Ultrastructure of the avian iris dilator muscle

The striate structure of the dilator iris muscle of diving and non-diving ducks was studied by light and electron microscopy. The myofibrils of both muscles are highly organized and resemble the typical appearance of fast striate muscle. The larger relative size of the dilator in the diver cannot be explained on the basis of differences in accommodation but is likely due to a difference in speed and magnitude of pupil changes.