In the space between the ciliary circle and the ciliary process the larger circumference of the iris is fixed. This is a muscular membrane, according to some writers, and vascular according to others, forming a vertical partition behind the cornea. The iris is pierced in the middle by a circular opening called the pupil. It represents exactly what is called a diaphragm in optical instruments. Its anterior surface is coloured in different shades according to the individual, but always remarkable for their delicacy or their intensity; the variety of which has given to the membrane the name of iris, or the rainbow. Its posterior face is covered with a layer of pigment, which is called the uvea.
It is well known that the pupil dilates in the dark, and contracts on the contrary in a bright light, only allowing that quantity of luminous rays to enter the eye which is necessary to vision. Certain substances also when taken into the system act similarly on the iris; such are opium and the Calabar bean, which cause the pupil to contract; belladonna, on the contrary, dilates it Changes in the diameter of the pupillary opening also result from certain affections of the eye and brain. Physiologists consider the dilatation and contraction of the pupil as belonging to muscular movements; in fact the microscope demonstrates the existence of muscular fibres in the iris; it contracts also under the influence of electricity.
It has been remarked that the posterior surface of the iris, the ciliary processes, and the choroid were covered with a layer of pigment. This name is also given to a dark brown substance, appearing black in a mass, which colours certain portions of the skin in the white man, and the whole tegument of the negro. In the eye this pigment plays the same part as the lamp-black in the interior of certain optical instruments, as the telescope and magic-lantern; it absorbs the luminous rays, and prevents them from being reflected, which would confuse the vision.