The best kept secret of the medical community is the existence of a second heart that beats in the chest of every living human. This secret is hidden in the same way that all the best kept secrets are hidden: in plain sight. For evidence of its existence, one need look no further than the second half of one’s own heartbeat. “Lub-dub. Lub-dub”. Though most are unaware of the existence of this second heart, few are surprised to learn of its existence; deep in our hearts, we all know the truth, even if we know not which heart houses it. For much of history, the Dual Heart Hypothesis was the subject of mysticism. Only fairly recently has it been accepted as medical fact. After all, the sound of one heart beating twice is indistinguishable from the sound of two hearts beating once each. The theory was long thought to be disproven by the existence of only a single heart in all dissected cadavers. This absence is due to the incorporeal nature of the second heart; the only physical evidence of its existence is the sound of its pumping, which ceases after death.
The common name for the second heart is the “Shadow Heart”, though whether it is the shadow of one’s heart, or the heart of one’s shadow, is still a hotly contested topic among umbral anatomists. The distinction between these may seem pedantic, but it is important to recognize that these relations do not always commute. A shadow of a doubt is certainly not a doubt of a shadow, for example. The exact function of the shadow heart is also not yet fully understood. It may play a role in the regulation of our emotions, especially fear, anger, and love, according to a theory that is especially popular among scholars of the supernatural. The medical community, on the other hand, is reticent to accept that emotions are the product of something other than neurons and hormones, especially without concrete evidence. Still, it is far from universally rejected by the scientific community, especially since it provides a compelling explanation for why so many cultures independently consider the heart to be responsible for our emotional temperament.
Though there is no consensus on what the shadow heart does for the human body, there is no questioning that it does something important; a failure of the shadow heart is as fatal as a failure of the physical heart. To most physicians, this is all they need to know, as their job is to heal, not to know why their patient will die if they don’t. The discovery of the shadow heart led to both practical and theoretical breakthroughs in medicine. For example, arrhythmias that were once understood as defects of the physical heart alone can be more accurately described as a difference in phase between the beating cycles of the two hearts. Though no medical technology yet devised is capable of directly influencing the behavior of the shadow heart, doctors can treat such conditions by helping the physical heart adapt to the shadow heart’s abnormal rhythm. Mathematically, the functioning of a dual heart system can be modeled as a wavefunction, where a thump of the physical heart generates a positive value, and the thump of the shadow heart generates a negative value. Much like a quantum mechanical wavefunction, it is not precisely known what this wavefunction represents, only that it works. In a healthy heart, the phase difference between the two produces a sort of constructive interference, allowing both hearts to better serve their respective purposes. This model gives rise to a theory explaining death of old age. While the physical heart is made of flesh, which tires and atrophies over the years, the shadow heart is unbound by such limitations. As the physical heart slows, the shadow heart’s cadence remains steady, until it catches up. When the two hearts beat simultaneously, destructive interference occurs. The two hearts cancel each other out, and the patient dies. For this reason, some occultists believe the shadow heart to be a memento mori from a cruel god: a second heartbeat to remind us of how many seconds we have left.