The iris dilator muscle, is a smooth muscle of the eye, running radially in the iris and therefore fit as a dilator. The pupillary dilator consists of a spokelike arrangement of modified contractile cells called myoepithelial cells. These cells are stimulated by the sympathetic nervous system. When stimulated, the cells contract, widening the pupil and allowing more light to enter the eye.
The pupillary dilator acts to increase the size of the pupil to allow more light to enter the eye. It works in opposition to the pupillary constrictor. Pupil dilation occurs when there is insufficient light for the normal function of the eye, and during heightened sympathetic activity, for example in the "fight-or-flight reflex".
History
Etymology
The English name dilator pupillae muscle as currently used in the list of English equivalents of the Terminologia Anatomica, the reference-work of the official anatomic nomenclature, can be considered as a corruption of the full Latin expression musculus dilatator pupillae. The full Latin expression exhibits three words that each can be traced back to Roman antiquity. The ClassicalLatin namemusculus is actually a diminutive of the Classical Latin name mus, and can be translated as little mouse. In the medical writings of Aulus Cornelius Celsus we can also find this specific name to refer to a muscle instead of its literal meaning. Latin musculus can be explained by the fact that a muscle looks like a little mouse that moves under the skin. In the writings of Greek philosopherAristotle the Ancient Greek word for mouse, i.e. μῦς is also used to refer to a muscle. Dilatator in the Latin expression musculus dilatator pupillae is derived from the classical Latin verb dilatare, to dilate, to spread out. Two possible explanations exist concerning the etymological derivation of this verb. The first explanation considers dilatare as frequentative of differere. The Latin verb differe can mean, to carry different ways, to spread abroad, to scatter, but also to delay. The other explanation considers dilatare as a compound from di- and latus, with the latter word meaning, broad or wide, hence the German nameErweiterer for Latin dilatator. The expression dilator pupillae muscle, as used in the list of English equivalents of the Terminologia Anatomica, is actually partly Latin, i.e. dilator pupillae, with pupillae, a noun in the genitive case modifying dilator, a noun in the nominative case, and partly English, i.e. muscle. In previous editions this muscle was officially called the musculus dilator pupillae, The Nomina Anatomica as authorized in 1895 in Basle and in 1935 in Jena used the full Latin expression.