A thorough understanding of the anatomy of the skull simplifies greatly the learning of the rest of the anatomy of the head. Begin by examining the skull as a whole.
Identify the individual bones of the skull. Notice which bones lie adjacent to each other. Paired bones are marked by an asterisk.
The following bones belong to the neural cranium (the part of the skull that surrounds the brain and the major sense organs):
The surface of the skull is marked by three sutures:
the coronal suture - between the frontal bone and parietal bones;
the sagittal suture - midline between the parietal bones;
the lambdoid suture - between the parietal bones and occipital bone (named because it resembles the Greek letter “lambda”)
The junction of the sagittal and coronal sutures is called the bregma (position of the anterior fontanelle of the infant skull); the junction of the sagittal and lambdoid sutures is called the lambda (position of the posterior fontanelle).
[Until about six years of age the frontal bone is divided into two halves by a suture which sometimes persists into adulthood as the metopic suture. This may be read erroneously as a skull fracture on an xray.]
The following bones belong to the visceral cranium (the part of the skull that supports the face and the jaws):
Inferior nasal conchae*
Locate the following cavities and their boundaries. Note the bones that take part in their formation:
Anterior, middle and posterior cranial fossae
Temporal fossa (depression on the side of the skull which contains the temporalis muscle; bounded superiorly by the temporal line and inferiorly by the zygomatic arch)
Infratemporal fossa (the space between the medial surface of the mandible and the skull which contains several muscles of mastication. It is bounded superiorly by the zygomatic arch, but has no bony inferior boundary).
2. OCCIPITAL BONE
On the outside of the occipital bone, locate the foramen magnum (for the spinal cord), along with the occipital condyles on either side of it. Directly anterior to the foramen magnum, identify the bony pharyngeal tubercle for the attachment of the pharynx.
Identify the external occipital protuberance, where the nuchal ligament attaches. The ridge extending laterally from the external occipital protuberance is where the trapezius muscle attaches.
Passing through the base of each occipital condyle, locate the hypoglossal canal, for the hypoglossal nerve (CN XII).
Lateral to the external opening of the hypoglossal canal, examine the large jugular foramen opening between the occipital and temporal bones. Several structures pass through the jugular foramen:
the sigmoid sinus drains into the internal jugular vein, which is the lateralmost structure in the jugular foramen,
the glossopharyngeal nerve (CN IX),
the vagus nerve (CN X), and
the accessory nerve (CN XI)
Locate the foramen lacerum between the occipital, temporal, and sphenoid bones. This "foramen" is covered by cartilage during life.
On the inside of the occipital bone, locate the S-shaped sigmoid groove (sulcus) running up from the jugular foramen. This groove contains the sigmoid sinus. Trace the sigmoid groove posteriorly into the transverse groove for the transverse sinus.
3. SPHENOID BONE
Within the cranial cavity, locate the body and the greater and lesser wings of the sphenoid bone. Note how they contribute to the structure of the skull.
Examine the depression on the body of the sphenoid (hypophyseal fossa), which contains the pituitary.
Projecting from the posterior end of the body, locate the dorsum sellae, which forms the posterior wall of the sella turcica.
Projecting laterally from the tip of the dorsum sellae, identify the posterior clinoid processes.
Find the anterior clinoid processes, which are projections from the lesser wing of the sphenoid.
At the base of the anterior clinoid processes, locate the opening for the optic canal, through which the optic nerve (CN II) and the ophthalmic artery pass.
The superior orbital fissure can be found between the greater and lesser wings of the sphenoid. This fissure allows for the following structures to pass between the middle cranial fossa and the orbit:
oculomotor nerve (CN III),
ophthalmic nerve (CN V1),
trochlear nerve (CN IV), and
the abducens nerve (CN VI).
At the base of the greater wings, you should identify a number of important foramina. From anterior to posterior:
Foramen rotundum for the maxillary division of the trigeminal nerve (V2)
Foramen ovale for the mandibular division of the trigeminal nerve (V3)
Foramen spinosum for the middle meningeal artery.
Medial to this row of foramina, examine the carotid canal and the carotid groove, through which the internal carotid artery runs.
Look at the skull from below, and locate the foramen ovale, the foramen spinosum and the carotid canal. Notice that the foramen rotundum is not easily visible from below. Can you locate where it exits the skull?
Now locate the pterygoid processes. Note that each process consists of a medial and a lateral plate with a pterygoid fossa located between them.
Identify the hamulus, which projects toward the lower jaw from the medial pterygoid plate.
If you look at the lateral side of the skull with the mandible removed, you can find a small opening anterior to the base of the pterygoid processes, posterior to the maxilla. This small opening is the fissure leading into the pterygopalatine fossa. The pterygopalatine fossa contains branches of the maxillary nerve (V2), the maxillary artery, and the small, but important, pterygopalatine ganglion.
Determine that the pterygopalatine fossa is continuous superiorly with the orbit through a long slit; the inferior orbital fissure. The infraorbital nerve, artery, and vein run through the inferior orbital fissure.
4. FRONTAL BONE
Look at the skull from the front, and locate the supraorbital notch (or foramen) in the upper margin of the orbit. The supraorbital nerves (from V1) and blood vessels exit this notch (foramen) to the forehead.
Look at the skull laterally, and note the zygomatic process of the frontal bone.
Remove the skull cap and look inside the cranial cavity. Identify the groove for the superior sagittal sinus.
5. PARIETAL BONES
Internally, note the grooves for the middle meningeal artery.
6. ETHMOID BONE
Remove the skull cap, and look within the cranial cavity. Locate the cribriform plate, which separates the anterior cranial fossa from the nasal cavity. Locate the crista galli.
Look within the orbit, and locate the orbital surface (lamina) of the ethmoid. Be careful; this bone is exceedingly thin! Deep to it are the ethmoid air sinuses.
Look within the nasal cavity, and locate the superior and middle nasal conchae, and the perpendicular plate. These features are most easily seen by turning the skull upside down, and looking into the nasal cavity from behind. The middle concha and perpendicular plate can be seen anteriorly in the nasal cavity. The inferior nasal concha is not part of the ethmoid bone.
7. TEMPORAL BONES
Look at the skull from below and locate:
Zygomatic process, which forms part of the zygomatic arch
Styloid process just lateral to the jugular foramen (this structure is often broken off)
The massive mastoid process.
The mandible articulates with the temporal bone at the mandibular fossa. Just medial to the foramen spinosum of the sphenoid is the rough opening for the bony portion of the auditory tube. Posterior to that opening is the carotid canal, where the internal carotid artery enters the cranial cavity. Between the styloid process and the mastoid foramina, find the stylomastoid foramen, through which the facial nerve (CN VII) exits the skull.
Remove the skull cap, and look within the cranial cavity to locate the groove for the middle meningeal artery extending from the foramen spinosum. Posteriorly on the temporal bone, locate the internal auditory (acoustic) meatus through which the facial nerve (CN VII) and the vestibulocochlear (acoustic) nerve (CN VIII) pass.
Look at the skull laterally, and locate the external auditory meatus. Note that it is NOT continuous with the internal auditory meatus.
8. THE ORBIT
Look at the orbit from the front. Notice that the roof of the orbit is composed largely from the frontal bone. Attempt to locate the trochlear notch (or spine) on the frontal bone in the upper medial corner of the orbit (this is the pulley for the superior oblique muscle of the eye).
Find the orbital plate of the ethmoid in the back of the orbit.
Move laterally, and identify the contribution of the sphenoid bone to the back and lateral wall of the orbit.
Locate the optic foramen and the superior and inferior orbital fissures.
Follow the infraorbital groove across the maxillary bone on the floor of the orbit.
Notice the contribution of the zygomatic bone to the lateral wall and floor of the orbit.
Finally, examine the medial wall of the orbit and find the paper-thin lacrimal bone. Identify the fossa for the lacrimal sac.
9. PALATINE BONES
NOTE: Be aware that the paired palatine bones consist of horizontal and vertical plates. Only the horizontal plate, that forms the posterior portion of the hard palate, is readily visible in the complete skull.
Look at the skull from below (with the mandible removed), and note the major (greater) and minor (lesser) palatine foramina of the palatine bones.
Look at the skull from the front, and locate the infraorbital groove in the floor of the orbit. Anteriorly, this groove becomes the infraorbital canal. This canal opens anteriorly as the infraorbital foramen. These structures contain the infraorbital nerve, artery, and vein.
Look at the skull from below. Locate the palatine processes that form the anterior part of the hard palate with the incisive foramen between them.
Note that the mandible consists of a horizontal body and a vertical ramus.
Examine the angle of the mandible between the body and the ramus.
The ramus has a condylar process posteriorly and a coronoid process anteriorly.
The alveolar process is the part of the mandibular bone which supports the teeth.
On the internal aspect of the ramus is the mandibular foramen, which opens into a canal. Inferior alveolar nerves and blood vessels to the teeth run through this canal. Cutaneous nerves, which are branches of V3, exit through the mental foramen on the external surface of the mandible.