1 about 18 minDeep Learning viewOpenStaxNetter p.9–18BodyParts3D

Orientation: finding your way around a skull

Before you name a single bone, fix the frame of reference: how the head is held, whose left and right you mean, and what each standard view is for. Everything later in the unit is described in these terms.

Anatomical position, applied to the head

Anatomy is described as if every body were standing the same way. In the anatomical position the person stands upright, faces straight ahead with the eyes on the horizon, and holds the arms by the sides with the palms forward. Every term of direction in this unit assumes that posture, however the patient is really lying and however you have turned the skull on your screen. The top of the head is still superior when the patient is flat on a CT table.

A skull on its own has no body to tell you when it is level, so the rule is written on the bone. A skull is in the anatomical position when two landmarks sit at the same height on each side: the lowest point of the bony rim under the eye, the infraorbital margin, and the top of the opening of the ear canal, the external acoustic meatus. The horizontal plane through those points is the Frankfort horizontal plane. Open the left lateral view and test it: a line from the lower orbital rim to the top of the ear opening should run level across the screen.

Whose left? The patient's left and right

Left and right always belong to the patient, the person the skull came from, and never to the observer. The left parietal bone is the one above the patient's left ear, wherever it happens to be on your screen.

The difficulty is that a picture does not turn with the patient. Face someone across a table and their left hand is on your right. The anterior view of the skull works the same way: the patient's left is on the viewer's right. Walk around to the back and the two agree again: in the posterior view the patient's left is on the viewer's left.

Where the patient's left appears in each view

  • Anterior

    You are looking from
    The front, at the face
    The patient's left is
    On the viewer's right

  • Posterior

    You are looking from
    Behind
    The patient's left is
    On the viewer's left

  • Left lateral

    You are looking from
    The patient's left side
    The patient's left is
    The side facing you. The face points to the viewer's left and the right side is hidden

  • Right lateral

    You are looking from
    The patient's right side
    The patient's left is
    The hidden side. The face points to the viewer's right

  • Superior

    You are looking from
    Above, down onto the vault
    The patient's left is
    On the viewer's left when the face is toward the top of the screen

  • Inferior

    You are looking from
    Below, up at the outside of the base
    The patient's left is
    On the viewer's right when the face is toward the top of the screen

  • Internal cranial base

    You are looking from
    Above, with the skullcap removed
    The patient's left is
    On the viewer's left when the face is toward the top of the screen, as in the superior view

  • Midsagittal section

    You are looking from
    The side, at a skull cut in the midline
    The patient's left is
    Not laid out as a side: the picture runs from front to back along the cut

Each atlas preset states where the patient's left is. Read it before you name a side.

The superior and inferior views are mirror images, because they look at the same skull from opposite directions. This is the trap at the cranial base: an opening that lies on the left of the screen from above lies on the right of the screen from below, and it has not changed sides.

In this unit a chip that names a side selects one bone and a chip that names a pair selects both. In the anterior view, tap left parietal bone and note which half of the screen it is on. Switch to the posterior view and tap it again. Then tap both parietal bones.

Directional terms on the skull

Directional terms come in opposite pairs, and each pair measures along one axis of the body in the anatomical position. Learn them by pointing: every row below names something you can tap on the specimen.

Directional terms, each with a skull example

  • Superior (cranial)

    Means
    Toward the top of the head
    On the skull
    The parietal bones are superior to the temporal bones

  • Inferior (caudal)

    Means
    Toward the feet
    On the skull
    The mandible is inferior to the maxillae

  • Anterior (ventral)

    Means
    Toward the front, the face
    On the skull
    The frontal bone is anterior to the parietal bones

  • Posterior (dorsal)

    Means
    Toward the back
    On the skull
    The occipital bone is the most posterior bone of the skull

  • Medial

    Means
    Toward the median plane
    On the skull
    The nasal bones are medial to the zygomatic bones

  • Lateral

    Means
    Away from the median plane
    On the skull
    The zygomatic arches are the most lateral parts of the face

  • Median

    Means
    On the median plane itself
    On the skull
    The vomer and the sagittal suture are median structures

  • External and internal

    Means
    Toward the outer surface, or toward the cavity inside
    On the skull
    The external surface of the occipital bone lies under the scalp; its internal surface faces the brain

  • Superficial and deep

    Means
    Nearer to, or farther from, the surface of the body
    On the skull
    The coronoid process of the mandible passes deep to the zygomatic arch

  • Ipsilateral and contralateral

    Means
    On the same side, or on the opposite side
    On the skull
    The left mastoid process is ipsilateral to the left ear opening and contralateral to the right one

Three sets of planes cut the head at right angles to one another. The median plane is the single vertical plane that divides the skull into right and left halves. Planes parallel to it are sagittal, and a cut exactly in the median plane is called midsagittal. Coronal (frontal) planes are vertical and divide front from back. Axial (transverse, horizontal) planes divide top from bottom. Two sutures are named for the plane they lie in: the coronal suture crosses the vault from side to side, and the sagittal suture runs from front to back along the midline.

The standard views and what each is for

OpenStaxNetter p.9–18

Anatomists look at the skull from a small set of agreed directions, so that “seen from below” means the same thing to everyone. The atlas beside this text has eight presets: six views of the outside and two that open the skull. Older books give the outside views Latin names beginning with norma; they are added here in case you meet them.

The standard viewsOpenStax

  • Anterior (frontal view, norma frontalis)

    What you see
    The forehead, both orbits, the nasal opening, the cheekbones and both jaws
    Use it for
    The facial skeleton and the frontal bone; comparing right with left

  • Lateral, left or right (norma lateralis)

    What you see
    The side of the braincase, the temple, the ear opening, the zygomatic arch and the jaws in profile
    Use it for
    The temporal bone, the sutures of the side wall, the pterion, the mandible and its joint

  • Posterior (norma occipitalis)

    What you see
    The back of the vault, with a mastoid process at each lower corner
    Use it for
    The occipital bone, the backs of the parietal bones and the lambdoid suture

  • Superior (norma verticalis)

    What you see
    The oval dome of the vault from above
    Use it for
    The frontal and parietal bones, the coronal and sagittal sutures and the point where they meet

  • Inferior (norma basalis)

    What you see
    The external surface of the base: the palate in front, the large opening for the spinal cord behind
    Use it for
    The hard palate, the processes that hang from the base, and the outer ends of its openings

  • Internal cranial base

    What you see
    The floor of the cranial cavity from above, after the skullcap has been lifted off
    Use it for
    The three cranial fossae and the inner ends of the openings (sections 4 and 5)

  • Midsagittal section

    What you see
    One half of a skull cut in the median plane, seen from the cut side
    Use it for
    The nasal septum, the air sinuses, and the thickness and slope of the base

The lower jaw covers part of the base from below, so atlases usually show the inferior view with the mandible taken away.

No view shows everything, and each bone has one view in which it is easiest to recognize. Section 3 names that view for every bone. Start the habit now: find a structure in its best view, then rotate to a second view and find it again.

Calvaria versus cranial base

OpenStax

The part of the skull that holds the brain has a roof and a floor. The roof is the calvaria (skullcap), the smooth dome you can feel through the scalp. It is made mainly by the frontal bone in front, the two parietal bones on top and at the sides, and the upper part of the occipital bone behind, with the thin upper plates of the temporal bones low on each side.

The floor is the cranial base (base of the skull). It is thicker and far more irregular, and it is pierced by the openings that let nerves and vessels in and out. Parts of five bones build it: the frontal, ethmoid, sphenoid, temporal and occipital bones. Like any floor it has two surfaces. The internal surface is stepped into three hollows, the anterior, middle and posterior cranial fossae (a fossa is a hollow), each lower than the one in front of it. The external surface faces down toward the neck and throat and carries the joints for the vertebral column and the lower jaw.

Roof and floor of the braincase

Calvaria

  • Domed roof of the cranial cavity
  • Broad curved plates of bone joined by long sutures
  • Few openings
  • Best views: superior, lateral, posterior

Cranial base

  • Floor of the cranial cavity
  • Thick ridges and thin plates, with three fossae inside
  • Many openings for nerves and vessels
  • Best views: inferior (outside) and internal base (inside)
OpenStax

The line between them is a convention, not a suture. Picture a horizontal ring around the head that passes just above the supraorbital margins (the upper rims of the orbits) in front and close to the external occipital protuberance (the midline bump at the back of the head) behind. What lies above the ring is calvaria. The internal cranial base view shows the skull with that cap removed.

Neurocranium versus viscerocranium at a glance

OpenStaxNetter p.9–10

A second way to divide the skull is by what the bones are for. The neurocranium (braincase) is the box around the brain: the calvaria and the cranial base together, built from 8 cranial bones. The viscerocranium (facial skeleton) hangs from the front of the base and frames the mouth, the nose and most of each orbit; it is built from 14 facial bones. Section 2 lists all 22.

Specimen still

Front of the skull. The bones of the braincase are tinted one color and the bones of the face another; the teeth are left plain.

Anterior view. Facing the skull: the patient's left is on your right.

Anterior view with the cranial bones and the facial bones tinted apart. The forehead belongs to the braincase; from the orbital rims downward, almost everything you see is facial skeleton.BodyParts3D

From the front, the boundary runs through the orbits. The forehead and the upper rim of each orbit are frontal bone, a cranial bone. The cheeks, the bridge of the nose and both jaws are facial bones. From the side, the braincase is the large rounded mass above and behind, and the face is the smaller block below and in front of it.

Specimen still

Left side of the skull, face pointing to the left of the picture. The bones of the braincase are tinted one color and the bones of the face another.

Left lateral view. The patient's left side: the face points to your left.

Left lateral view with the same tint. The zygomatic arch is the bridge between the two groups.BodyParts3D

One landmark shows how closely the two parts are joined. The zygomatic arch, the bar of bone you can feel running from the cheek to just in front of the ear, is half facial and half cranial. Its front part belongs to the zygomatic bone and its back part to the temporal bone.

How to use the specimen beside the text

BodyParts3D

The skull beside this text is a three-dimensional model of one adult skull. All 22 bones are separate pieces, so every suture shows as the seam between two bones, and the teeth are separate from the jaws. Use it as you would a real skull on the bench: look, turn, name, and check.

  1. Tap a chip. A highlighted name in the text, such as frontal bone, selects that structure on the specimen. A chip for a pair, such as temporal bones, selects both sides. A chip that names a side, such as right temporal bone, selects one.
  2. Start from a preset. When the text names a view, switch to it before you look. Then rotate a little away from the preset and watch how the outline of the structure changes.
  3. Read the side. Every preset states where the patient's left is. Check it before you call anything left or right.
  4. Describe before you move on. Place the structure with two directional terms and one neighbor: “the mastoid process is posterior and inferior to the ear opening, on the temporal bone”.
  5. Find it a second way. Look for the same structure from another direction. A bone you can recognize in two views is a bone you know.

Knowledge check · 1

Answer, read why, and try again whenever you like. Answers are kept in both study modes.

0 of 8 correct
  1. Q1Left and right in the anterior viewBodyParts3D
    Front of the skull with one cheekbone highlighted.

    Anterior view. Facing the skull: the patient's left is on your right.

    This is the anterior view. One cheekbone (zygomatic bone) is highlighted. On which side of the patient is it?

  2. Q2Choosing a view: the vault from aboveOpenStax

    You want one picture that shows both parietal bones, the suture between them, and the suture that separates them from the frontal bone. Which preset is the best choice?

  3. Q3External versus internal views of the baseOpenStaxNetter p.17–18

    A classmate wants to see the hard palate and the two occipital condyles together. Which view shows them?

  4. Q4Directional termsOpenStax

    Which statement uses directional terms correctly?

  5. Q5Calvaria versus cranial baseOpenStax

    Which of these belongs to the cranial base rather than to the calvaria?

  6. Q6Neurocranium versus viscerocranium: the ethmoidOpenStax

    Which bone is counted with the cranial bones although most of it lies between the orbits and in the walls of the nasal cavity?

  7. Q7Left and right in the posterior viewBodyParts3DOpenStax
    Back of the skull with one bone highlighted.

    Posterior view. From behind: the patient's left is on your left.

    This is the posterior view. Name the highlighted bone.

  8. Q8Zygomatic arch: one facial bone, one cranial boneOpenStax

    The zygomatic arch joins the face to the braincase. Which two bones form it?

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