An anatomical analysis of the relationships between the intercostal nerves and the thoracic and abdominal muscles in man. I. Ramification of the intercostal nerves.

Minute dissection was performed on the ventral primary rami of the first thoracic to first lumbar nerves in two halves of an adult male cadaver in order to determine points of origins of muscular branches from the intercostal nerves. Generally, the muscular branches originated independently, but the branches supplying the intercostalis intimus and internus tended to originate as a common trunk. The muscular branching patterns were compiled in an illustration, and based on these patterns the serial homology of the thoracic and abdominal muscles was proposed. Although the obliquus externus abdominis has been thought to correspond to the intercostales externi, from these innervation findings it appears to correspond to the intercostales interni. Further, the quadratus lumborum seems homologous to the intercostales externi.