Rectus capitis lateralis muscle revisited: a histological study using human fetuses.

Background: Rectus capitis lateralis muscle (RCLM) is located at the border between the ventral and dorsal muscle groups, but the nerve topographical anatomy around the muscle is obscure.

Materials and methods: We observed the RCLM in histological sections of 12 midterm and 10 near-term fetal heads (9-18 and 26-40 weeks of gestational age).

Results: At midterm, the RCLM wrapped around the inferiorly protruding inferolateral corner of the cartilaginous occipital bone. The muscle was adjacent to, or even continued to, the intertransversarius muscle between the atlas and axis. At near-term, the jugular process of the occipital bone, that is, the RCLM upper insertion, was either cartilaginous or bony, depending on age. The process formed a collar supporting the internal jugular vein from the inferior side. Moreover, the muscle is tightly attached to or inserted into the venous wall itself. The cartilaginous jugular process was adjacent to Reichert's cartilage, and the uppermost muscle fibers passed through a narrow space between these cartilages. The RCLM appeared to accelerate the jugular process elongation, resulting in complete union of the occipital and temporal bones. The ventral ramus of the first cervical nerve passed between the RCLM and rectus capitis anterior muscle to reach the longus capitis muscle. No nerve passed between the RCLM and the obliquus capitis superior muscle (a muscle at the suboccipital triangle).

Conclusion: The dorsoventral position of the RCLM seemed to correspond to the scalenus posterior muscle in a laminar arrangement of the cervical axial musculature.