Commentary Inf XII 73-75

More numerous than the classical centaurs (see C.Inf.XII.56-57), Dante's number in the thousands, firing their arrows at any who rise more out of the river of blood than their guilt allows. We come to understand that degree of sinfulness controls the depth of immersion: sinners are variously swathed in blood up to their eyebrows (v. 103: murderous tyrants); throats (116: murderers); waists (121: plunderers); feet (125: unspecified).

Some commentators believe that Dante's conception of this river of blood was influenced by his experience as a cavalryman at the battle of Campaldino in 1289, when the slaughter made the rivulets crossing the terrain run with blood. For a description and analysis of that battle see Oert.1968.1.