A portion of the cerebral hemispheres, enveloped by the cortex, and consisting of projection fibers passing upward and downward between the cortex and the thalamus, interspersed among cell bodies of the basal ganglia. The white fibers and gray cells give the structure its striped appearance.In the course of evolution, the corpus striatum became established as a motor center in fishes. Its importance continued in amphibia and reptiles, and reached its highest development in birds. In mammals, however, the cerebral cortex gradually took over its motor functions, and in man it is primarily a transmission area rather than a control center. However, some authorities believe it is still involved in motor control, since injuries located in it are associated with certain forms of chorea, a disorder characterized by spasmodic, involuntary movements of the limbs or facial muscles. This suggests that the corpus striatum may play a role in smoothing and integrating motor activities.