At the most ancient part of the human brain lies the spinal cord; the medulla and pons, which comprise the hindbrain; and the midbrain. This cobination of spinal cord, hindbrain, and midbrain MacLean calls the neural chassis. It contains the basic neural machinery for reproduction and self-preservation, . . . MacLean has distinguished three sorts of [more recent brain structures controlling] the neural chassis. The most ancient of them surround the midbrain. . . . We share it with the other mammals and the reptiles. It probably evolved several hundred million years ago. MacLean calls it the reptilian or R-complex. Surrounding the R-complex is the limbic system, so called because it borders on the underlying brain. (Our arms and legs are called limbs because they are peripheral to the rest of the body.) We share the limbic system with other mammals, but not, in its full elaboration, with the reptiles. It probably evolved more than one hundred fifty million years ago. Finally, surrounding the rest of the brain and clearly the most recent evolutionary accretion, is the neocortex.