Hey everyone! This is Nicole from Kenhub, and in this tutorial, we'll be looking at the bones of the skull. In this tutorial, we'll be mainly looking at these two images of the skull – a frontal view on the left and a left lateral view on the right. There are many bones that comprise the skull. Broadly, they can be divided into two categories – the bones of the neurocranium and the bones of the viscerocranium.
The neurocranium – sometimes called the cranial vault – forms the skeletal casing around the brain. We can see the neurocranium highlighted on both of our frontal and lateral views of the skull. The neurocranium itself is made up of two parts – the calvaria or skullcap which consists of the frontal, parietal and occipital bones and is considered its roof and the basicranium or the cranial base which is made up of the frontal, ethmoid, paired sphenoid, paired temporal bones and occipital bone and can be divided into three parts – an anterior, middle and posterior part. The basicranium is considered the floor of the neurocranium.
The viscerocranium which again we can see in our frontal and lateral views of the skull is the facial skeleton. This is the portion of the skull that forms the structure of the face. The bones that comprise it are those of the anterior aspect of the cranium which form the mouth, nose and part of the orbits. In this tutorial, we’ll first look at the bones of the neurocranium and then move on to the viscerocranium. But before we begin, let's look at how the bones of the skull articulate.
Almost all of the bones of the skull are connected by sutures which can be defined as the immovable joints between two cranial bones. Sutures are further classified as fibrosynarthroses joints otherwise known as immobile joints composed of matrices of connective tissue called Sharpey's fibers. During childhood, cranial sutures are somewhat mobile which allows the brain and head to grow. As we age, however, sutures become increasingly ossified. There are many sutures of the skull but for the neurocranium, there are four important ones to remember – the sagittal suture along the midline of the calvaria, the coronal suture anteriorly, the lambdoid suture posteriorly, and the squamous suture which is bilaterally paired. For the other articulations of the skull, just note that each suture is named according to the structures involved.
Now that we've finished discussing sutures, let's take a look at the bones of the neurocranium.
The neurocranium is formed by eight bones – four singular bones along the midline: the frontal bone, the ethmoid bone, the sphenoid bone and the occipital bones; as well as two bilateral pairs of bones: the temporal bone and parietal bones. Let's begin by talking about the singular bones.
The frontal bone is found on the anterosuperior aspect of the skull and is considered the center of the forehead. It is comprised of three parts – the squamous part which is the largest part of the bone and encompasses the area of the forehead; the orbital part which makes up a large part of the superior wall of the bony orbit, and finally the nasal part which we can't see in this image but lies behind the maxilla to form the most superior part of the nasal septum.
The frontal bone has three main functions – to protect the brain, to give shape to the skull, and to provide a site for muscle attachments for numerous cranial muscles. Anterior to the sphenoid bone lies the ethmoid bone. It contributes to the anterior basicranium and separates the brain from the nasal cavity. Note that the ethmoid bone is also considered part of the viscerocranium. This is because it contributes to the orbit, the nasal cavity and the nasal septum. Note that we're keeping our notes on the ethmoid bone brief for now as we're going to take a look at the ethmoid a little bit later.
The sphenoid bone makes up a small portion of the temporal fossa. However, as you can see from the superior view of the skull in transverse section, you can see that it also comprises the middle third of the basicranium. The sphenoid bone can be divided into four parts – the body which is the most central aspect of the bone, the lesser wings which forms the optic canal, the greater wings which articulates with the temporal bones on the lateral aspect of the skull, and proximal to the greater wings are the pterygoid processes from which the pterygoid muscles originate and which we can see highlighted in this inferior view of the skull.
The final singular bone of the neurocranium is the occipital bone. The occipital bone is an unpaired bone that covers the back of the head and makes up a large portion of the basilar part of the neurocranium. The occipital bone is the only cranial bone that articulates with the cervical spine. You should also remember from before that the occipital bone articulates with the parietal bones at the lambdoid suture.
The occipital bone can be divided into four parts – the basilar part which you can see in this sagittal section of the lower portion of the skull and the upper portion of the spine as this green section quadrilateral in shape and located just anterior to the foramen magnum which is the major opening through which the spinal cord passes; two condylar parts which in this view of the base of the skull we can see on either side of the foramen magnum and which are the protuberances that articulate with the atlas vertebra at the top of the spine; and the squamous part which is the rest of the bone highlighted in green. All four parts of the occipital bone are arranged around a large opening at the base known as the foramen magnum which we mentioned before and you can see highlighted in green just here. Several structures pass through this opening including the brainstem, the spinal branch of the accessory nerve, the anterior and posterior spinal arteries, the vertebral artery and the spinal vein.
Now that we've looked at the singular bones of the neurocranium, let's look at the paired bones.
The temporal bones are bilaterally paired bones of the neurocranium. Breaking down the temporal bones a bit further, we can see that the temporal bone has two parts – a squamous part contributing to the temporal fossa which the temporal muscle attaches to and the petrous part which contributes to the basicranium.
Coming back to our image of the temporal bone, we can see that each temporal bone has four notable features – the mastoid process and the styloid process which are both attachment site for numerous muscles and the zygomatic process contributing to the zygomatic bone or cheek bone which we’ll discuss with the viscerocranium. Also note that the temporal bone is in the region where the ear is located. The opening here is called the tympanic part of the temporal bone and contributes to the external acoustic meatus. However, we won't be discussing the ear in this tutorial.
On either side of the neurocranium are two paired bones known as parietal bones which articulate with each other along the sagittal suture at the midline of the calvaria. The parietal bones also form articulations with – the frontal bone anteriorly via the coronal suture, the temporal bones laterally via the squamosal suture, the sphenoid bone via the sphenoparietal suture and with the occipital bone posteriorly via the lambdoid suture.
Now that we've finished talking about the neurocranium, let's move on to the viscerocranium.
The viscerocranium is the facial skeleton. It's comprised of fifteen bones – three singular bones along the midline the ethmoid bone which as we've mentioned before contributes to the neurocranium and the viscerocranium, the vomer and the mandible; as well as six bilaterally paired bones: the nasal bones, the maxillae, the inferior nasal concha, the zygomatic bones, the lacrimal bones, and the palatine bones.
Let's begin with the singular bones.
The perpendicular plate of the ethmoid bone comprises the superior portion of the bony nasal septum which divides the nasal cavity into left and right. It's important to note that on a skeleton, the nasal cavity begins at the anterior nasal aperture. The vomer which is the thin trapezoidal bone in this image forms the inferior and posterior portion of the nasal septum; and in this image we can see the nasal septum appearing as a line down the center of the anterior nasal aperture. The mandible is also known as the lower jaw. The mandible has a horseshoe shape and is bilaterally symmetrical. It is comprised of a body, rami, condylar processes, coronoid processes and the alveolar process which houses the lower dentition. The mandible is unique in that it is the only bone of the cranium that does not articulate with adjacent bones by sutures because it is the movable portion of the jaw. It does however articulate with the temporal bones via the temporomandibular joint or TMJ which is a modified hinged synovial joint.
Now that we've finished talking about the singular bones of the viscerocranium, let's talk about the paired bones of viscerocranium.
The nasal bones are paired bones that form the bridge of the nose. The nasal bones articulate with the frontal bone superiorly and the maxilla laterally. The nasal bones also posteriorly articulate with the perpendicular plate of the ethmoid bone which is highlighted in this image in green and you can see the perpendicular plate articulating with the nasal bone which is anterior just here. The paired maxillary bones form the immobile maxillae or the upper jaw and house the upper dentition. The body of each maxilla has four main parts named according to the articulating bone – the frontal process, the zygomatic process, the orbital surface and the palatine process shown from an inferior view looking up at the roof of the mouth. The many articulations of the maxillae mean that they contribute not only to the formation of the jaw but also to the formation of the medial and inferior orbit, nasal cavity and the palate.
Within the nasal cavity, there are three pairs of nasal conchae. These are projections of thinly rolled bone which humidify, warm, filter and directs the air we breathe. The superior nasal concha which are not visible from the anterior nasal aperture and the middle nasal concha which can be visible directly inferior to the nasal bones are projections of the ethmoid bone and are considered part of the neurocranium. However, the inferior nasal concha do not project from the ethmoid bone and instead are considered separate facial bones. They are also typically the most visible conchae through the anterior nasal aperture.
Moving on to the other walls of the orbit, as you can see in this image, the inferior and lateral borders of the orbit are formed by the paired zygomatic bones. Remember that the superior borders of the orbit are formed by the frontal bone which is part of the neurocranium. The zygomatic bones protrude laterally forming eminences on the face called cheekbones. Each zygomatic bone is comprised of three parts which are named according to the bone they articulate with – the frontal process, the temporal process and the maxillary part which is not a process per se but refers more to the part of the bone that communicates with the maxillary bone just here.
The lacrimal bones are the smallest and most fragile bones of the face. As you can see, they are located in the medial wall of the orbit. The lacrimal bones contain the foramen of the nasolacrimal ducts also known as tear ducts. The palatine bones lie posterior to the palatine process of the maxillary bones and have an L shape that is a little bit difficult to see in this image. Hopefully, you'll be able to identify it in the following images. The palatine bone is made up of three parts – a horizontal plate which you can see just here in the sagittal section and also in this inferior view of the basicranium. Note that together with the palatine process of the maxilla, they form the bones of the hard palate; a perpendicular plate and it's important to note that the perpendicular plate excludes the horizontal plate just here; and a pyramidal process which are not visible in these images. Located between the sphenoid bone and the maxilla, the palatine bone helps form part of the nasal cavity and the hard palate.
That's all we have time for today. Thanks for watching.
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