Within the depth-psychology and somatic-therapy corpus, the parasympathetic nervous system occupies a position far exceeding its classical physiological definition as the ‘rest-and-digest’ counterpart to sympathetic arousal. The literature treats it as a layered, evolutionarily stratified system whose internal divisions carry distinct psychological and relational implications. The pivotal contribution is Porges’s Polyvagal Theory, which fractures the parasympathetic branch into two phylogenetically distinct pathways: the ancient dorsal vagal circuit, mediating immobilization, collapse, and dissociation; and the evolutionarily recent ventral vagal circuit, underwriting social engagement, safety, and co-regulation. This bifurcation has profound consequences for trauma theory, requiring clinicians to distinguish between parasympathetic calm and parasympathetic shutdown — states phenomenologically opposite yet sharing the same branch. Schore’s developmental neurobiology adds a temporal dimension, tracing how sympathetic and parasympathetic systems mature on different schedules in infancy and how their imbalance underwrites divergent psychopathological trajectories. Heller situates the parasympathetic as the homeostatic restorer that modulates sympathetic arousal, while Craig maps its afferent re-representation in the insular cortex as the substrate of interoceptive self-awareness. Across all these voices, the parasympathetic is not simply inhibitory machinery but the neurobiological ground of relational safety, affect regulation, and embodied well-being.