Within the depth-psychology corpus centred on Polyvagal Theory, ‘vagal’ functions as the master adjective organizing an entire neurophysiological grammar of emotion, safety, and social relatedness. Porges establishes the foundational architecture: the vagus nerve is not a unitary structure but a phylogenetically stratified system whose two principal branches — the dorsal vagal (unmyelinated, reptilian) and the ventral vagal (myelinated, mammalian) — instantiate qualitatively distinct modes of organismic being. The dorsal vagal pathway mediates immobilization, metabolic conservation, and dissociative collapse; the ventral vagal pathway underwrites social engagement, co-regulation, and the capacity for felt safety. Between these poles operates the ‘vagal brake,’ a rapid-modulation mechanism by which myelinated vagal tone to the sinoatrial node is transiently withdrawn or restored, allowing flexible mobilization without full sympathetic recruitment. Porges insists that vagal tone, measurable via respiratory sinus arrhythmia, is not a unidimensional quantity but indexes distinct functional subsystems whose confounding has historically distorted the literature. Dana translates this neuroscience into clinical idiom: ventral vagal energy becomes a therapeutic resource, a quality of presence the clinician cultivates and transmits. The central tension across the corpus is between the precise, systems-physiological meaning of ‘vagal’ in Porges and its metaphorical extension — the ‘compassion nerve,’ the ‘smart vagus’ — in clinical-popular writing.