This naturalist, Marcus Aurelius Severinus, professor of anatomy and surgery at Naples, was born in 1580, and died of the plague in 1656. In his Zootomia Democritæa, printed in 1645, he gave the figure, with a description of a leaf of the _Ficus Opuntia_ reduced to a skeleton. Of the particular process employed to prepare this leaf, the figure of which is very coarse and indistinct, he gives no account. He says only that the soft substance was so dissolved that the vessels or nerves alone remained; and that he had been equally successful with a leaf of the palm-tree. A piece of a leaf of the like kind he sent by Thomas Bartholin to Olaus Wormius, who caused it to be engraved on copper, in a much neater manner, without saying anything of the method in which it had been prepared[477]. The process Severin kept secret; but he communicated it to Bartholin, in a letter, on the 25th of February 1645, on condition that he would not disclose it to any one. At that period, however, it excited very little attention, and was soon forgotten, though in the year 1685 one Gabriel Clauder made known that he had reduced vine-leaves, the calyx of the winter cherry, and a root of hemlock, to a net or tissue by burying them in sand during the heat of summer, and hanging them up some months in the open air till they were completely dried. This art was considered to be of much more importance when it was again revived by the well-known Dutchman, Frederick Ruysch. That naturalist found means to conduct all his undertakings and labours in such a manner as to excite great wonder; but we must allow him the merit of having brought the greater part of them to a degree of perfection which no one had attained before. By the anatomy of animals, in which he was eminently skilled, he was led to the dissection of plants; and as it seemed impossible to fill their tender vessels, like those of animals, with a coloured solid substance[478], he fell upon a method of separating the hard parts from the soft, and of preserving them in that manner. For this purpose he first tried a method which he had employed with uncommon success, in regard to the parts of animals. He covered the leaves and fruit with insects, which ate up the soft or pulpy parts, and left only those that were hard. But however well these insects, which he called his little assistants, may have executed their task, they did not abstain altogether from the solid parts, so that they never produced a complete skeleton. He dismissed them, therefore, and endeavoured to execute with his own fingers what he had before caused the insects to perform, after he had separated the soft parts from the hard by decomposition. In this he succeeded so perfectly, that all who saw his skeletons of leaves or fruit were astonished at the fineness of the work and wished to imitate them.