Iliocostalis (IC) muscles were studied in four dolphin embryos, three goat embryos and four Japanese adult cadavers through macroscopic dissection. The IC muscles of the dolphin were located on the lateral aspect of the trunk and displayed an intercostal arrangement. In contrast, the IC muscles in both the goat and human showed a double-layered architecture formed by a multisegmental muscle-tendon complex and were located on the lateral and medial sides of the costal angle, respectively. Generally, the nerve to the iliocostalis (NIC) in the dolphin and goat did not form a common trunk with the nerve to the longissimus on the epaxial plane, whereas in humans the NIC ran parallel to the nerve to the longissimus part of the way. The individual NIC ran caudolaterally, innervating the one lower (caudal) metameric division of the IC muscle in the dolphin and piercing the fascia of the IC muscles at a point in the next caudal intercostal level in the goat and human. In the upper thoracic part of the goat and human, the caudal shift of innervation was obscured, where the IC muscles were close to the vertebrae. The course of the NIC was closely related to that of the lateral cutaneous branch. The present study shows that the NIC is commonly destined for the one lower intercostal level among the three mammalian species, with their respective IC muscles having distinctly different structural complexity.

Detailed studies on intramuscular innervation of human thoracic iliocostalis (IC) muscles were done through macroscopic dissection of six sides of three Japanese adult cadavers. Human IC muscles possessed a segmental nature in that muscular segments sequentially originated from individual ribs, although their structure was complex, with many long sequentially overlapping multisegmental bundles and their diverse multilayered insertions. Nerves to IC muscles (NICs) arose metamerically from the spinal nerve at every thoracic axial level. Nerves to IC muscles ran caudally, and their entry points into the fascia of IC muscles were shifted inferiorly by one segment level compared with their origin. Each NIC supplied a few muscular segments that were defined by their costal origin, but the distribution boundary often did not match the muscular segmental boundary. At the distribution boundaries of individual NICs, the muscular layers were often innervated by two NICs from adjacent levels. The level of costal origin of the IC muscular segment was lower than that of the segmental origin of the NIC, with the difference in level, which is one at the higher thoracic region, increasing caudally up to three on average at the lower thoracic region. It is thus noteworthy that while the IC muscles and NICs were both segmental in nature, their segmentations exhibited level discrepancies, which were not coordinated with each other due to their indistinct boundaries as well as the inconsistent differences in level.