MYOFIBROUS ORGANIZATION OF THE THYROHYOID MUSCLE OF FUR-SEAL

Myofibrous organization of the thyrohyoid (TH) muscle from nine female fur-seals was analyzed and compared with that of other vertebrates including man. Hematoxylin eosin (HE) stained sections and Sudan Black B (SBB) stained sections were analyzed. Results: 1. Myofibrous organization of HE stained sections of seal TH muscle tended to be similar to the human mylohyoid muscle. This suggests that the TH muscle of the seals is well developed. 2. The cross sectional area of the muscle belly of seal TH muscle was larger and there were more muscle fibers than in any SBB stained suprahyoid muscles of the crab-eating macaques compared. The thickness and number of muscle fibers in a 1 mm² block were similar to those in the mylohyoid and the anterior belly of the digastric, which were the largest of the macaque suprahyoid muscles. The density was greater than any of the macaque muscles compared. The proportion of white muscle fiber was greater in the TH than in any of the muscles compared. The thickness of white muscle fiber was similar to that of the mylohyoid and anterior belly of the digastric, and was remarkably larger than those of any other muscles compared. 3. The following results were obtained from comparison with the constrictor pharyngis inferior (CPI) muscle. Although the cress sectional area of the TH muscle belly was smaller than that of the CPI muscle of the fur-seals from the same sample stock, the density of TH muscle was greater than that of the CPI muscle. The three fiber types in TH muscles occurred with frequency similar to those in the seal CPI muscles. The thickness of each fiber type in TH muscles was larger than those in CPI muscles. The thickness of white muscle fiber of seal TH muscles was larger than those of human CPI muscles and the density of seal TH muscle was greater than that of the human CPI muscles. 4. Analysis of the ages of the samples revealed that the seals were in the developing stage (during their first 3 years) by the number and size of the muscle fibers.