The author performed comparative histological observations on the “chiai” (M. superficialis trunci) and the lateral muscle of 15 species of fish. In cross section “chiai” muscle fibres are smaller in size than the lateral muscle. Their shapes are round or oval, while the latter angled. The lateral muscle fibres have a great amount of cylindric, or band-shaped myofibrils within themselves and their arrangement pertains to LANSIMAKI's category. Therefore, they contain a smaller amount of sarcoplasm among their myofibrils. The nuclei are always situated just beneath the sarcolemma. The “chiai” muscle fibres, on the other hand, contain a bundle of cylindric, prismatic or short bandshaped myofibrils in transverse section which wind and fuse to form a wreath or network. Based on their shape, they are designated by the author as muscle fibres of serpentine, garland-like, reticular and annular type. Consequently, the sarcoplasm is abundant among the myofibrils. Moreover there is a greater mass of sarcoplasm (sarcoplasm mantle) between myofibrils and sarcolemma. The nuclei are usually present within this layer.
In several fish (Carassius auratus, Mugil cephalus, Lateolabrax japonicus, Sparus swinhonis, Platycephalus indicis) the author observed transitional types of muscle fibres between the “chiai” and the lateral muscle. Muscle fibres of this layer show histologically transitional figures from the “chiai” to the lateral muscle, and their size are intermediate between the two.
The author compared muscle fibres of fish that move with alacrity with those that do not. And it seems to the author that both the “chiai” and lateral muscle fibres of the former show a tendency to be smaller in size than the latter. The “chiai” muscle fibres in the former possess a greater mass of fine myofibrils and sarcoplasm than in the latter. The perimysium internum also develops remarkably in the “chiai” of the former.
The connective tissues in the “chiai” are generally present in much more abundance than in the lateral. The capillaries of the “chiai” are also very numerous and they run in the endomysium.
