What are the physical paths towards consciousness? How do humans transition out of deep anesthesia, deep sleep, or traumatic brain injury? This book presents a new argument that expands past theories centered on the cerebral cortex, and instead emphasizes the longitudinally-integrated brainstem systems that are essential to the mechanism of consciousness. The workings of these vertical pathways that 'wake up the brain' are examined in neurobiological and molecular detail. Mirroring the evolution of this system from fish to humans, chapters in the book move from hindbrain to forebrain and from animal brain to human brain, developing the unified approach involved in the brain arousal mechanism. Considering consciousness through an array of neuronal structures, this book provides a new physical explanation of the phenomenon. Written for neurologists, neuroscientists, psychologists and psychiatrists, the book's succinct and readable tone means it is also suitable for readers interested in the workings of the brain.