The pupillary mechanism of the teleostean fish U ranoscopus scaber

Abstract

The old opinion that the pupil of telsostean fish never varies in size was disproved by the work of Brown-séquard (1847), who showed that in certain species (eels, flat fish) there was a very considerable light reflex. Steinach (189o and 1892) showed that there were in fact changes in the size of the pupil in all fish, although in many cases the variations were very slight and only detectable by careful measurement. Steinach also showed that in Teleosts and Amphibia the incidence of a bright light caused narrowing of the pupil even after the eye had been removed from the head, and he concluded that in animals the movements of the iris were produced by the direct effect of the incidence of the light on the pigmented muscles cells of the sphincter. The opinion has persisted to the present day (Kuntz, 1929), in spite of the discovery Magnus (1899) of the fact that the curve of tire pupil reaction in respect to spectral light resembles the absorption curve of the visual purple of the retina and not that of the pigment in the iris. Magnus therefore concluded that some intra-ocular nervous reflex was involved in closure of the pupil. However, Langley and Orbeli (1910) showed that in the frog stimulation of the sympathetic chain caused dilatation of the pupil, so that in the Amphibia at any rate there is certainly an extra-ocular nervous mechanism responsible for changes in the size of the pupil, in addition to an intra-ocular mechanism (whether nervous or not), responsible for the closure of the pupil of the isolated eye. Apparently no effort has been made to discover whether there is an extra-ocular mechanism in fish.