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Up the throat of an F-1 engine.

At the center is the injector. Farther outward from the center is the point at which the regenerative cooling tubes which comprise the thrust chamber split in two, or bifurcate. At the lower right is the point at which the nozzle extension attaches to the thrust chamber. At this interface, the exhaust ports expel the relatively cool turbine exhaust gasses (supplied by the turbine exhaust manifold surrounding the outside of the thrust chamber) to prevent the nozzle extension from melting.

Several of the F-1s on Michoud's S-IC have a "spike" sticking out of the center of the injector plate; this is a shaft which would have supported the thrust chamber throat security closure.