The cranium or skull is the uppermost part of the axial skeleton and plays an important role in protecting the brain and supporting the face. It is composed of 22 bones divided into two primary regions: the neurocranium which houses and protects the brain and the viscerocranium which forms the face. The intricate structure of the cranium provides not only protection for the brain but also critical support for tendinous muscle attachments, neurovascular passageways, and the formation of various facial structures. 

Structure of the Cranium 

The cranium consists of 22 bones in most adults. These bones are connected by sutures which are immovable joints that allow for the growth of the skull in childhood and eventually fuse in adulthood. These bones serve two primary functions: structural support and protection for the brain and sensory organs and attachment points for muscles that control facial expressions, chewing, and head movements. 

The Calvaria and Skull Base

The calvaria or skullcap is the upper part of the skull that protects the brain. It is composed of the frontal bone, parietal bone, temporal bone, and occipital bone. Various sutures including the coronal, sagittal, and lambdoid sutures form the boundaries between these bones. The skull base on the other hand consists of the sphenoid, ethmoid, frontal, temporal, and occipital bones which together create passageways for important neurovascular structures. 

Important anatomical landmarks include: 

• The pterion: where the frontal, parietal, temporal, and sphenoid bones meet. 
• The asterion: the junction of the parietal, temporal, and occipital bones. 
• The glabella: a key middle landmark located between the superciliary ridges of the frontal bone. 
• The bregma and lambda: junctions of the coronal, sagittal and lambdoid sutures. 

Cranial Fossae

The cranium houses three cranial fossae that support the brain’s lobes: 

• Anterior Cranial Fossae: Houses the frontal lobes of the brain formed by the frontal, sphenoid, and ethmoid bones. It contains the cribriform plate which allows the passage of the olfactory nerves. 
• Middle Cranial Fossa: Houses the temporal lobes formed by the sphenoid and temporal bones. This fossa contains the optic canal, superior orbital fissure, and various foramina that transmit critical neurovascular structures. 
• Posterior Cranial Fossa: Contains the cerebellum and brainstem. It is formed by the occipital and temporal bones and features the foramen magnum through which the spinal cord exits the skull. 

Facial Bones 

The 14 facial bones contribute to the structure of the face and support sensory functions like vision, hearing, and breathing. These include the nasal conchae, nasal bones, maxilla, mandibles, and others. The temporomandibular joint (TMJ) which connects the mandible to the skull is critical for mastication and plays a significant role in clinical anatomy. 

Development of the Cranium 

Embryology and Ossification 

The cranium begins its development during the embryonic phase with mesodermal and neural crest cells contributing to different parts of the skull. The bones of the neurocranium including the frontal, ethmoid, sphenoid, and temporal bones arise from neural crest cells. In contrast the parietal and occipital bones derive from mesoderm. As development progresses the cranium undergoes intramembranous ossification in the vault bones and endochondral ossification in the skull base. 

Branchial Arches 

The development of cranial structures is influenced by five branchial arches which contribute to the formation of the jaw, ear, and throat. These arches also give rise to various nerves including the trigeminal, facia,l, and glossopharyngeal nerves which play a key role in facial sensation and expression. 

Blood Supply and Nerve Pathways 

Blood Supply 

The primary blood supply to the skull is provided by the common carotid arteries (anterior circulation) and vertebral arteries (posterior circulation). The external carotid artery supplies the skull bones, facial muscles, and meninges, while the internal carotid artery provides blood to the brain. The middle meningeal artery, a branch of the maxillary artery, is of particular clinical relevance, as trauma to the lateral skull can lead to an epidural hematoma.

Cranial Nerves and Foramina 

The cranium is also a conduit for cranial nerves that control various functions such as facial sensation, vision, and motor control. These nerves pass through specific foramina in the skull. For example, the optic nerve (CN II) passes through the optic canal, while the facial nerve (CN VII) exits via the stylomastoid foramen.

Clinical Significance 

Craniotomy and Surgical Considerations 

Neurosurgical procedures such as craniotomy (removal of a portion of the skull) are crucial for accessing intracranial structures, including the brain, meninges, and blood vessels. Indications for craniotomy include conditions such as intracranial hemorrhage, brain tumors, and congenital malformations. Surgeons rely heavily on knowledge of cranial anatomy to minimize complications and ensure proper exposure during surgery.

Epidural Hematoma 

Epidural hematoma is a potentially life-threatening condition often resulting from trauma to the skull, particularly at the pterion. This type of hemorrhage typically arises from a lacerated middle meningeal artery, leading to the rapid accumulation of blood between the dura mater and the skull. Early diagnosis and surgical intervention are essential to prevent brain herniation and other complications.

Craniosynostosis and Abnormalities 

Craniosynostosis, the premature fusion of cranial sutures, can lead to abnormal skull shapes such as brachycephaly and plagiocephaly. This condition can also result in developmental delays and increased intracranial pressure if left untreated. Genetic and environmental factors, including nutrient deficiencies, can contribute to cranial malformations.

The anatomy of the cranium is not only foundational to the physical structure of the head but also critical to many functions essential for life. From protecting the brain to supporting facial muscles, the skull plays an indispensable role. Understanding its development, structure, and clinical significance is crucial for clinicians, particularly in the context of surgical interventions and the diagnosis of skull-related abnormalities. As research continues to uncover new aspects of cranial development and function, our understanding of its complexity and importance will continue to grow.

Frequently Asked Questions: 

1. What is the anatomy of the cranium bone?

The cranium (from the Greek word krania, meaning skull) is the most cephalad aspect of the axial skeleton. The cranium, or skull, is composed of 22 bones and is divided into two regions: the neurocranium (which protects the brain) and the viscerocranium (which forms the face).

2. What are the 8 bones of the cranium?

The brain, which performs these various functions, is protected by a part of the skull called the cranium. We’ll turn our attention to the eight bones that form it: the ethmoid bone, the sphenoid bone, the frontal bone, the occipital bone, two parietal bones, and two temporal bones.

3. What is the function of the cranium?

The primary function of the cranium, or neurocranium, is to protect the brain and its delicate tissues, including the meninges and cerebral vasculature, and to provide a bony framework for the head. 

4. How many joints are in the cranium?

In the neurocranium these are the occipital bone, two temporal bones, two parietal bones, the sphenoid, ethmoid and frontal bones. There are about 230 or more joints in an average person. On the whole total amount of joints in the skull is 86.