We were told it all long ago by Plato. As the king governs by his executive, so Reason in man must rule the mere appetites by means of the 'spirited element.' The head rules the belly through the chest--the seat, as Alanus tells us, of Magnanimity, of emotions organized by trained habit into stable sentiments. The Chest--Magnanimity--Sentiment--these are the indispensable liaison officers between cerebral man and visceral man. It may even be said that it is by this middle element that man is man: for by his intellect he is mere spirit and by his appetite mere animal.
C. S. Lewis, The Abolition of Man, Collier (New York: 1955) p. 34.
The footnotes are to Plato's Republic and to Alanus ab Insulis's De Planctu Naturae Prosa (the latter is certainly getting the idea from Plato's Timaeus).