Anatomy
Written By EZmed
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Introduction
The skull is made up of 22 bones that articulate with each other - 8 cranial bones and 14 facial bones.
The remaining 7 bones in the head (6 auditory ossicles and 1 hyoid bone) do not articulate with the rest of the skull, and they are often referred to as accessory bones of the skull as a result.
In this post, we will discuss the cranial bones and sutures along with their anatomy and landmarks using labeled diagrams.
By definition, the cranial bones form the surrounding cranium that encloses and protects the brain.
Every EZmed post is filled with simple tricks to learn the material, and today we will use an easy mnemonic to remember the cranial bone names and anatomy.
Make sure to also check out the EZmed facial bone mnemonic which will help you remember the names and anatomy of the facial bones as well!
Let’s get started!
Cranial Bone Mnemonic
The skull has 8 cranial bones as mentioned above.
The cranial bones are attached to one another by sutures, which are fibrous bands of tissue that connect the bones to form the cranium around the cerebrum, cerebellum, and brainstem.
The names of the cranial bones can be remembered using the following mnemonic:
“Eight Osseous Parts Form The Skull”
Eight = Ethmoid Bone
Osseous = Occipital Bone
Parts = Parietal Bones (2)
Form = Frontal Bone
The = Temporal Bones (2)
Skull = Sphenoid Bone
This mnemonic not only helps you remember the cranial bone names, but also that there are 8 cranial bones (osseous parts) that form the skull.
We are now going to discuss the anatomy and important features of each cranial bone in the order of the mnemonic.
Ethmoid Bone
We will start with the ethmoid bone shown in red on the diagram below.
Since the ethmoid bone is obscured by other bones, let’s look at an inside sagittal view of the skull to understand its shape and location.
You can see the ethmoid bone shown in green and yellow on the 2 images below.
The ethmoid bone sits at the roof of the nose between the 2 orbits or eye sockets, and it makes up the medial aspect of each orbit.
The ethmoid bone separates the nasal cavity from the brain, and it articulates with the frontal bone and sphenoid bone.
The skull contains several different sinuses or cavities within the bones of the face and cranium.
One of these sinuses is called the ethmoid sinus, and it is located in the ethmoid bone as the name suggests.
The ethmoid bone has 2 important features - the cribriform plate and the crista galli.
The cribriform plate is where the olfactory nerves travel through.
The olfactory nerves are involved in smell, so it makes sense they will need to travel through the ethmoid bone which sits at the roof of the nose and separates the nasal cavity from the brain.
The crista galli is a sail-like projection located at the top/superior portion of the ethmoid bone.
The crista galli is where the membrane around the brain attaches, thereby preventing the brain from moving around in the skull.
It essentially serves as an anchor point for the brain.
Occipital Bone
The occipital bone is next in the mnemonic.
The occipital bone is located at the back of the skull and protects the underlying cerebellum, brainstem, and occipital lobe of the cerebrum.
There are several important features to know about the occipital bone.
First, the lambdoid suture connects the occipital bone to both parietal bones.
As mentioned above, sutures are fibrous bands of tissues that attach the cranial bones to one another.
The lambdoid suture is easy to remember because it is shaped like the lambdoid symbol as shown below.
The next important feature to know about the occipital bone is the external occipital protuberance (EOP).
The EOP is the “bump” you can feel at the back of your skull, and it serves as an attachment point for some of the neck muscles.
Third, the occipital condyles are protuberances located at the base of the occipital bone, and they articulate with the 1st cervical vertebra (C1) or the atlas.
The final important feature to know about the occipital bone is called the foramen magnum.
The foramen magnum is a large hole at the base of the occipital bone where the brainstem travels through and connects the brain to the spinal cord.
Parietal Bones
The parietal bones are next as we continue through the mnemonic.
There are 2 parietal bones, one located on either side of the skull, shown in blue.
The parietal bones are surrounded by several different sutures.
The most anterior suture is the coronal suture which connects the parietal bones to the frontal bone.
The coronal suture is easy to remember because it travels along the coronal plane.
Next, the inferior suture is called the squamous suture and it attaches the parietal bones to the temporal bones.
The most posterior suture is the lambdoid suture, and it connects the parietal bones to the occipital bone as mentioned above.
The final suture is called the sagittal suture, and it connects the 2 parietal bones together at the top of the skull.
The sagittal suture is easy to remember because it travels along the sagittal plane.
Frontal Bone
Next is the frontal bone shown in yellow below.
The frontal bone is the forehead, and it connects to the parietal bones via the coronal suture as discussed above.
The frontal bone makes up the superior aspect of each orbit.
There is a foramen (hole in the skull) located above each orbit, known as the supraorbital foramen, in which the supraorbital nerve travels through.
Temporal Bones
The temporal bones are next in the mnemonic, and one of them is shown in orange below.
There are 2 temporal bones, one located on either side of the skull in the “temple” region.
There are several features to know about the temporal bones.
First, the squamous suture connects the temporal bones to the parietal bones as discussed above.
Next, there is an anterior projection coming off the temporal bone called the zygomatic process.
The zygomatic process joins with the zygomatic bone, specifically the temporal process of the zygomatic bone, to form the zygomatic arch which is the cheek bone you can feel on the side of your face.
The zygomatic bone is one of the facial bones discussed in the EZmed facial bone blog.
Third, the auditory meatus is a canal that travels through the temporal bone and connects to the inner ear.
Next is the mastoid process, and it serves as an attachment point for some of the neck muscles.
Fifth, the styloid process is an inferior projection where some of the tongue and larynx muscles attach.
Lastly, the mandibular fossa is the point of articulation between the mandible and temporal bone.
Pterion
The region on the side of the skull where the frontal, temporal, parietal, and sphenoid bones join together is known as the pterion.
This part of the skull is thin, and the middle meningeal artery travels underneath it.
Blunt trauma to the area can cause damage to the middle meningeal artery and subsequent epidural hematoma formation.
Sphenoid Bone
The final cranial bone is the sphenoid bone shown in light purple/pink below.
It is the only cranial bone that articulates with all of the other cranial bones.
The sphenoid bone is difficult to see on the side view of the skull, but it is actually shaped like a butterfly
Let’s look at other slices of the skull to better understand the shape.
The sphenoid bone has a depression in it, called the sella turcica, in which the pituitary gland sits.
There are several anatomical features of the sphenoid bone including the greater wing, lesser wing, pterygoid process, and body.
Conclusion
Hopefully this mnemonic and overview helped you better understand the anatomy and important features of each cranial bone.
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