The commissural fibers or transverse fibers are axons that connect the two hemispheres of the brain. Huge numbers of commissural fibers make up the commissural tracts in the brain, the largest of which is the corpus callosum.
In contrast to commissural fibers, association fibers form association tracts that connect regions within the same hemisphere of the brain, and projection fibers connect each region to other parts of the brain or to the spinal cord.
The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. It is the largest site of neural integration in the central nervous system, and plays a key role in attention, perception, awareness, thought, memory, language, and consciousness.
The six-layered neocortex makes up approximately 90% of the cortex, with the allocortex making up the remainder. The cortex is divided into left and right parts by the longitudinal fissure, which separates the two cerebral hemispheres that are joined beneath the cortex by the corpus callosum and other commissural fibers. In most mammals, apart from small mammals that have small brains, the cerebral cortex is folded, providing a greater surface area in the confined volume of the cranium. Apart from minimising brain and cranial volume, cortical folding is crucial for the brain circuitry and its functional organisation. In mammals with small brains, there is no folding and the cortex is smooth.
Association fibers are axons (nerve fibers) that connect cortical areas within the same cerebral hemisphere.
In human neuroanatomy, axons within the brain, can be categorized on the basis of their course and connections as association fibers, projection fibers, and commissural fibers. Bundles of fibers are known as nerve tracts, and consist of association tracts, commissural tracts, and projection tracts.