The human larynx at the end of the embryonic period proper. I. The laryngeal and infrahyoid muscles and their innervation.

The larynx at stage 23 (about 27-32 mm C-R, 8 postovulatory weeks) is being studied in detail, and findings on the musculature and its innervation are presented here. Serial sections of six embryos all belonging to the one developmental stage were investigated and compared, and graphic reconstructions were made from five of the specimens. All the infrahyoid and most of the major laryngeal muscles are present, and their innervation follows closely the adult pattern. The ary-epiglottic and thyro-epiglottic apparently may or may not be identifiable. The vocalis is beginning to differentiate and is noted here in the embryonic period proper for the first time. The histological degree of development of the intrinsic laryngeal muscles is not as advanced as that of adjacent muscles in the neck, including the infrahyoid group. Variation within this one stage of development are small but definite. They include differences in the position, level, and shape of the skeletal components, and the precise position and extent of development of some of the muscles.