Fig. 21. Normal outflow system. A much broader appreciation of limbal anatomy is necessary for deep sclerectomy procedures than for standard trabeculectomy. To understand how nonpenetrating filtration surgery works, a review of outflow anatomy is essential. Normal outflow resistance is approximately distributed in the following fashion: 60% in the trabecular meshwork (TM) and juxtacanalicular (JC) tissue, 25% in the canal and immediate collector systems, and 15% in the scleral venous collector system (VP). The TM is 750 μ in meridional width, Schlemm's canal (SC) is 36 mm in circumference with an average width of 300 μ, and roughly 30 collector channels connect the canal to the venous plexus (VP). Approximately 6 to 8 aqueous veins (AV) exit Schlemm's canal and bypass the collector channels directly connecting into the venous plexus.