The essential change found in all cases of primary glaucoma is the blocking of the angle of the anterior chamber owing to the root of the iris being applied to the back of the cornea, and thus preventing the filtration of the fluid into the canal of Schlemm, as a result of which the tension of the eye is raised, either acutely (acute glaucoma) or slowly from time to time (chronic glaucoma) (Fig. 111). The aim of every operation for the permanent relief of glaucoma is the opening up of Schlemm’s canal at the angle of the anterior chamber or the creation of a new lymph channel between the anterior chamber and the subconjunctival tissue (filtrating cicatrix). Although this latter condition is not unattended by the risk of the spread of inflammation from the conjunctiva to the interior of the globe, it is not an inadvisable condition to obtain in some cases of chronic glaucoma if the scar be small and free from iris tissue; in this disease the opening up of the canal of Schlemm by iridectomy is often impossible. (See Sclerectomy, p. 231.) =Indications.= Since the days of von Graefe, who first performed iridectomy empirically for the relief of glaucoma, the operation has held the first place in its treatment. (i) =In primary glaucoma.= Iridectomy should be undertaken as early as possible in the disease. _In acute cases_, unless the tension is relieved, the disease ends in rapid destruction of the sight. Operation should always be undertaken as quickly as possible, provided the patient has not lost his perception of light for longer than about ten days. [Illustration: FIG. 110. THE NORMAL ANGLE OF THE ANTERIOR CHAMBER. A. Cornea. B. Ciliary processes. C. Iris. D. Ciliary muscle. E. Pectinate ligament, to the right of which is the angle of the chamber. F. Canal of Schlemm. G. Lens. H. Posterior chamber. I. Anterior chamber. ] Whilst waiting for the operation, the pupil should be put under the influence of eserine (2 to 4 grains to the oz.) with the idea of reducing the tension by contraction of the pupil. Some surgeons, in addition to using eserine, perform a posterior scleral puncture with the idea of temporarily reducing the tension and allowing the acute symptoms to subside, and do the iridectomy some twenty-four to forty-eight hours later. This method is extremely useful (_a_) in cases where a general anæsthetic is inadvisable, since the reduction of tension allows cocaine to diffuse into the eye; (_b_) in cases liable to subsequent intra-ocular hæmorrhage, a more gradual reduction of tension being obtained, rupture of a choroidal vessel is less likely to occur; (_c_) a deeper anterior chamber is often obtained, and hence there is less risk of wounding the lens during the operation; (_d_) in cases where the operation has been performed in one eye and the lens has been subsequently extruded on the dressings.