In 1661, he went to Leyden, for the purpose of attending the lectures at the celebrated university of that city. There he remained two years, studying surgery with Van Horne, and medicine with Franciscus Sylvius de le Boe, with as much diligence as he had previously displayed in his other pursuits. During the whole of this time he enjoyed the friendship of Steno and De Graaf; and, becoming much attached to the study of anatomy, he exerted his utmost ingenuity in devising means for effectually preserving his preparations. He then went to Paris to improve himself in his profession. There he continued the examination of insects, and had the good fortune to discover the valves in the lymphatic vessels. After this he resided for some time at Lyons, where he lived on terms of intimacy with Thevenot the celebrated traveller, who introduced him to the learned men by whom his house was frequented. In their society he usually remained a listener, and could not be prevailed upon to communicate his ideas; but, being repeatedly urged to exhibit one of his minute dissections, he gratified the wishes of his friends, and, by the profound knowledge which he displayed, acquired at once their esteem and admiration. Thevenot recommended him to Van Beuningen, a senator at Amsterdam, who, on his returning to that city, obtained permission for him to examine the bodies of patients dying in the hospital,--an opportunity of increasing his knowledge which he took care not to neglect. In his native town he frequented a society of medical men, who met once a-fortnight for the purpose of discussing subjects connected with their profession, and made observations on the structure of the spinal marrow and nerves, on respiration, and on the effects produced by the injection of fluids into the blood-vessels of animals. About the end of 1666, he returned to Leyden with the view of obtaining his medical diploma, and there continued his researches in company with his former teacher, Van Horne, in whose house he injected the uterine vessels with wax,--a method of showing the distribution of the arteries and veins afterwards greatly improved by him, and which has been productive of much advantage in the study of anatomy. In February 1667, he received the degree of doctor, and in March published his thesis on respiration, which he dedicated to Thevenot. He next invented a new method of preserving anatomical subjects by inflating them with air. But the eagerness with which he engaged in these occupations was prejudicial to his health, and he was seized with a quartan ague, which reduced him to a state of extreme debility. On recovering from this disease, he remitted his professional studies for two years; resuming the investigation of insects, the structure of which he unfolded with astonishing precision and success.