Book XIII examines the significance of the segmental development of the human body, including the creation of the integrated neuro-muscular-skeletal system, which in turn has practical implications for the head-neck relationship. The way in which "the head leads" is investigated: in movement (e.g., birth, Douglas’ “equipoise of the joints”) and in ontogenetic and phylogenetic development (e.g., as Dart writes, that the parts first developed are those for seeing, smelling, food-seeking and seizing, tasting, swallowing, and breathing.)
The mechanisms that allow the head to take this role of ‘leading’ are also examined. This includes how the head is stabilized and supported (e.g., spinal curves, accessory nerve innervating the sternomastoid and trapezius) and how its balance is affected (e.g., position of jaw, suspensory muscles of the larynx).
In connection to all other topics covered throughout these books, the cranial nerves and their ramifications are again presented. As is central to all Alexander Technique work, the book concludes with the psychophysical characterization of the human system, using Zahn’s discussion of learning to align the intellectual mind with the autonomic nervous system.