Later anatomists knew the function of the tricuspid valve after reading the Hippocratic Corpus.In the 4th century BCE, Aristotle and several contemporaries produced a more empirically founded system, based animal dissection. Around this time, Praxagoras is credited as the first to identify the difference between arteries and veins, and the relations between organs are described more accurately than in previous works. The first use of human cadavers for anatomical research occurred later in the 4th century BCE when Herophilos and Erasistratus gained permission to perform live dissections, or vivisection, on criminals in Alexandria under the auspices of the Ptolemaic dynasty. Herophilos in particular developed a body of anatomical knowledge much more informed by the actual structure of the human body than previous works had been.GalenThe final major anatomist of ancient times was Galen, active in the 2nd century. He compiled much of the knowledge obtained by previous writers, and furthered the inquiry into the function of organs by performing vivisection on animals. Due to a lack of readily available human specimens, discoveries through animal dissection were broadly applied to human anatomy as well. His collection of drawings, based mostly on dog anatomy, became the anatomy textbook for 1500 years. The original text is long gone, and his work was only known to the Renaissance doctors through the careful custody of Arabic medicine.