Iris

The iris is the continuation of the choroid through the ciliary body, and extends down to the pupil, its free edge resting on the anterior surface of the lens. The iris is composed of a vascular and fibrous anterior portion, and a muscular and pigmented posterior portion. In consequence of its vascularity, the iris is the frequent site of inflammation. When inflamed it pours out lymph which may cause it to adhere to the lens behind, forming a posterior synechia. An anterior synechia is where, on account of a perforation of the cornea, the iris washes forward and becomes attached to the cornea in front.

The circular muscle fibres surrounding the pupil are anterior, and form the sphincter pupillae muscle; it contracts the pupil. The radiating muscular fibres, which lie posteriorly, form the dilator pupillae; it dilates the pupil. The dark pigment layer is on the posterior surface of the iris, and after an attack of iritis, as the adherent iris is torn loose from the lens, it leaves patches of pigment adhering to the anterior capsule.

The iris, as it rests at its pupillary margin on the lens, divides the space anterior to the lens into two parts. The part between the posterior surface of the iris and the anterior surface of the lens forms the posterior chamber. The anterior chamber lies between the anterior surface of the iris and the posterior surface (Descemet' s membrane) of the cornea. The two chambers communicate through the pupil. The anterior surface of the iris toward its periphery is of the nature of a coarse mesh-work, the spaces of which are the spaces of Fontana. They communicate with a venous or lymph canal which passes around the eye at the sclerocorneal junction {canal of Schlemm).

Aqueous Humor And Anterior Lymph Circulation

The aqueous humor is of the nature of lymph. It is secreted by the ciliary processes and posterior surface of the iris. It passes through the pupil to the anterior chamber, and enters the spaces of Fontana to empty into the canal of Schlemm. The canal of Schlemm empties its contents into the anterior ciliary veins. In iritis and glaucoma the lymph-current is seriously interfered with. In iritis, the swelling and outpouring of lymph blocks the spaces of Fontana and prevents a free exit of the aqueous humor from the anterior chamber, therefore in this condition the anterior chamber is deep, and the iris is seen to lie far beneath the cornea.

Glaucoma

Glaucoma is a disease accompanied by increased intra-ocular tension. The eyeball feels hard to the touch. It is supposed to be due to disease of the ciliary region interfering with the canal of Schlemm and obstructing it. Therefore, the drainage of the eye and the circulation of the aqueous humor is interfered with. In iritis the anterior chamber becomes deeper, but' in glaucoma, as the intra-ocular tension increases, it pushes the lens forward, and it is seen to lie close up to the cornea; so that a shallow anterior chamber causes the ophthalmologist to suspect glaucoma and a deep anterior chamber iritis. The increased pressure within the eye pushes the optic nerve backward at its point of entrance, so that it is seen sunk below the surface of the adjoining retina, forming a distinct cup-shaped cavity or pit. This is cupping of the disk.