Fig. 8. Trabeculotomy ab externo. A. The surgeon is creating a radial incision in the bed of the partial-thickness scleral flap with a microsurgical blade. B. The dissection is carried downward layer by layer until Schlemm's canal is unroofed. C. Schlemm's canal can be positively identified by cannulation with a 6-0 nylon suture. D. Intraoperative photograph demonstrating cannulation of Schlemm's canal with a 6-0 nylon suture. Note the subconjunctival hemangioma in this eye with Sturge-Weber-associated glaucoma.