The fibers connecting with the cell-bodies in the brain and spinal cord
are gathered into bundles or tracts, and these pass through different
parts somewhat as follows:
1. _In the cerebrum_ they extend in three general directions, forming
three classes of fibers. The first connect different localities in the
same hemisphere, and are known as _association_ fibers (_A_, Fig. 134).
The second make connection between the two hemispheres, and form the
corpus callosum. These are known as _commissural_ fibers (_C_, Fig. 134).
The third connect the cerebrum with the parts of the nervous system below,
and are called _projection_ fibers (_P_, Fig. 134).
2. _In the cerebellum_ both association and commissural fibers are found.
Bands of fibers, passing upward toward the cerebrum and downward toward
the cord, connect this part of the brain with other parts of the nervous
system.
[Fig. 134]
Fig. 134--*Semi-diagrammatic representation of a section through the right
cerebral hemisphere*, showing fiber tracts. _A._ Association fibers. _C._
Commissural fibers. _P._ Projection fibers. The cell-bodies with which the
fiber bundles connect are in the surface layer or cortex.
3. _In the midbrain, bulb, and spinal cord_ fibers are found: first, that
connect these parts with the cerebrum(101) and cerebellum above; second,
that pass into and become a part of the nerves of the body; and third,
that connect the opposite sides of these parts together.
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