The Skull

1. Introduction to the Skull
2. The Facial Skeleton
3. The Calvaria
4. The Cranial Floor
5. Skull Fractures
6. Quiz

• The skull is made up of 22 individual bones joined by sutures.
• The skull has two parts: the neurocranium and the viscerocranium.
• The viscerocranium is made up of 14 bones: the zygomatic, lacrimal, nasal, palatine, and vomer bones, inferior nasal conchae, maxillae and mandible.
• The neurocranium has a roof (calvaria), a floor and a cranial cavity.
• The calvaria is arranged in 3 layers: the inner table, diploe and outer table.
• The cranial floor is divided into 3 fossae: anterior, middle and posterior.
• The cranial floor has a series of canals, foramina and fissures which transmit important nerves and blood vessels.
• Fractures of the calvaria can be linear or depressed.
• A fracture of the pterion can shear the middle meningeal artery, resulting in an extradural haemorrhage.
• Basal skull fractures are rare but important and present with specific signs.
• Facial fractures are common: the nasal bones, zygomatic arches and mandible are most commonly affected.

Introduction to the Skull

The skull, or cranium, is the bony skeleton of the head. It is made up of 22 individual bones which are joined by fibrous joints called sutures. Most of these bones form by intramembranous ossification, although parts of the cranial base develop by endochondral ossification.

The skull can be broadly divided into two parts:

• Neurocranium (8 bones) - this is the part which encases the brain. It has a roof (calvaria), a floor and a cranial cavity (which holds the brain).
• Viscerocranium (14 bones) - this is the skeleton of the face. It supports the soft tissue of the face and houses the eyes, oral cavity and upper respiratory tract.

The Facial Skeleton

The facial skeleton is made up of 14 bones:

• Zygomatic Bones (2) - these are the cheekbones. They articulate with the frontal, sphenoid, temporal bones and the maxillae.
• Lacrimal Bones (2) - these are the smallest bones of the facial skeleton, roughly the size a fingernail. They each have a groove for the nasolacrimal duct.
• Nasal Bones (2) - these form the bony root of the nose.
• Palatine Bones (2) - these form part of the hard palate of the oral cavity.
• Inferior Nasal Conchae (2) - these protrude into the nasal cavity and enhance the warming and humidification of inspired air by increasing surface area.
• Vomer Bone - this forms part of the nasal septum posteriorly.
• Maxillae (2) - these bones make up the majority of the upper facial skeleton. It forms the upper jaw, which is fixed to the cranial base. It also forms part of the hard palate.
• Mandible - this U-shaped bone forms the lower jaw. It is moveable as it articulates with the cranial base at the temporomandibular joint (TMJ).

Diagram - Osteology of the viscerocranium

Public Domain Source by العربيَّة [Public domain]

The Calvaria

The calvaria is the roof of the neurocranium. It comprises 4 bones: frontal, occipital and two parietal bones.

The bones of the calvaria are joined by sutures. Important suture lines to note are:

• Coronal Suture - joins the frontal bone with the two parietal bones.
• Sagittal Suture - joins the two parietal bones in the midline.
• Lambdoid Suture - joins the occipital bone to the two parietal bones.

Diagram - Sutures of the skull: 1) Coronal; 2) Sagittal; 3) Lambdoid

Public Domain Source by Julius Kollmann [Public domain]

In infants, the skull is immature, leaving membranous gaps between ossified bone. This permits enlargement of the skull in line with the growth of the brain. These unossified areas are termed fontanelles: there is an anterior fontanelle and posterior fontanelle. These fontanelles typically fuse at 18-24 months and 1-3 months, respectively.

The areas of the adult skull which correspond to the previous locations of the anterior and posterior fontanelles are called bregma and lambda, respectively.

Diagram - The anterior and posterior fontanelles

Creative commons source by BruceBlaus, edited by Dr. Joshua Bray [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

The bone of the calvaria is arranged in 3 layers:

• Inner Table - the innermost layer of cortical bone.
• Diploe - the middle layer composed of spongy bone.
• Outer Table - the outermost layer of cortical.

This trilaminar arrangement ensures adequate protective strength without excessive weight.

The outer table of bone is covered with periosteum (connective tissue), which is bound to the suture line. The inner table is lined by the dura mater (endosteal layer), which is continuous across the cranial cavity.

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