A model of fiber tracts
Notice the relationship between these arrows and the wall of the circle. The arrow origin or head may be located at the perimeter of the circle, or outside of the circle. If we replace the arrows for tracks and the circle for the brain (the perimeter the cerebral cortex) we will understand the tracts called "Projection Fibers." They carry information in and out of the brain. Some examples of "Projection Fibers" are: Visual Pathways, and the Cortico-spinal or pyramidal tract.
In this other example the arrows have both origin and end point within the circle "connecting" different points of the perimeter. Using the same model, this represents fibers that connect different areas of the cerebral cortex, within the same hemisphere or between them. These types of fibers are called "Associative Fibers." They carry information between similar areas of processing, or from lower to higher levels of cortical complexity. Two examples of these fibers are the Corpus Callosum and the arcuate fasciculus.
In this model we have arrows that have either the origin or end point within the circle but one of the ends is not located in the perimeter of the circle.
This represents fibers connecting the cerebral cortex with other gray matter areas of the CNS (e.g.: the thalamus, the cerebellum). Fibers connecting cerebral non-cortical gray matter areas are also considered within this group. These are the "Integrative Fibers." Two examples of fibers of this type are the thalamic-frontal fibers and the rubro-thalamic fibers.