The human skull consists of 22 bones including the cranial bones and the facial bones. The cranial bones are responsible for creating the cavity in which the brain is located. These bones are present in humans as well as most animals including dogs, cats and horses. This article discusses about the human cranial bones, their location and functioning.
Definition of the Cranial Bones
The cranial bones form the human neurocranium (also referred to as the braincase) which is a bony structure that surrounds and protects the brain as well as the brain stem. The neurocranium is one of the 2 categorical parts of an adult human skull, the other one being the viscerocranium. These categorical parts have different embryological origins.
List and Location of Cranial Bones
There are eight cranial bones in a human skull. Following are the locations of the different bones:
- Ethmoid Bone (1) : It is located at the cranium-floor, in front of the sphenoid and below the frontal bone. The nontechnical location of the bone is behind the nose, at the center of the face.
- Frontal Bone (1) : This bone is located is located in the forehead, at the anterior side of the skull roof. The bone extends down and forms the orbit’s upper surfaces.
- Occipital Bone (1) : It is located at the lower back side of the head.
- Parietal Bones (2) : These bones are found at the posterior part of the skull roof, forming the top and the sides of one’s cranium.
- Sphenoid Bone (1) : It is located behind the orbits, at the base of the cranium and in front of the temporal bones. The sphenoid bone consists of 2 wing-like structures and 2 pterygoid processes that project downwards.
- Temporal Bones (2) : These paired bones can be found at the sides of one’s skull, above and behind both ears, below the 2 parietal bones.
The ethmoid bone, frontal bone and sphenoid bone are the three bones containing paranasal sinuses.
Development and Anatomy of Cranial Bones
The human skull is a very complex structure. Both intramembranous ossification and endochondral ossification plays important roles in the formation of the skull bones. The roofs and sides of the neurocranium develop through intramembranous ossification. But the two temporal bones develop from endochondral ossification. The endocranium, including the bones that support the brain (the ethmoid, occipital and sphenoid bones) are mainly formed by endochondral ossification. So, it can be said that the frontal bones and the parietal bones are primarily membranous.
The basic anatomy of the human cranial base as well as its fossas (the anterior, middle and posterior fossas) changes rapidly, mainly during the 1st trimester of pregnancy. Due to this reason, the 1st trimester is very important for proper skull growth. Various skull defects in newborn infants develop during this period.
The human skull consists of forty-four separate bony elements at the time of birth. Some of these bony components fuse together gradually after birth to form solid bones (e.g. frontal bone). The bones in the skull roof are separated by six fontanels in newborn babies. These regions are moveable and fibrous at birth which is necessary for birth as well as later growth.
The connective tissues of the fontanels are gradually replaced by sutures with the progression of the ossification process. The posterior fontanel generally closes by the time the baby is eight weeks old. However, the anterior fontanel remains open until eighteen months.
Mobility of the Cranial Bones
Most theories suggest that the cranial bones fuse together with the growth of an individual and are not capable of movement. However, a considerable amount of medical studies have documented a small and rhythmic movement of these bones.
Functions of the Cranial Bones
The general functions of these eight bones include:
- Protecting the brain is one of the principal functions.
- The inner surfaces of the bones are attached to certain membranes for stabilizing the positions of one’s brain, nerves and blood vessels.
- Their outer surfaces play an important role in keeping the moving muscles in the head attached in their proper positions.
- The cranial bones also provide support and protection to the sensory organs for vision, hearing, smell, taste and equilibrium (balance).
Some of these bones have certain specialized functions as well:
Ethmoid Bone – The functions of the ethmoid bone include:
- It forms an essential part of the orbits and nasal cavity
- It is the principal support structure of human nasal cavity
Occipital Bone – It has the following functions:
- The occipital condyles, the rounded surfaces at the bone’s base, articulate with the first vertebra of human spine (atlas). This allows movement of one’s head relative to spine.
- The Foramen Magnus is the large opening of the occipital bone which is the passage for the spinal cord.
- It is the base and back of the cranium, shaping the skull’s back.
- The sphenoid bone joins the parietal, frontal and temporal bones by articulating with them.
Disorders of the Cranial Bones
There are a number of conditions that can affect the bones forming the neurocranium. Some of the well known skull bone disorders include:
- Cranial Neuropathies
- Paget’s disease
- Fibrous Dysplasia of the cranium
- Hyperostosis cranialis interna
- Disorders related to trauma and head injuries
- Bone overgrowth resulting in headache and hearing loss
- Bone cancer
- Aneurysmal bone cysts (rare)
Diagnosis and Treatment of the Cranial Bone Diseases
Various imaging tests such as x-ray, magnetic resonance imaging (MRI), Computed tomography (CT) scan and positron emission tomography (PET) scan as well as other tests like biopsy helps to diagnose the above mentioned bone conditions of the cranium. The skull base surgery is one of the commonly used surgical procedures used for treating various skull bone ailments. Craniotomy is another surgical procedure in which a portion of the cranium is removed for treating certain disorders. The removed portion of the bone is replaced again once the treatment completes. Craniotomy is often used for treating a number of brain diseases.