The iris is a very vascular body which lies in a closed cavity occupied by the aqueous humour. The cavity is lined by a serous membrane, and the diseases are like those of serous cavities.

Inflammation occurs in the iris as a result of traumatic, rheumatic, or still more frequently of syphilitic irritation. The inflammation is usually acute, but its severity varies. Different forms have been described according as the exudation is serous, fibrinous, or purulent. Descemet's membrane, which covers the posterior surface of the cornea, takes part in the inflammation. Serous iritis is the mildest form, and leads to distension of the anterior chamber by the accumulated fluid. This fluid contains leucocytes, which occasionally render it somewhat turbid. Fibrinous iritis (also called Plastic iritis) is more severe and is accompanied by a fibrinous exudation, which appears most commonly at the edge of the pupil, but also presents itself on the posterior surface, and sometimes on the anterior. The exudation fixes the iris by forming a membrane in the pupil, or more commonly by gluing it to the lens. This adhesion to the lens (called Synechia) is a serious part of the lesion, as it is apt to become permanent. As the inflammation progresses the fibrinous exudation becomes replaced by the formative cells of granulation tissue, and new-formed blood-vessels pass into it. The regular development into spindle-celled tissue and connective tissue occurs, and, just as in the case of the pleura, permanent adhesions are the consequence. Sometimes the new-formed membrane fills the pupil and obscures the vision. The iris itself is the seat of new-formation of connective tissue, which may cause considerable atrophy of its proper tissue. On the other hand, hypertrophy of the iris sometimes results. Purulent iritis is more severe. It causes an opening out and softening of the tissue of the iris, while the anterior chamber is filled with pus.

Inflammations of the iris are liable to extend to the ciliary body and choroid (Irido-choroiditis).

Syphilis has already been mentioned as a cause of iritis. It is so in the secondary stage, producing an acute inflammation usually of the fibrinous kind. A formation of gummata is an exceedingly rare occurrence.

Tuberculosis of the iris is also rare, but the condition called Granuloma is really a tuberculosis. There is inflammation with development of tubercles and destruction of the eyeball.