Ancient physician Herophilus is considered the father of anatomy. And while he made significant discoveries during his practice, it's how he learned about internal workings of the human body that lands him on this list.
Herophilus practiced medicine in Alexandria, Egypt, and during the reign of the first two Ptolemaio Pharoahs was allowed, at least for about 30 to 40 years, to dissect human bodies, which he did, publicly, along with contemporary Greek physician and anatomist Erasistratus. Under Ptolemy I and Ptolemy II, criminals could be sentenced to dissection and vivisection as punishment, and it's said the father of anatomy not only dissected the dead but also performed vivisection on an estimated 600 living prisoners [source: Elhadi].
Herophilus made great strides in the study of human anatomy — especially the brain, eyes, liver, circulatory system, nervous system and reproductive system, during a time in history when dissecting human cadavers was considered an act of desecration of the body (there were no autopsies conducted on the dead, although mummification was popular in Egypt at the time). And, like today, performing vivisection on living bodies was considered butchery.