At the organ level, the nervous system is composed of brain regions, such as the hippocampus in mammals or the mushroom bodies of the fruit fly.[1] These regions are often modular and serve a particular role within the general pathways of the nervous system. For example, the hippocampus is critical for forming memories. The nervous system also contains nerves, which are bundles of fibers that originate from the brain and spinal cord, and branch repeatedly to innervate every part of the body.