Atropine

A poisonous alkaloid extracted from atropa-belladonna, or deadly night-shade; it temporarily paralyzes the sphincteriridis (circular muscles of the iris) and the ciliary muscles within the eye, consequently the pupil is dilated and accommodation fully relaxed, leaving the eye adjusted for its far pointof vision in a state of rest. For this reason itis known as a mydriatic and a cycloplegic, and is used more than any other to suspend accommodation and dilate the pupil. Atropine paralyzes the sphincter muscle of the iris and theciliary muscle, and hence results in dilatation of the pupil, and also an inability to see clearly near by.

Source: Ophthalmic Dictionary including pronunciation, derivation and definition of the words used in Optometry and Ophthalmology by James J. Lewis, Oph. D. Professor of Optometry in the Northern Illinois College of Ophthalmology and Otology, Chicago.