Judging from his writings, Andreas Vesalius must have had dozens of bodies at his disposal, thirteen of which were definitely from before 1543. They came from cemeteries, places of execution or hospitals. Not only did his students help him obtain the bodies, but also public and judicial authorities. At first, he used the corpses for his own learning purposes, and later to teach his students and to write De humani corporis fabrica, his principal work. Clearly he had an eye for comparative anatomy. He observed anatomical variants and studied foetal anatomy. Occasionally, he would dissect a body to study physiological processes, while the post-mortems on the bodies brought in by the families of the deceased gave him an insight into human pathology. Some of his dissection reports have been preserved.