Inferior View of the Base of the Skull
This article will describe the anatomy from the inferior view of the skull base. We will explore the many foramina and projections that enable arteries and nerves to both enter and leave the skull. Structure is closely related to function, and having an awareness of the location of a structure as well as its function gives an all-round knowledge of the anatomy.
At the anterolateral aspect of this view, we can find the zygomatic Arch, which is a plate of narrow bone that is formed by the zygomatic process of the temporal bone and the zygomatic bone itself. From the inferior aspect the process has a pronounced lateral convexity and the lower part acts as the site of origin of the masseter muscle.
The zygomatic bone can be seen articulating with the zygomatic process of the maxilla medially as well as with the temporal bone laterally. The bone forms the bony foundation of the cheek as well as part of the lateral and inferior sections of the orbital fossa.
In the anterior midline is the maxilla, which forms the anterior two-thirds of the hard palate and connects with the palatine bone to complete the hard palate. It can also be seen articulating with the maxillary process of the zygomatic bone.
The hard palate is formed by the palatine process of the maxilla (the anterior two thirds) and the horizontal plate of palatine bone the posterior one third). The palatine bones articulate with the vomer posteriorly and the sphenoid bone laterally. In the most anterior part of the palatine process of the maxilla is the Incisive Foramen. This foramen is also known as the anterior palatine foramina, and can be found in the anterior most section of the midline of the maxillary bone. They allow the nasopalatine nerves and the sphenopalatine artery to enter from the floor of the nasal cavity.
Forming the roof of the nasopharynx are the choanae superior to the hard palate. They are also known as the posterior nasal aperture. They are separated in the midline by the vomer. It is the opening that lies between the naso and oropharynx. It lies superior to the hard palate and is anterior and inferior to the sphenoid bone.
Between the two maxillae is the median palatine suture. It lies between the two maxillae in the midline, both in the anterior two thirds of the hard palate and between the two palatine bones in the posterior third of the hard palate.
A second suture is the transverse palatine suture, which divides the maxillae forming the hard palate from the palatine bones posteriorly. It is usually in line with the 2nd molar tooth, posterior to the hard palate and closely associated with the medial pterygoid plate. Posterior to the palatine bones is the Vomer. It is an unpaired bone that forms the posterior inferior part of the bony nasal septum.
The sphenoid bone sits within the centre of the skull base like a wedge. This bone articulates with the vomer inferiorly, and the greater wings extend laterally to form part of the anterior pterion joint.
From the inferior aspect of the sphenoid bone, can be found the pterygoid processes. The lateral plate of pterygoid process is a thin plate of bone sits laterally to the medial plate and posterior to the upper teeth. The ridge allows for the attachment of the medial pterygoid muscle (which closes the jaw). A few centimetres medially is the medial plate of pterygoid process. This plate of bone lies behind the hard palate and anterior to the most anterior part of the occipital bone. The lower edge curves laterally to form the pterygoid hamulus, which is a small lateral facing projection of bone from the inferior most part of the process. This acts as a point which the tendon of the tensor veli palatini glides across.
Between the zygomatic process of the maxilla and the greater wings of the sphenoid bone is the inferior orbital fissure. This bony opening allows the inferior orbital vessels and nerves to leave the skull as well as the zygomatic branch of the maxillary nerve and the ascending branches of the pterygopalatine ganglion.
In the middle section of the skull is the foramen Lacerum, which is a jagged opening that is filled with cartilage in life. It lies directly above the ICA (internal carotid artery) during its course through the carotid canal. It can be described as the window through which the ICA can be seen (from above). Lateral to the opening of the internal carotid artery is the foramen Ovale. This foramen lies slightly anterior and medial to the foramen spinosum. It allows the mandibular nerve to exit the skull. This nerve is the V3 branch of the trigeminal nerve, and its inferior alveolar branch is responsible for supplying sensation to the lower teeth. It also supplies sensation to the lower part of the face. The foramen spinosum lies posterior and lateral to the foramen ovale, and anterior to the carotid canal. It transmits the middle meningeal artery, middle meningeal vein and meningeal branch of mandibular nerve to enter the skull, and supply the meninges with blood.
The stylomastoid foramen lies in between the two landmarks in its name (the styloid and mastoid process). It allows the facial nerve to exit the skull. A closely associated structure is the Mastoid Notch. It is a deep groove which can be found just posterior to the mastoid process, and allows for the attachment of the digastric muscle. At the posterior margin of the temporal bone is the Mastoid Foramen. It allows the mastoid emissary vein and small dural branch of occipital artery to supply the dura mater.
Just posterior to the middle of the skull is the foramen magnum. This is Latin for large hole. It allows the spinal cord to pass inferiorly out of the cranial vault, and also the vertebral arteries to enter the skull and provide the posterior input to the circle of Willis. The anterior and posterior spinal arteries also descend through this foramen, as well as the alar ligaments (that stabilise the dens), tectorial membrane and the spinal part of the accessory nerve.
Deep to the foramen lacerum is the carotid canal, which is a tunnel within the inferior part of the skull. The internal carotid artery does not pass through the foramen lacerum (which is filled with cartilage during life) but instead emerges from a circular opening which exists at the cartilage edge. The canal has its external opening posterior to the foramen spinosum and medial to the styloid process.
Behind the articular tubercle and in front of the tympanic section of the temporal bone is the petrotympanic fissure. The lingual nerve and chorda tympani (branches of the facial nerve) also run in this fissure for part of their course. An important adjacent bone is the Temporal Bone which articulates with the temporal process of the zygomatic bone laterally, as well as the occipital bone posteriorly and the sphenoid bone anteriorly. It has important sections, but is chiefly known for containing the organs of hearing in its petrous portion. Posterior to the external acoustic meatus is a large pyramidal bump known as the mastoid process. It gives attachment to sternocleidomastoid and longissimus capitis among other muscles. The Jugular Foramen is a large j shaped foramen found in the recesses of the posterior cranial fossa, inferior to the petrous portion of the temporal bone.
On the outer surface of the occipital bone is the inferior nuchal line. This is a small ridge of bone that projects from the posterior portion of the occipital bone. It gives attachment to a number of neck muscles including the rectus capitis posterior major and minor as well as the obliquus capitis superior. These muscles form the boundaries of the suboccipital triangle, which contains the vertebral artery before it enters the skull. Above this is the Superior Nuchal Line. It gives attachment to the splenius capitis, trapezius, sternocleidomastoid and occipitalis muscle. At the posterior midline of the external surface of the occipital bone is the external occipital protuberance. This is a small bump on the midline of the external surface of the occipital bone and extends into the superior nuchal lines laterally. The ‘inion‘ is the most superior part of the structure.
The occipital bone can be seen to articulate with the temporal bones laterally, as well as with the sphenoid bone anteriorly via its basilar part. The occipital condyles project inferiorly and articulate with the lateral masses of the first cervical vertebrae (atlas). Just posterior to the occipital condyles are the condylar canals. They allow the occipital emissary veins to join the venous system which also receives venous blood from the sigmoid sinus, suboccipital venous plexus and occipital sinus.
There is a small ridge of bone which arises from the squamous part of the occipital bone known as the external occipital crest. It acts as a site of attachment for the nuchal ligament. The parietal bones are difficult to visualise from the inferior view of the skull, however they can be seen articulating with the temporal and occipital bones. They form the posterosuperior part of the skull.
- Young children who present with cleft palate have a failure of the two maxillae to unite in the midline. This causes problems with the separation of the oropharynx and nasopharynx resulting in feeding and breathing issues.
- The middle meningeal artery (a branch of the maxillary artery) lies just deep to the anterior pterion (where the frontal, parietal, sphenoid and temporal bones meet), trauma of which can cause fatal extra dural haematoma. This is also referred to as ‘talk and die’ syndrome, as after trauma the patient feels fine but suddenly dies, as the bleed raises intracranial pressure.
- In raised intracranial pressure, the brain stem can herniate inferiorly out of the skull via the foramen magnum, which causes the vital respiratory centres to be damaged and perhaps suffer ischaemia.
- When the internal carotid is revealed for surgery e.g. carotid endarterectomy, the styloid process and posterior belly of digastric act as landmarks which are superficial to it. Reflecting these two structures reveals the vessel in the upper cervical region.