The reptilian brain is the complex found in mammal and bird brains that is analogous with the brain of a reptile. In humans, the reptile brain, or R-complex, is situated inferior to the cerebrum and includes the brain stem and the cerebellum.
The comparison of this section of the brain with that of reptiles emerged in the 1960s as the triune model of brain development. This model attempts to describe the evolution of the mammal brain as a series of advances that took place without erasing the brain's evolutionary history. Under this model, the modern mammalian brain is essentially a reptile brain that has had two other sections, the limbic system and the neocortex, added to it over the 250 million years since mammals emerged as a distinct lineage.
The R-complex regulates many functions that mammals and reptiles have in common. Blood pressure, eye movements and swallowing are all controlled by the R-complex. Certain basic emotions also emanate from this area in the brain. Aggression, territoriality and some voluntary movements are managed by the midbrain. Another component of the R-complex, the hypothalamus, is directly involved in sexual behavior. These actions and emotions are common to mammals, birds and reptiles, and all seem to derive from the same basic structure.