Part I · Chapter 3 · Lectures 2 & 3
Organizing & dissecting the brain
The six cortical layers, the directions and planes anatomists use, the four ways to carve up the brain, and what you saw in the sheep-brain dissection.
1. Cortical columns and the six layers
The cerebral cortex is the thin, folded outer layer of the cerebrum. Cut straight down through it and you see six distinct layers, stacked like floors in a building, organized into columns. Each floor has a different job in the building's mail system:
Who talks to whom in a cortical column — click a layer group
What to notice: input and output live on different floors. The
thalamus is both where most input comes from and where layer 6 sends output back.
Common mistake — the source changed
The Lecture 2 study guide says "Layer 1 = cortex-to-cortex, Layer 6 = output". Lecture 3 opened by
correcting this after exit tickets showed confusion: Layers 1, 2, 3 =
connections from one cortical region to another; Layers 5, 6 = output (to thalamus);
Layer 4 = input (from thalamus and other cortex). Use the Lecture 3 version. You'll use
"LGN → layer 4 of V1" again in Chapter 10.
2. Directions and planes
"Top", "front" and "side" are ambiguous — top of what, facing which way? Anatomists use fixed terms. Learn them as pairs:
| Term | Means | Hook |
|---|---|---|
| Dorsal | Top of the brain | A dolphin's dorsal fin is on top |
| Ventral | Bottom of the brain | "Venter" = belly |
| Rostral | Front of the brain | Rostrum = beak / nose |
| Caudal | Back of the brain | Cauda = tail |
| Lateral | Side of the brain | Lateral = side (as in a football lateral) |
| Plane | Definition (lecture) |
|---|---|
| Horizontal | Orthogonal to the main axis of the body — parallel to the floor |
| Sagittal | Aligned with the central tissue that splits the brain into two hemispheres — the "arrow" plane, front to back |
| Coronal | Cuts through the two ears and the top of the head — like a headband / crown |
Slice the brain — pick a plane
You look at a slice showing both hemispheres side by side, with the top of the head at the top and both temporal lobes hanging down on either side. Which plane is it?
Why: A slice that shows both hemispheres and runs top-to-bottom
goes through both ears — coronal. A sagittal slice shows only one hemisphere (front-to-back). A
horizontal slice would show both hemispheres but viewed from above, front at one end and back at the
other.
3. Four ways to divide the brain
Why four? Because "where does one part end and the next begin" has different answers depending on what you measure — tissue colour, cell shape, folds, or function.
Anatomical division: the four lobes (lateral view, front = left)
Click a lobe
Schematic, not to scale. What to notice: the lobes are defined by landmarks
(the big sulci), which is exactly why the lecture says sulci and gyri are used as landmarks for
surgery.
| Division | Based on | Details from lecture |
|---|---|---|
| 1. Grey vs white matter | Tissue colour | Grey = cell bodies (somas). White = axons — white because of the fatty myelin. |
| 2. Cytoarchitectural | Cell composition | Brodmann areas: divisions based on the size, shape and density of neurons in different regions of cortex. |
| 3. Anatomical | Folds | Four lobes: frontal, parietal, occipital, temporal. Gyri (ridges) and sulci (grooves) form to maximize surface area; used as surgical landmarks. |
| 4. Functional | What a region does | Specific locations have specific functions — but general functions like language are distributed across regions. Historical (wrong) version: phrenology — skull bumps linked to personality (chart from 1883; idea goes back hundreds of years). |
Across species
Compared with other animals, humans have more grey matter (neurons), more
folds (convolutions), and more white-matter pathways.
Across people
Human brains differ in size and shape, but general organization is consistent. Because we often study
group-average brains, we can miss small indentations (tertiary sulci). Manually
labelling 4,511 sulci in 572 hemispheres found a new one — the inframarginal sulcus,
absent from atlases.
Intuition
Why fold at all? A sheet of cortex needs area (more neurons), but the skull is a fixed box. Crumpling a
sheet lets a large area fit in a small volume — like crumpling paper into a ball. More folds = more
cortex per skull.
4. The sheep-brain dissection
The three meninges
The meninges are the protective outer layers of the brain. From outside in — mnemonic "DAP", or "the brain is protected by a Dad, an Aunt and a Parent":
Meninges in cross-section (skull at top, brain at bottom)
What to notice: only the pia dips into the sulci; the arachnoid bridges over them.
Cerebrospinal fluid sits beneath the arachnoid — a favourite exam detail.
What you identified in each view
| View | Structures |
|---|---|
| Dorsal (from above) | Meninges; motor cortex and somatosensory cortex (on either side of the central sulcus) |
| Ventral (from below) | Underside of the brain (brainstem, optic nerves/chiasm) |
| Sagittal (cut down the middle) | White matter (myelinated axons) vs grey matter (cell bodies); cerebral cortex as the thin folded outer layer; subcortical structures |
| Coronal | Thalamus, basal ganglia, hippocampus |
The slides were mostly photographs with labels; the table rebuilds the labels that came through in text. The ventral-view contents are my reconstruction — check against your lab notes.
Key idea — why these three coronal structures matter
They reappear throughout the midterm material: basal ganglia → Parkinson's and
Huntington's (Ch. 7); hippocampus → Alzheimer's (Ch. 7); thalamus →
layer 4 input, the basal-ganglia output target, and the LGN relay in vision (Ch. 10).
5. Check yourself
White matter looks white because it is mostly…
Why: Myelin is fatty and pale. Cell bodies are the grey matter.
A map dividing cortex by the size, shape and density of its neurons is a…
Why: "Cyto" = cell; cytoarchitecture = cellular composition. Lobes are
defined by folds; functional maps by what regions do; phrenology by skull bumps.
A tumour sits at the caudal end of the brain. It's most likely in the…
Why: Caudal = back ("tail"). The occipital lobe is the back of the
cerebrum. Frontal would be rostral; top of parietal would be dorsal.
Axons arriving from the thalamus mostly terminate in cortical layer…
Why: Layer 4 is the input layer. Layer 6 is where output goes
back to the thalamus — the tempting mix-up.