Distribution of contractile forces in the iris of cats and dogs

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Abstract
By means of a newly devised technique, the irides of cats and dogs were dissected into segments usually containing only one of the three alleged intraocular muscles. Slits centered in the segments were oriented sometimes radially and sometimes circumferentially. Isometric tensions developed by muscle fibers parallel to the central slits were measured. Graded concentrations of autonomic drugs or graded intensities of electrical current stimulated responses in the sphincter pupillae and in the ciliary muscles. The dose-response curve had an exponential form, reaching an asymptote at maximal response. The strength of the sphincter was markedly greater circumferentially than radially. The ciliary muscles contracted equally and moderately in both directions tested. On the other hand, the ‘dilator pupillae’ never developed tension in either direction to any stimulus. The distribution of contractile forces were, therefore, consistent with the arrangement and density of histologically identifiable muscle fibers, the ‘dilator’ not resembling muscle in either appearance or behavior.

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