Corpus Callosum
The bridge that makes two hemispheres behave like one mind.
You are looking at the flat, cut face of the brain model — the midsagittal surface, the view you get when the brain is sliced straight down the middle. The thick, C-shaped band arching across the centre of that face is the corpus callosum, and the rounded bulge at its back end is the splenium, where this pad sits.
The splenium carries signals from the back half of the brain — vision, hearing and memory areas — between the left and right hemispheres. That is why this pad is also a doorway to the temporal lobe: much of what the temporal lobe sends to the opposite hemisphere crosses right here.
See it for yourself: open the 3D brain model and drag the Cutaway slider to slice the near half away.
Your brain is split into two halves, a left hemisphere and a right hemisphere. The corpus callosum is the bridge between them. It is a thick bundle of about 200 million nerve fibres, and it is the largest connection in the entire brain. Every time the two sides of your brain need to compare notes, the message travels across it.
That teamwork happens constantly. If you hold something behind your back with your left hand, the right side of your brain feels it — but the language areas that let you say "it's a key" sit on the left side. The corpus callosum carries the message across so the two halves work as one brain instead of two.
The corpus callosum is not all the same. The front end curves down toward your forehead and carries messages between the thinking and planning areas. The back end is thicker and rounder, and it is called the splenium. The splenium handles traffic from the back of the brain: what you see, what you hear, and the memories tied to those things.
When surgeons cut the corpus callosum — something once done to treat severe epilepsy — the results are striking. People act normally most of the time, but each half of the brain can end up knowing something the other half does not. Those "split-brain" patients taught scientists an enormous amount about what each hemisphere does on its own.
The corpus callosum is the largest white matter commissure in the human brain, containing an estimated 200–300 million myelinated axons that interconnect homologous and heterologous cortical regions of the two cerebral hemispheres. Composed almost entirely of projecting fibres, it appears on a midsagittal section as a dense, pale, arching band above the lateral ventricles and the fornix.
Anatomists divide it front to back into the rostrum, genu, body (truncus), isthmus and splenium. This organisation is topographic: prefrontal fibres sweep forward through the genu as the forceps minor, motor and somatosensory fibres cross through the body, and posterior fibres pass through the splenium as the forceps major, which projects to the occipital lobes, and the tapetum, which arcs downward alongside the lateral ventricle to reach the temporal lobes.
The splenium — the thickened posterior extremity, and the region marked on this model — is therefore the principal conduit for interhemispheric transfer of visual, auditory and posterior temporal information. It carries the connections between the visual association cortices and between the posterior temporal language and memory regions.
Its clinical signature reflects that role. A splenial lesion combined with damage to the left occipital cortex produces alexia without agraphia: the patient can still write but can no longer read what they have written, because visual information arriving in the intact right hemisphere can no longer reach the left-hemisphere language areas. Splenial lesions also appear in demyelinating disease and as reversible cytotoxic lesions following seizures.
Callosal fibres myelinate front-to-back into early adulthood, one of the slowest maturational trajectories in the brain. In agenesis of the corpus callosum the commissure never forms, yet affected individuals often function far better than the anatomy would predict, because pathways such as the anterior commissure take on some of the load.
Surgical section of the commissure — corpus callosotomy, performed for intractable epilepsy — gave rise to the classic split-brain studies of Sperry and Gazzaniga, which demonstrated hemispheric specialisation directly: a stimulus presented to one visual field could be named only when it reached the language-dominant hemisphere, while the disconnected hemisphere could still respond to it nonverbally.