He himself dissected chiefly apes and pigs.

His osteology was admirable, and his little tractate "De Ossibus" could, with very few changes, be used today by a hygiene class as a manual.

His description of the muscles and of the organs is very full, covering, of course, many sins of omission and of commission, but it was the culmination of the study of the subject by Greek physicians.
His work as a physiologist was even more important, for, so far as we know, he was the first to carry out experiments on a large scale.

In the first place, he was within an ace of discovering the circulation of the blood.

You may remember that through the errors of Praxagoras and Erasistratus, the arteries were believed to contain air and got their name on that account: Galen showed by experiment that the arteries contain blood and not air.