Why were Vesalius's writings about human anatomy far more accurate than earlier writings on the same topic?

Andreas Vesalius did animal dissections himself and encouraged his students at the University of Padua to the same hands-on work. He then also pursued dissection of human cadavers which had not been done before.

His 1543 book, De Humani Corporis Fabrica ("On the Fabric of the Human Body"), was based on his work with human dissection.