“…2 In 1968, using vector force analysis, Mapstone theorized that pupillary block results from the iris sphincter pulling the iris toward the lens, overcoming resistance of the dilator, and creating a ''pupil blocking force.'' 1 Huang and Barocas concurred, having used steady-state computer simulations to model the mechanical interaction between the aqueous humor and the iris. 3 However, in vivo measurement of iris muscle vector magnitude and direction is difficult, if not impossible, to obtain with traditional methods.…”