832. The delicacy and transparency of the lacteals and lymphatics long concealed them from the view of the anatomist. The lacteals had indeed been occasionally seen in ancient times, but their office was altogether unknown. In the year 1563 Eustachius discovered the thoracic duct, but did not perceive its use. About half a century afterwards, in the year 1622, the lacteals were again one day by chance seen by Asellius, in Italy, while investigating the function of certain nerves. Mistaking the lacteals for nerves, he at first paid no attention to them; but soon observing that they did not pursue the same course as the nerves, and “astonished at the novelty of the thing,” he hesitated for some time in silence. Resolving in his mind the doubts and controversies of anatomists, of which it chanced that he had been reading the very day before, in order to examine the matter further, “I took,” he says, “a sharp scalpel to cut one of these chords, but scarcely had I struck it when I found a liquor white as milk, or rather like cream, to leap out. At this sight I could not contain myself for joy; but turning to the by-standers, Alexander Tadinus and the senator Septalius, I cried out Εὕρηκα! with Archimedes; and at the same time invited them to look at so rare and pleasing a spectacle; with the novelty of which they were much moved. But I was not long permitted to enjoy it, for the dog now expired, and, wonderful to tell, at the same instant the whole of that astonishing series and congeries of vessels, losing its brilliant whiteness, that fluid being gone, in our very hands, and almost before our eyes, so evanished and disappeared that hardly a vestige was left to my most diligent search.” The next day he procured another dog, but could not discover the smallest white vessel. “And now,” he continues, “I began to be downcast in my mind, thinking to myself that what had been observed in the first dog must be ranked among those rare things which, according to Galen, are sometimes seen in anatomy.” But at length recollecting that the dog had been opened “athirst and unfed,” he opened a third “after feeding him to satiety; and now everything was more manifest and brilliant than in the first case.” The zeal with which he followed out the clue he had obtained is indicated by the number of dogs, cats, iambs, hogs, and cows which he dissected, and by the statement that he even bought a horse and opened it alive; but, he adds, “a living man, however, which Erasistratus and Herophilus of old did not fear to anatomize, I _confess_ I did not open.” 833. Nearly thirty years elapsed before the lacteals, which were long thought to terminate in the liver, were traced to the thoracic duct; and it was not until the year 1651, about eighty years after the discovery of Asellius, that the lymphatics were discovered, and that the whole of this portion of the absorbent system was brought to light.