Limbic system of the brain. An illustration of the structures of the limbic system of the brain, the complex network of nerve pathways involved with expression of human instinct and mood. Regions are colour-coded as follows: the reticular formation (purple); the thalamus (pink); the hypothalamus (orange); green body overlying top of hypothalamus is the mamillary body; yellow body below the hypo- thalamus is the amygdala; yellow body beneath the amygdala is the hippocampus; large yellow body at top, left of the thalamus, is the caudate nucleus. Activities governed by the limbic system include self-preservation, reproduction, expression of fear, rage, pleasure, and the formation of memory.