Abstract
The fine structure of the sphincter and dilator of the pupil of the mouse was studied by light and electron microscopy. The sphincter of the pupil is composed of circumferentially-oriented smooth muscle cells. The dilator of the pupil consists of a large number of myoepithelial cells and a small number of completely-differentiated smooth muscle cells containing no melanosomes. Gap junctions and desmosomes are often observed among the muscle cells of both the sphincter and dilator.
