Anatomy of the cranial base

The cranial base is the most inferior part of the skull.It consists of the floor of the cranial cavity plus the inferior aspect of the viscerocranium, minus the mandible.Together with the cranial vault, which is the part of the skull that protects the brain, it contributes to the neurocranium.Now, the cranial base is a highly irregular surface from both an external and internal view.So let’s begin with the external surface of the cranial base, which is formed by the maxillary, palatine, vomer, sphenoid, and occipital bones, in the midline; and the zygomatic and temporal bones laterally.In the most anterior part of the cranial base, there are the alveolar arch and the hard palate.The hard palate makes up both the roof of the mouth and the floor of the nasal cavity, and it’s made up of the palatine processes of both maxil lary bones and the horizontal plates of both palatine bones behind them.The maxillary teeth border the hard palate anteriorly and on both sides.Right behind the incisor teeth there’s the incisive fossa, which has one or more incisive foramina, that allow the nasopalatine nerves and the terminal branches of the sphenopalatine arteries to pass through.Posteriorly and laterally, on the horizontal plate of each palatine bone, there’s the greater palatine foramen; and right behind it, the lesser palatine foramen through which the greater and lesser palatine nerves and arteries emerge.Now, superior to the posterior end of the hard palate are the posterior nasal apertures - or choanae - which are also bounded by the body of the sphenoid bones superiorly, the medial pterygoid plates laterally, and the vomer medially.The choanae allow air to pass from the nasal cavities into the pharynx.Speaking of which, the vomer is unpaired and located in the midline.It's flat and thin, and forms the nasal septum together with the ethmoid bone, to which the vomer articulates superiorly.Also superiorly, but posterior to the ethmoid, the vomer articulates with the sphenoid bone; and inferiorly, it articulates with the maxillary and palatine bones.Now let’s move on to the middle part of the cranial base.In this part we can see right behind the hard palate: the vomer in the midline; and the sphenoid bone laterally.The sphenoid bone is an unpaired bone that has three pairs of processes: the greater wings and the lesser wings which spread laterally and form the lateral aspect of the body, and a third pair of pterygoid processes which possess a medial and lateral plate, and extend inferiorly.Now, behind the vomer and sphenoid bones are the occipital and temporal bones.The occipital bone has four parts and forms the foramen magnum, which is where the spinal cord connects to the brain, and also serves as a passage for the vertebral arteries, the spinal arteries, the tectorial membrane and the spinal accessory nerves.Laterally, there are the occipital condyles, which articulate with the first cervical vertebra - or atlas - through a mobile joint that allows us to move our head forward, backward, and laterally.Now, anterior to each condyle, there’s the hypoglossal canal where the hypoglossal nerve passes through.Finally, behind the foramen magnum there’s the external occipital protuberance.

In the midline, there’s the basilar part of the occipital bone which has the pharyngeal tubercle, where the pharyngeal raphe and superior pharyngeal constrictor muscle attach.

In regards to the temporal bone, within the petrous part,there’s the carotid canal, through which the internal carotid artery passes.Behind the carotid canal, there’s the jugular foramen, formed by the petrous part of the temporal bone anteriorly, and the occipital bone posteriorly; it is famous for allowing the passage of the cranial nerves IX, X, and XI, and where the internal jugular vein emerges from the cranium as the continuation of the sigmoid sinus.Lateral to the jugular foramen is the styloid process and the mastoid process.Between the styloid and the mastoid processes, there’s the stylomastoid foramen through which the facial nerve and stylomastoid artery pass.Finally, anterior to the mastoid process is the external acoustic meatus, and anterior to that is the mandibular fossa, which articulates with the mandibular condyles to form the temporomandibular joint.Looking at the cranial base from the inside, it can be divided into three staggered depressions which lie at different levels, with the anterior fossa being the highest level and the posterior fossa being the lowest level, similar to the stairs on a staircase: the anterior, middle, and posterior cranial fossa.The anterior cranial fossa houses the frontal lobes of the brain.It’s delimited by the squamous part of the frontal bone anteriorly and laterally, and the anterior margin of the lesser wings of the sphenoid, posteriorly, with the ethmoid bone forming the central part of the fossa.From front to back there’s the frontal crest of the frontal bone, the crista galli, and the body of the sphenoid in the midline.Lateral to the crista galli, there’s the cribriform plate of the ethmoid bone on each side where the olfactory nerves travel through, and articulating posteriorly with the lesser wings of the sphenoid bone are the orbital parts of the frontal bone which form most of the fossa and support the frontal lobes of the brain.Now, the middle cranial fossa houses the temporal lobes of the brain.It’s delimited by the the posterior borders of the lesser wings of the sphenoid anteriorly; the greater wings of the sphenoid, the squamous part of the temporal bone, and the parietal bones, laterally; and the dorsum sellae of the sphenoid bone and the superior borders of the petrous part of the temporal bone, posteriorly.In the midline, from front to back, there’s the tuberculum sellae; the hypophyseal fossa, where the pituitary gland, or the hypophysis, sits; and dorsum sellae, all coming together to collectively form the sella turcica which looks like a horse’s saddle.Laterally, on each side of the tuberculum sellae, there’s the optic canal and the superior orbital fissure, delimited by the lesser and greater wings of the sphenoid and open anteriorly into the orbit.On each side of the body of the sphenoid there are three foramina that travel through the greater wing of the sphenoid bone; the foramen rotundum, foramen ovale, and foramen spinosum.Finally, right where the sphenoid, occipital, and temporal bones join, there’s the foramen lacerum.All of these foramina serve as passageways and landmarks for important structures passing in and out of the cranium.Finally, the posterior cranial fossa houses the brainstem and the cerebellum.It’s delimited by the dorsum sellae of the sphenoid bone and the superior borders of the petrous part of the temporal bone, anteriorly; the parietal bones laterally; and the squamous part of the occipital bone posteriorly.In the midline, there’s the clivus of the occipital bone, the foramen magnum, the internal occipital crest, and the internal occipital protuberance.Laterally, in the petrous part of the temporal bone is the internal acoustic meatus; and posteroinferiorly to this is the jugular foramen, from which the sigmoid sinus travels to in the groove for the sigmoid sinus before exiting as the internal jugular vein; and medial to the jugular foramen, is the hypoglossal canal.OK, let’s take one quick break before we recap, and try to identify the openings labeled on the following image?Alright, as a quick recap…The cranial base is the most inferior portion of the neurocranium and viscerocranium - excluding the mandible.In the most anterior part of the cranial base, there are the alveolar arch and the hard palate.In the middle of the cranial base, we can see the vomer in the midline; and the sphenoid bone laterally.From the inside, the cranial base can be divided into three staggered depressions which lie at different levels.From highest to lowest, these are the anterior, middle, and posterior cranial fossa.The anterior cranial fossa houses the frontal lobes of the brain.The middle cranial fossa houses the temporal lobes of the brain.Finally, the posterior cranial fossa houses