In this early study, written in the form of a letter to his friend and mentor Imperial Physician, Nicolaus Florenas, who had encouraged him to study medicine, Vesalius reported his study of the venous system of the human body, motivated by the need to determine where to bleed in the treatment of disease. At this time venesection was, of course, a mainstream therapy. Translated into English by John B. de C. M. Saunders and Charles Donald O'Malley as The bloodletting letter of 1539. An annotated translation and study of the evolution of Vesalius's scientific development (New York: H. Schuman, [1947]). Digital facsimile of the 1539 edition from Google Books at this link, of the English translation from the Hathi Trust at this link.