Sherlock who

John dropped to his knees and placed his index and middle finger against the side of Sherlock's neck, checking for a pulse. Thankfully, he was still breathing; the steady beat came strongly through the vein. But hang on—there was something wrong. He kept his fingers there and was soon able to isolate the abnormality: instead of a single, straightforward beat, there were two, one after the other. Frowning, he pulled his hand away. The rhythm was unlike any cardiovascular irregularity he'd ever encountered.Hesitantly, he leaned down, pressing his ear against the man's chest. He could hear the unmistakable sound of not one but two pulses, thumping out a steady four-beat rhythm.  He has two hearts.