The dark arrow points to President
Kennedy, just before he first reacts to being hit by something in the throat.
Abraham Zapruder is filming on the pedestal in the background, as the
limousine at this point goes behind the Stemmons Freeway sign in the Z-film.
Notice the position of the "flanking" motorcycle policemen, who were
supposedly to remain in this trailing position at the request of JFK. The
president is looking towards the freeway sign(at least in the proximity of it)
when he is struck in the throat.
