Under conditions of normal life the canal of Schlemm contains a colorless fluid. Various experimental procedures cause gonioscopically visible entrance of blood into the canal. Decrease in ocular tension and rises in venous pressure appear to be the outstanding factors that bring on the filling phenomenon. If it has been produced by induction of ocular hypotony, compression of the globe will cause the filling phenomenon to disappear promptly.
Reference: Kronfield KC. Further gonioscopic studies on the canal of Schlemm. Arch Ophthalmol. 1949;41:393-403.
A look at the past . . .. Arch Ophthalmol. 1999;117(11):1495. doi:10.1001/archopht.117.11.1495