Polarized light microscopy of the smooth muscle anococcygeus of the rat.

1. The birefringence of the rat anococcygeus muscle has been studied in the presence of solutions which dissolve myosin and actin filaments. About 80% of the muscle anisotropy disappear after this treatment. 2. The 'form' and 'intrinsic' components of the birefringence of muscles fixed either at rest or during maximal isometric contraction were studied by inhibition with media of different refractive indices. At rest, the 'form' birefringence accounts for about 67% of the total birefringence. In contraction, the increase of total birefringence is primarily (80%) due to an increase of the 'form' component, indicating that new filamentous material appears in this condition.