A segmental surface
A dermatome is the area of skin whose sensory supply arrives through a single spinal nerve root. Because the embryo develops in segments, the adult body surface retains a segmental organisation: regular bands across the trunk, and elongated, oblique strips along the limbs.
In the thorax the pattern is at its clearest. Each band lies roughly at the level of its own rib, giving reliable surface references such as the nipple line at T4 and the umbilical line at T10.
Overlap is the rule
Adjacent dermatomes overlap substantially. Published maps differ from one another because they were derived using different methods, and each is a schematic summary rather than a precise border.
For a surface concept this matters: fields should be defined by anatomical landmarks and generous margins, not by hairline boundaries copied from a diagram.
Why SIRIAM uses the segmental map
Organising formulations by region rather than by symptom keeps the vocabulary anatomical and verifiable. It also makes protocols reproducible: the same landmarks produce the same field in a different room, with a different practitioner, months later.