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View into the exhaust manifold inlet receiving turbine exhaust gases from the fuel turbopump.

We see that the exhaust manifold is a duct which is open to the exterior of the thrust chamber only through these relatively small, circular cut-outs. The turbine exhaust flows through these openings and enter the thrust chamber in gaps caused by the introduction of the 180 half-length, "down" tubes comprising the thrust chamber.

Although looking into the LOX turbopump exhaust manifold inlet affords a view of more of these cut-outs, from this view into the fuel turbopump exhaust manifold inlet, it appears that the cuto-outs are evenly-spaced throughout the exhaust manifold.