The limbic system (or Paleomammalian brain) is a set of brain structures including the hippocampus, amygdala, anterior thalamic nuclei, septum, limbic cortex and fornix, which seemingly supports a variety of functions including emotion, behavior, long term memory, and olfaction. The limbic system is the set of brain structures that forms the inner border of the cortex. It operates by influencing the endocrine system and the autonomic nervous system. It is highly interconnected with the nucleus accumbens, the brain's pleasure center, which plays a role in sexual arousal and the "high" derived from certain recreational drugs. These responses are heavily modulated by dopaminergic projections from the limbic system. It is also tightly connected to the prefrontal cortex. Some scientists contend that this connection is related to the pleasure obtained from solving problems. Its putative role in emotion was developed only in 1937 when the American physician James Papez described his anatomical model of emotion.