Pupil Contraction and Dilation
On the left (a) the pupil is wide open (dilated), while on the right (b) the pupil is contracted. The iris regulates the opening and closing of the pupil and it has two sets of fibers, straight and circular. When the pupil is open the circular fibers of the iris are relaxed while the straight ones are open. When the pupil is contracted, the straight fibers of the iris are relaxed while the circular ones are contracted.