In a nutshell
Thomas Lewis pioneered the development of the electrocardiograph, sometimes even using himself as a guinea pig.
Thomas Lewis was a medical scientist and a heart specialist, occasionally consulted by the royal family. His interest in medicine was initially sparked by two family doctors who did conjuring tricks to entertain him during their visits to his house. He was convinced that if he did medicine he'd be able to compete with them in conjuring.
His life's work was studying the operations of the human heart and the associated features of the system of blood flow in the body. At University College Hospital London he had a ward and a lab, both essential for studying illness.
Often he used himself as a test case for his experimental trials. We don't know whether this contributed to the three heart attacks he suffered, the third of which killed him. His hobbies were mostly to do with the great outdoors, including gardening, walking, bird-watching and bird-sketching.
The human heart is a four-chamber muscle pump that maintains a daily flow of approximately 2,000 gallons (9,000 litres) of blood around the body. Clean, oxygenated blood goes out and impure, de-oxygenated blood comes back and so the cycle goes.
Doctors get some crude indication of the state of the heart's condition by measuring the pulse and using a stethoscope. For a more detailed picture which can help decide treatment, including surgery, you need more complex techniques.
Thomas Lewis carried out important work on developing the early electrocardiograph. The basis of this was that contraction of muscle is associated with electrical properties - known for centuries through the work of Galvani, who investigated movement of the legs of frogs.