The reference here is to Plato's Timaeus (69d). The Phaedrus had contained the parable of the charioteer (reason) with his two horses (the noble and obedient passions and the wild, disobedient ones). The Timaeus goes on to ascribe the rational part of the soul to the head, the faculty of courage and anger to the part of the body near the heart, and desire to the lower part of the body.
This was the doctrine, popularized by Cicero, which Erasmus quoted from Plato in the Enchiridion Militis Christiani. Plato himself, who regarded ethical activity as determined by rational judgement, ascribed desire to the liver, and by widening the 'lower part of the body' to include the sexual organs, Erasmus retained plausibility for Plato's ascription of the soul's faculties to the three regions of the body. It is only under the influence of St Augustine that the later Renaissance authors will reverse Plato's doctrine and put love into the heart.