(Chapter 6) Footnote 41:

The reference is to Plato’s contrast between the rational, spirited and appetitive parts of the soul. Here, the “two desirous spirits” are Plato’s appetitive (to\ )epiqumhtiko/n)and spirited (to\ qumoeide/j) principles, lodged below the head with the neck serving as an isthmus or boundary separating the desires from the intellect. These are distinguished in Timaeus 70, where the spirited desires are placed above the midriff and the appetitive are situated between the midriff and the navel. See Galen De placitis Hippocratis et Platonis “Plato placed the rational part of the soul in the head, the spirited in the heart, and the appetitive around the navel.”