Fig. 18. Electron micrograph of a multiply innervated, tonic fiber. The Z line is zigzag and irregular; very little sarcoplasmic reticulum (SR) or T system (T) is evident. Inset shows irregular attachment of thin filaments to the Z line. (Mukuno K: The fine structure of the human extraocular muscle: II. The classification of muscle fibers. Jpn J Ophthalmol 12:111, 1968.)