The structure of the femoral muscle of the cockroach, Leucophaea maderae, was investigated by light and electron microscopy. The several hundred fibers of either the extensor or flexor muscle are 20 to 40 µ in diameter in transverse sections and are subdivided into closely packed myofibrils. In glutaraldehyde-fixed and epoxy resin-embedded material of stretched fibers, the A band is about 4.5 µ long, the thin filaments are about 2.3 µ in length, the H zone and I band vary with the amount of stretch, and the M band is absent. The transverse sections of the filaments reveal in the area of a single overlap of thick and thin filaments an array of 10 to 12 thin filaments encircling each thick filament; whereas, in the area of double overlap in which the thin filaments interdigitate from opposite ends of the A band, the thin filaments show a twofold increase in number. The thick filament is approximately 205 to 185 A in diameter along most of its length, but at about 0.2 µ from the end it tapers to a point. Furthermore, some well oriented, very thin transverse sections show these filaments to have electron-transparent cores. The diameter of the thin filament is about 70 A. Transverse sections exhibit the sarcolemma invaginating clearly at regular intervals into the lateral regions of the A band. Three distinct types of mitochondria are associated with the muscle: an oval, an elongate, and a type with three processes. It is evident, in this muscle, that the sliding filament hypothesis is valid, and that perhaps the function of the extra thin filaments is to increase the tensile strength of the fiber and to create additional reactive sites between the thick and thin filaments. These sites are probably required for the functioning of the long sarcomeres.

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