Each cerebral hemisphere has three basic regions. The outermost region is called the cerebral cortex. It is a 2 to 4 mm thick layer of gray matter composed of neuron cell bodies, dendrites, associated glia, and blood vessels. The inner region of the cerebrum is the white matter. It is primarily composed of myelinated nerve fibers. These fibers are stacked into three types of large fibers or tracts. The association fibers connect different regions within the same cerebral hemisphere. In contrast, the commissures, such as the corpus callosum, connect the two hemispheres. The projection fibers connect the cerebral cortex to the thalamus, brainstem, and spinal cord. The innermost region, deep within the white matter, contains collections of neuron cell bodies called basal nuclei, which are involved in motor functions. The first two nuclei, the globus pallidus and the putamen, are adjacent to each other and collectively called the lentiform nucleus. The third is the caudate nucleus. When viewed laterally, it appears as an arch with a large head and a slender tail-like structure above the diencephalon.