Limbic system. Illustration of structures of the limbic system of the brain, with a cross-hatched pattern superimposed. Areas of the limbic system are colour-coded: reticular formation (purple); the thalamus (pink, at centre); the hypothalamus (orange); green body overlying top of hypothalamus is the mamillary body; green body below hypo- thalamus is the amygdala; green body beneath amygdala is the hippocampus; large green body at top is the caudate nucleus. The limbic system is a complex network of nerve pathways involved with expression of instinct and mood. It governs the expression of fear, rage, pleasure, reproduction, self-preservation, and the formation of memory.