Science is a field based largely on theory and experimentation, which is why you have to be pretty darn certain you are right if you're willing to test your ideas on yourself. For example, in the picture above:
In 1929 in the basement of the Eberswaled Hospital in Germany, surgical resident Werner Forssmann inserted a ureteral catheter tube into his elbow, feeding it through a vein up to his heart. He used a mirror as his assistant, since he had restrained his nurse to the operating table. He then took an x-ray of his chest (at left) to determine the catheter had indeed made it to the right atrium.
Learn about more dedicated, and brave, scientists over at Mental Floss.