The fiber tracts are bundles of axonal fibers which connect different parts of the brain and central nervous system. There are three different types of fiber systems: 1) projection fibers, which provide connections between the cerebral cortex and the subcortical centers, either as ascending systems terminating in the cortex or as descending systems extending from the cortex to the deeper-lying centers; 2) association fibers, which connect different cortical areas of the same hemisphere; 3) commissural fibers, which provide connections between the cortices of the two hemispheres, being really nothing else but interhemispheric association fibers.
The pathways descending from different cortical areas merge and form a fanlike structure known as the internal capsule. The ascending fibers pass through the internal capsule and then radiate outward like a fan. In this way, ascending and descending fibers form a radiating crown of fibers beneath the cortex, the corona radiata. The internal capsule appears in horizontal sections as an angle consisting of an anterior limb, bordered by the head of the caudate nucleus, globus pallidus and putamen, and a posterior limb, bordered by the thalamus, globus pallidus and putamen. Between both limbs lies the genu of the internal capsule.
The most important projection pathways include the acoustic radiation and the optic radiation. The fibers of the acoustic radiation originate in the medial geniculate body, extend across the lateral geniculate body, and cross the internal capsule at the inferior margin of the putamen. In the white matter of the temporal lobe, they ascend almost vertically to the anterior transverse gyrus (Heschl’s convolution). The optic radiation originates in the lateral geniculate body. The fibers fan out into a wide medullary lamina and run to the temporal lobe, where they form the temporal genu of the visual pathway. They then pass in an arch around the inferior horn of the lateral ventricle and through the white matter of the occipital lobe to the calcarine sulcus.