Commentary Inf XII 127-132

As he crosses toward the far side of the river (the one nearer to the center of hell) Nessus looks back over the area they have traversed (to their right), where the river grew increasingly shallow, and then looks left, where, he knows, the river bed gradually deepens until it reaches its lowest point, which coincides with the first place Dante saw, that in which the tyrants are punished. This information suggests that Dante and Virgil have traversed a semi-circle in order to reach this shallowest point, where they have forded the river. The unexplored run of the river thus also occupies 180^o of the circle.