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Human Anatomy Lesson 14

Robert is an anatomy professor at Benedictine University in Lisle, IL.

human-anatomy-lesson-14

Lesson 14 - Skull

The skull is the "skeleton" of the head. In an older adult, the skull is composed of two bones, the cranium (the part which houses the brain and upper jaw) and the mandible, or lower jaw. However, the skull is really composed of 22 different bones which fuse together during the course of development to form the cranium and mandible. The 22 bones of the skull are the frontal, parietal (2), temporal (2), occipital, sphenoid, zygomatic (2), nasal (2), lacrimal (2), ethmoid, vomer, maxilla (2), palatine (2), inferior nasal concha (2), and the mandible. The distinction between bones is somewhat arbitrary, since many of the 22 cranial bones form from a larger number of bones during embryonic development and early postnatal development - the occipital bone, for example, forms from four different parts and the frontal bone is divided into two halves at birth.

The cranium is often split into three parts - the neurocranium, which houses the brain; the viscerocranium, or face; and the splanchnocranium, or cranial base - to distinguish parts that come from different embryonic precursors. Another term for the neurocranium is the cranial vault, and the viscerocranium and splanchnocranium are more commonly referred to as the face (or upper jaw) and cranial (or skull) base, respectively. Embryology of the skull is very interesting, and an understanding of embryological development is necessary to understanding growth processes and structure of the adult skull. We will consider that topic briefly in this lesson.

A good atlas covering head and neck anatomy

The only embryology text to focus exclusively on the head

Learning Objectives - By the end of this lesson, you should be able to ...

  1. List the bones of the skull and understand how they articulate with one another
  2. Understand which bones make up the cranial vault, face, and basicranium, and which are paired and which unpaired
  3. List and describe the three cranial fossae
  4. List the foramina in the basicranium, and understand which nerves and vessels pass through each one
  5. Understand embryonic development of the bones of the cranial vault, face, and basicranium

Bones of the Skull

The neurocranium is made up of seven different bones: the frontal, parietal (2), temporal (2), sphenoid, and occipital. The majority of the viscerocranium is made up of the two maxillae (plural of maxilla), but many smaller bones contribute to its structure: the nasals (2), lacrimals (2), zygomatics (2), palatines (2), mandible, small parts of the frontal and temporal bones at the peripheries of the face, and the inferior nasal conchae (2) and ethmoid and sphenoid bones in the orbits and nasal cavity. The splanchnocranium is made up of the frontal, ethmoid, vomer, sphenoid, occipital, and parts of the temporal bones (2). It should be evident from the above lists that many of the bones of the cranium overlap with one another and cross from one region to another. The cranium is a complex, integrated, 3-D structure. The bones of the skull are listed below, and we will consider each separately by region, listing various features of the skull where they occur on each bone.

  1. Frontal
  2. Parietal (2)
  3. Temporal (2)
  4. Sphenoid
  5. Occipital
  6. Maxillae (2)
  7. Nasals(2)
  8. Lacrimals (2)
  9. Zygomatics (2)
  10. Palatines (2)
  11. Inferior nasal conchae (2)
  12. Mandible
  13. Ethmoid
  14. Vomer

Cranial Vault Bones

As noted above, the neurocranium, or cranial vault, is made up of the frontal, parietal (2), temporal (2), sphenoid, and occipital bones.

  1. Frontal bone: forms the roof of each orbit and the forehead, and articulates with the parietal bones at the coronal suture, the greater wing of the sphenoid bone at the sphenofrontal suture, the zygomatic bone at the frontozygomatic suture, the maxilla at the frontomaxillary suture, and other bones in and around the orbit, such as the nasal bones, lacrimal bones, and ethmoid. Relevant features include the superciliary arches over each orbit and the supraorbital foramen, which passes the supraorbital nerves and vessels, in the medial part of each supraorbital rim.
  2. Parietal bones: Each parietal bone is a curved but roughly rectangular shape, and articulates with the frontal bone anteriorly at the coronal suture, with each other in the midline at the sagittal suture, with the occipital bone posteriorly at the lambdoid suture, the temporal bone at the squamous suture and parietomastoid suture, and sometimes with the greater wing of the sphenoid bone at the sphenoparietal suture (although the connections between bones in this region are highly variable). Relevant features on the outer surfaces of each parietal bone include a superior temporal line and an inferior temporal line where the temporalis muscle and aponeurosis attach to the side of the cranial vault. Meningeal vessels mark the internal surface of the two parietal bones.
  3. Temporal bones: Each temporal bone is made up of a squamous part, zygomatic part, styloid part, mastoid part, and petrous part. The squamous part of the temporal bone articulates with the parietal bone at the squamous suture and the greater wing of the sphenoid at the sphenosquamous suture, and the mastoid part articulates with the parietal bone at the parietomastoid suture. The zygomatic part articulates with the zygomatic bone at the zygomaticotemporal suture. On the underside and inside of the cranium, the temporal bone can be seen to articulate with the sphenoid and occipital bones. The petrous part of the temporal holds all the structures of the inner and middle ear and the tympanic tube to the external ear. The temporal bone features many of the holes and foramina that pass cranial nerves, arteries and veins into the neck, including the jugular foramen, stylomastoid foramen, and carotid canal.
  4. Sphenoid bone: this bone is the 'keystone" of the skull, since it sits in the middle of the cranium and articulates with 12 bones: the frontal, ethmoid, occipital, vomer, and the two parietal, temporal, zygomatic, and palatine bones. It makes a brief appearance on the cranial vault, where it articulates with the frontal, ethmoid, parietal, and temporal bones in a highly variable region referred to as pterion. We will consider the sphenoid at greater length along with the other bones that make up the basicranium.
  5. Occipital bone: a large bone that surrounds the foramen magnum and can be split, for descriptive purposes, into a squamous part and a nuchal part (both posterior to the foramen magnum), the occipital condyles, and a basioccipital part (anterior to the foramen magnum). The squamous part is positioned superiorly, and it articulates with the parietal bones at the lambdoid suture. The nuchal part is positioned more inferiorly, and articulates with the mastoid part of the temporal bone at the occipitomastoid suture. It has a "scalloped" surface that is marked by the attachment areas for nuchal muscles, including the superior nuchal line, inferior nuchal line, and external occipital protuberance. The squamous and nuchal parts of the occipital bone can be appreciated in posterior view. Anterior to this, on the external surface of the cranial base just anterolateral to the foramen magnum, are the occipital condyles, which articulate with the atlas vertebra. The basioccipital part is anterior to the foramen magnum on the external portion of the cranial base, and articulates with the temporal bone laterally (but variably separated from it by foramina lacerum on either side) and the spheno-occipital synchondrosis (SOS) anteriorly.

Bones of the Face and Basicranium

  1. Maxillae: this pair of bones makes up the bulk of the face. The superior part of the maxilla forms the medial part of the floor and medial wall of the orbit, and runs up the side of the nasal opening to articulate with the frontal bone superiorly and the nasal bone medially. The maxilla articulates with the zygomatic bone laterally and the other maxilla medially, and the ethmoid bone articulates with the two maxillae at their junction in the nasal cavity. The maxillae make up most of the hard palate, holding the upper teeth and forming the floor of the nasal cavity. The maxillae articulate with the palatine bones at the posterior edge of the hard palate and the sphenoid bone of the cranial base. The infraorbital foramen is just inferior to the orbit and lateral to the nasal opening.
  2. Zygomatics: Each zygomatic articulates with the frontal bone, maxilla, and temporal bone, forming much of the lateral wall of the orbit and part of the zygomatic arch. The zygomaticofacial foramen is just lateral to the orbit in the body of the zygomatic bone.
  3. Nasals: These two bones at the superior border of the nasal opening articulate with one another in the midline, with the frontal bone superiorly, and with the maxillae laterally.
  4. Lacrimals: the lacrimal is a small, irregularly-shaped bone in the medial wall of the orbit that articulates with the frontal, maxilla, and ethmoid bones. The lacrimal gland is positioned in the lacrimal canal in the anterior part of this bone.
  5. Palatines: The palatine bones articulate with the maxillae anteriorly to contribute to the palate, to the sphenoid bone posteriorly and laterally, and to the ethmoid and vomer bones posteriorly and superiorly, making up a small part of the back of each orbit.
  6. Ethmoid: this bone has a component in the midline that forms the median nasal septum and articulates with the nasal cartilage anteriorly and the vomer posteriorly. This portion runs superiorly to become the cribriform plate, which is surrounded by the frontal bone, and terminates superiorly in the crista galli. It also has lateral components that hold the superior nasal concha and middle nasal concha and are invested with ethmoid air cells.
  7. Inferior nasal conchae: The superior and middle nasal conchae are parts of the ethmoid bone, but the inferior nasal conchae are separate bones with a separate embryological origin. They are also known as the inferior turbinates.
  8. Vomer: This bone articulates with the ethmoid bone to make up the posterior portion of the median nasal septum, and inserts onto the external portion of the basicranium, either to the sphenoid bone or to the basioccipital.
  9. Mandible: in the dental and orthodontic literature, the orbits make up the upper face, the nasal cavity and surrounding parts of the maxilla and zygomatic and the upper teeth make up the midface, and the mandible and lower teeth make up the lower face. The two main parts of the mandible are the ramus, which connects the mandible to the temporal bone of the cranium at the temperomandibular joint (TMJ), and the body, or corpus, of the mandible, which holds the lower teeth and to which various muscles of the tongue and pharynx attach.

Cranial Cavity

The cranial fossae are set up, one behind the other, starting immediately posterior to the frontal bone on the endocortical side of the forehead, and ending immediately anterior to the occipital bone on the endocortical side of the back of the head. They are at different vertical levels, with the anterior cranial fossa highest and the posterior cranial fossa lowest, at the level of the foramen magnum where the spinal cord enters the brainstem.

  1. Anterior cranial fossa (ACF): made up of mostly frontal bone, with some contribution from the ethmoid in the cribriform plate and the lesser wing of the sphenoid bone at its posterior edge. The frontal lobe of the brain rests atop the anterior cranial fossa.
  2. Middle cranial fossa (MCF): made up mostly of sphenoid bone anteriorly, with some contribution from the temporal bone posteriorly. The anterior edge of the MCF is made up of the lesser wing of the sphenoid bone, and the posterior edge is made up of the petrous part of the temporal bone. The temporal lobe of the brain rests atop the MCF.
  3. Posterior cranial fossa (PCF): made up almost exclusively of occipital bone, although there is some contribution of temporal bone at its anterior edge. The brainstem rests atop the PCF.

Foramina

The foramina in bold pass cranial nerves.

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  1. Foramen caecum: this small hole between the frontal bone and crista galli of the cribriform plate passes emissary veins to the nasal cavity in a small percentage of individuals.
  2. Foramina in cribriform plate: multiple holes in the cribriform plate that pass rootlets of olfactory nerves (CN I)
  3. Optic canal: passes the optic nerve (CN II) and ophthalmic artery from the middle cranial fossae to the orbit.
  4. Superior orbital fissure: Passes the oculomotor nerve (CN III), trochlear nerve (CN IV), ophthalmic division of the trigeminal nerve (CN V1), and the abducent nerve (CN VI), in addition to ophthalmic veins, from the middle cranial fossae into the orbit. Cranial nerves III, IV, and VI provide motor innervation to muscles of the eyeball, and CN V1 runs sensory fibers out to the maxilla through the superior orbital fissure.
  5. Foramen rotundum: passes the maxillary division of the trigeminal nerve (CN V2) from the middle cranial fossa into the orbit in close approximation to the inferior orbital fissure, at which point it runs through the orbit to enter the external portion of the face.
  6. Foramen ovale: passes the mandibular division of the trigeminal nerve (CN V3) from the middle cranial fossa to the infratemporal fossa, where it provides motor and sensory innervation to the muscles of mastication.
  7. Foramen spinosum: passes the middle meningeal artery through the cranial base into the brain.
  8. Foramen magnum: passes the spinal cord/brainstem in addition to vertebral arteries and spinal roots of the accessory nerve (CN XI), which then leave the cranial cavity through the jugular foramen (see below).
  9. Internal acoustic meatus: passes the facial nerve (CN VII), vestibulocochlear nerve (CN VIII), and the labyrinthine artery. CN VIII provides special sensory innervation for hearing and balance, and CN VII runs into the internal acoustic meatus, gives off a few nerves, and exits the cranial cavity through the stylomastoid foramen.
  10. Jugular foramen: passes the glossopharyngeal nerve (CN IX), vagus nerve (CN X), and spinal accessory nerve (CN XI), in addition to the inferior petrosal sinus and sigmoid sinus, which form the internal jugular vein inferior to the external basicranium.
  11. Hypoglossal canal: passes the hypoglossal nerve (CN XII) and the meningeal branch of the ascending pharyngeal artery.

Embryonic Development

There are two aspects of embryonic development that are important for understanding growth and structure of the skull. First, some bones of the skull develop from cartilage, as happens in most of the rest of the skeleton, but other bones develop in membrane, without a cartilaginous precursor. Many of the bones of the face and basicranium develop from a cartilaginous precursor, via endochondral ossification, whereas the bones of the cranial vault develop in a membrane without a cartilaginous precursor via intramembranous ossification. A second aspect of embryonic development that is important is what cells form different parts of the cranium. Much of the cranial vault is formed from paraxial mesoderm, which also forms the basioccipital and petrous portion of the temporal, but the bones of the face, ethmoid, greater and lesser wings of the sphenoid, and squamous temporal are formed from neural crest cells. Neural crest cells also form the clavicle, but really no other bones in the skeleton; and they form the spinal nerves, as we talked about in a previous lesson.

Upcoming Lessons

This content is for informational purposes only and does not substitute for formal and individualized diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed medical professional. Do not stop or alter your current course of treatment. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.

© 2014 Robert McCarthy

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