But as they advanced flushed with victory, Caesar's fourth line, which he had held in reserve, and which was made up of the flower of his legions, appeared in their way.

So fierce was their attack that the Pompeians wavered, turned, and fled.

They never rallied.

The fourth line threw themselves upon Pompey's left wing, which was now unprotected.

This wing, composed of Caesar's old veterans, was probably in no mood to fight its former comrades in arms.