Abstract
The myofibrous organization of the seal inferior constrictor was analyzed to elucidate functional characteristic of the pharyngeal muscle. Subjects were 10 female seals (average 7.0 years and average body weight 31.6 kg) . The pharynx with the larynx was observed macroscopically, and muscle slices of the inferior constrictor were embedded in gelatine and stained with Sudan Black B. The muscle fibers of these transverse sections were classified into three fiber types : white, intermediate and red. The following results were obtained. 1) The inferior constrictor contacts with the middle constrictor at the cranial side but they do not overlap. The muscle fibers are arranged circularly in all parts of the inferior constrictor. 2) The length of the pharyngeal muscle at the raphe pharynx was 58.7 mm and about 52 % of it con-sisted of the inferior constrictor. The average thickness of the muscle layer was 2.8 mm, which was 1.6 times that of man. The average bilateral muscle weight of the inferior constrictor was 6.5 g. 3) The area of the transverse section was 305.0 mm2. There were 718 muscle fibers per mm2, and a total of 236, 885 fibers in the transverse sectional area. The percentages of white, intermediate, and red muscle fibers were 55.7 %, 26.4 %A and 17.9 %, respectively. In all cases, the white muscle fibers exceeded 50 %. 4) The mean area of the muscle fibers consisted of 1271.7, im2 of white muscle fibers, 720.3 im2 of intermediate, and 551.3μm2 of red. In the normal distribution of the muscle fiber size, white fiber was largest and red fiber was smallest. Occasionally the size of the white muscle fiber had a double low peak pattern. 5) The average density of the muscle fibers was 76.4 % ; consisting of 54.4 % white muscle fibers, 14.3 % intermediate, and 7.7 % red. The fast constriction and strong vermicular movement of the seal inferior constrictor is thus possible because of the remarkable development of the white muscle fibers, the circular arrangement of the muscle fibers and the thickness of the muscle layer.
