The spinal cord is not uniform in thickness, but tapers slightly, though not uniformly, from the upper toward the lower end. At the places where the nerves from the arms and legs enter the cord two enlargements are to be found, the upper being called the _cervical_ and the lower the _lumbar enlargement_. These, on account of the difference in length between the cord and the spinal cavity, are above—the lower one considerably above—the places where the limbs which they supply join the trunk (Fig. 133). *Arrangement of the Neurons of the Brain and Cord.*—The cell-bodies in the brain and spinal cord are collected into groups, and their fibers extend from these groups to places that may be near or remote. Guided by the white and gray colors of the nervous tissue, and also by the structures revealed by the microscope, physiologists have made out three general schemes in the grouping of cell-bodies, as follows: 1. _That of surface distribution_, the cell-bodies forming a thin but continuous layer over a given surface. This is the plan in the cerebrum and cerebellum, and here are found devices for increasing the surface: the cerebrum having convolutions, the cerebellum transverse ridges. 2. _That of collections of cell-bodies into rounded masses._ Such masses are found in the bulb, the pons, the midbrain, and the base of the cerebrum. 3. _That of arrangement in a continuous column._ This is the plan in the spinal cord. It matters not at what place the spinal cord be cut, a central area of gray matter, resembling in form the capital letter H, is always found. 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.