Our central nervous system consists of three major parts, the cerebrum, the cerebellum, and the brainstem. The largest part is the cerebrum, which consists of two nearly symmetrical halves called the cerebral hemispheres.
If we were to cut through the cerebral hemispheres in the coronal plane, we would see that the outermost area is the cerebral cortex, which consists of gray matter that contains billions of neuronal cell bodies.
Deep to the gray matter is the subcortical white matter, which is made up of myelinated axons that are extensions of the neuronal cell bodies, allowing them to send and receive signals.
The term white matter is used because the myelination of the axon fibers gives this area a white appearance on gross inspection.
Axon fibers can be divided into three main groups - commissural fibers, association fibers, and projection fibers - depending on the target of the axons.
The cerebral commissural fibers connect the left and right cerebral hemispheres and consist of the corpus callosum, the anterior commissure, the posterior commissure, and the hippocampal commissure.
Association fibers connect different regions within the same hemisphere and they include the U-fibers, uncinate fasciculus, cingulum bundle, arcuate fasciculus, superior longitudinal and inferior longitudinal fasciculi, and the occipitofrontal fasciculus.
Finally, projection fibers connect the cerebral cortex with more caudal structures in the central nervous system, like the thalamus, brainstem, and spinal cord.