Identify structures on the specimen

Finding a structure on a skull is a procedure. People who do it reliably are not quicker at recognizing shapes. They ask the same four questions in the same order every time, so a difficult target is found by the same steps as an easy one. Learn the order once and use it for every prompt in this section.

1. Choose the view. Ask which surface of the skull the structure is on, and open the preset that faces that surface. If a prompt states a starting view, begin there.
2. Find an anchor. An anchor is a landmark too large to mistake, such as an orbit or the foramen magnum. Find the nearest one first and work outward from it. Do not hunt for a small structure directly.
3. Fix the side. Say where the patient's left is in this view before you look at either half. Then look only at the half you need.
4. Confirm with the neighbors. Name two things that must lie beside your answer and check that they do. If the neighbors are wrong, the answer is wrong.

Step 1: the view. Every structure is on a surface, and each preset faces one surface. The table repeats the views of section 1, sorted this time by what you are looking for.

Step 2: the anchor. Six landmarks are large enough to find at a glance, and between them they fix every region of the skull. For each one, learn what lies next to it. The neighbors in this table are the routes you will use in practice.

Specimen still

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

Step 3: the side. Left and right belong to the patient, never to the viewer (section 1). Two rules cover every preset. Seen from the front or from below, the picture is swapped: the patient's left is on your right. Seen from behind or from above, the picture agrees with you: the patient's left is on your left. The internal cranial base is a view from above, so with the face toward the top of the screen the patient's right is on your right. In a lateral view the side facing you is the side the view is named for, and the other side is hidden. In the midsagittal section of this specimen the left half has been removed, so you look at the cut surface of the right half with the face toward your left.

Step 4: the neighbors. A structure is identified by where it is, and a position is a statement about neighbors. Before you tap, finish a sentence of the form “it lies between … and …” or “it is just behind and lateral to …”. This is the step that separates look-alikes, and the next topic is built on it.

Specimen still

Internal cranial base view. Skull cap removed, looking down into the cranial cavity: the face is toward the top, the patient's right is on your right.

Most wrong answers in identification are not wild guesses. They are swaps between two structures that share a bone, a region, a name or a nerve. Each trap below has one cue that separates the pair, and in every case the cue is a position. Shape on its own is a poor guide, for a reason given at the end of this topic.

Specimen still

Internal cranial base view. Skull cap removed, looking down into the cranial cavity: the face is toward the top, the patient's right is on your right.

Rotundum, ovale, spinosum: count along the wing. Do not try to tell them apart by shape. “Round” and “oval” are hard to judge on an opening a few millimeters across. Start at the superior orbital fissure and move backward and outward along the greater wing. The first opening, just below the medial end of the fissure, is the foramen rotundum. It faces forward, toward the back of the upper jaw. The second, the foramen ovale, is the largest and passes straight down. The third, the foramen spinosum, is the smallest and lies just behind and lateral to the ovale. Turn the skull over and the count changes. On the under-surface only the ovale and the spinosum have a mouth, behind the root of the lateral pterygoid plate, and the spinosum is the one next to the pointed spine of the sphenoid. The rotundum cannot be found from below, because it opens forward into the pterygopalatine fossa.

Internal acoustic meatus and stylomastoid foramen: ask which surface. The temporal bone has three openings whose names are easy to cross. The external acoustic meatus is the ear opening on the side of the skull, seen in the lateral view. The internal acoustic meatus is inside the skull, on the back slope of the petrous part, and is found from the internal cranial base. The stylomastoid foramen is on the under-surface, between the styloid process and the mastoid process, and is found from the inferior view. The facial nerve links the last two. It goes in at the internal acoustic meatus together with the vestibulocochlear nerve and comes out, alone, at the stylomastoid foramen. When a prompt mentions the facial nerve, read on for enters or leaves, and check which view you are in.

Optic canal and superior orbital fissure: the words give the shape. A canal is a tunnel through bone and a fissure is a gap between two pieces of bone (section 5). At the back of the orbit the round tunnel is the optic canal. It lies medially, at the front outer corner of the sella turcica, and opens at the very apex of the orbit. The slit lateral to it is the superior orbital fissure, with the lesser wing above it and the greater wing below. Looking into the orbit from the front there is a second slit to keep apart from it. The inferior orbital fissure runs lower down, along the angle between the lateral wall and the floor, and does not lead to the cranial cavity.

The conchae: two belong to the ethmoid, one is a bone. Look into the nasal opening in the anterior view. On each side wall the lowest and largest shelf is the inferior nasal concha, one of the 14 facial bones. Above it is the middle nasal concha, and higher and further back, out of sight from the front, is the small superior nasal concha. Those two are not separate bones: they are scrolls of the ethmoid. The usual errors are to count all three as bones or to give all three to the ethmoid. A prompt for “the inferior nasal concha” asks for a bone. A prompt for “the middle nasal concha” asks for part of one.

Coronoid and condylar processes: the one near the ear makes the joint. In a lateral view the ramus of the mandible ends above in two projections separated by the mandibular notch. The front one, the coronoid process, is thin and pointed. A chewing muscle attaches to it, and it slips up under the zygomatic arch, so part of it is hidden in this view. The back one, the condylar process, has a narrow neck and a rounded head. The head sits in the mandibular fossa of the temporal bone, just in front of the ear opening, and forms the temporomandibular joint. If the names blur, use their roots: coronoid comes from the Greek for a crow, after its beak-like point, and condyle from the Greek for a knuckle.

Three short activities put the routine to work on the live specimen. The labels are off, nothing is selected for you, and you answer by tapping the structure itself. Each prompt is precise about what is wanted and states the view it starts in. You are free to rotate and zoom before you answer.

1. Read the prompt. It names one structure and, where it matters, a side, for example “Select the left zygomatic bone.” The starting view is stated with it.
2. Run the routine. View, anchor, side, neighbors. Say the neighbors to yourself before you tap.
3. Tap the structure on the specimen.
4. Read the feedback. It shows the correct structure and explains the cue that locates it, usually a neighbor or a landmark in that view.
5. Retry. A missed prompt can be tried again. Afterward, find the same structure from a second view.