Blood (Symbol)
The speaker symbolizes her own sexual desire through blood, and in doing so links that desire to the bodily, the irrational, the vital, and the animalistic. The word "blood" also carries a slight connotation of harm or violence, suggesting that the speaker feels her desire for the addressee to be slightly dangerous. The language with which this symbolic blood is described personifies it and intensifies these connotations. It is described as "treasonous," again hinting at unpredictability and danger. The word "stout," meanwhile, connotes an unglamorous healthiness, again hinting at the liveliness and strength of the speaker's lust. In total, the use of blood as a symbol helps to depict sex as an expression of animal instinct.
Brain (Symbol)
In contrast to the use of blood as a symbol of desire, Millay uses the brain as a symbol of rational decision-making. Thus, her blood's behavior is "treasonous" against the brain, causing the speaker to feel desire against her better judgment. However, the specific choice of the brain as a symbol allows Millay to portray rationality as somewhat powerless in the face of instinct. It is the brain, not the mind, that she mentions; the brain remains a body part, such that the animal body remains the realm in which this conflict as a whole takes place. Moreover, Millay personifies the brain as well as the blood with the word "staggering," implying drunkenness, frailty, or fear. In the end, the brain's rationality cannot completely overcome the speaker's instincts.
