The life of a military surgeon attached to the army of Charles the Fifth, which was the life that Vesalius led during the following year or two, was not sufficiently attractive to divert his mind seriously from his favorite study; and it is therefore not surprising that we find him, at the age of twenty-three, accepting from the Senate at Venice the appointment of the professorship of anatomy at the University of Padua. When he entered upon this new work Vesalius felt considerable uncertainty as to the correctness of the anatomy which he was then teaching, and it is therefore easy to understand why his first three lectures were based entirely upon the teachings of Galen; but, before he had finished the third one of the series, he made up his mind that he would cut loose from the anatomy of the ape and confine himself to that of the human subject, as was then being revealed to him more and more perfectly from his own dissections. The stock of knowledge which he had thus begun to accumulate, increased steadily until, after seven years of teaching at Padua, Bologna and Pisa, at each of which schools of medicine he gave courses in anatomy of seven weeks’ duration, and after conducting the most painstaking dissections of a number of human cadavers, he finally declared that he was ready to publish his great treatise on anatomy. Some of his friends, foreseeing clearly what a storm of protest the new book would arouse among the followers of Galen, urged him to postpone for a time its publication; but a few others agreed with him that it should be issued without further delay. Accordingly Vesalius sent the manuscript of his work at once to the printers at Basel, and the book was finally published in June, 1543, before its author had attained his twenty-ninth year. Its title was “_De corporis humani fabrica_,” and it was provided with exceptionally fine pictorial illustrations, most of which were drawn, as is generally believed, by John de Calcar, one of Titian’s pupils. A second edition, superior in every respect to the first, was published in 1555. In comparison with this great work the few treatises written by Vesalius in later years are of minor importance.