_Fourth step._ With the knife-point again resting on the membrane, a second puncture is made and the incision is carried rapidly forward by the sawing movement to meet the extremity of the first incision at the apex of the triangle, thus making a V-shaped cut. Care must be taken that the pressure of the knife-edge on the tissue shall be most gentle, and that the second incision shall terminate a trifle inside the extremity of the first, in order that the last fibres may be severed and thus allow the apex of the flap to fall down behind the lower part of the iris membrane (Fig. 106). When the operation has been completed the knife is turned on the flat and withdrawn. IRIDECTOMY The operation of iridectomy differs widely in its performance, according to the different conditions for which it is used. Hence it is better to prefix the condition for which it is employed, thus: preliminary iridectomy, optical iridectomy, glaucoma iridectomy. [Illustration: FIG. 105. IRIDOTOMY BY ZIEGLER’S METHOD. Showing the first incision and the position of the second.] [Illustration: FIG. 106. IRIDOTOMY BY ZIEGLER’S METHOD. Final step; the triangular flap of iris attached at its base is turned downwards.] Apart from being one of the stages of removal of a cataract, already described, it is performed as an independent operation in the following conditions:-- 1. For optical purposes (optical iridectomy). 2. For the relief of glaucoma, primary and secondary (glaucoma iridectomy). 3. For small growths at the free margin of the iris. 4. For prolapse of the iris through a wound. OPTICAL IRIDECTOMY