexts.org