FREDERICK, one of the most eminent anatomists of which Holland can boast, was born at the Hague in 1638. After making great progress at home, he repaired to Leyden, and there prosecuted the study of anatomy and botany. He next studied at Franeker, where he obtained the degree of doctor of physic. He then returned to the Hague, and marrying in 1661, dedicated his whole time to the study of his profession. In 1665 he published a treatise entitled Dilucidatio Valvarum de varis Lymphaticis et Lacteis, which raised his reputation so high that he was chosen professor of anatomy at Amsterdam. This honour he accepted with the more pleasure, because his situation at Amsterdam would have given him easy access to every requisite help for cultivating anatomy and natural history. After he settled in Amsterdam, he was perpetually engaged in dissecting, and in examining with the most inquisitive eye, the various parts of the human body. He improved the science of anatomy by new discoveries; and, in particular, he found out a way to preserve dead bodies many years from putrefaction. His anatomical collection was curious and valuable. He had a series of fetuses of all sizes, from the length of the little finger to that of a newborn infant. He had also bodies of full-grown persons of all ages, and a vast number of animals of almost every species on the globe, besides a great many other natural curiosities. Peter the Great of Russia, in his tour through Holland in the year 1698, visited Ruysech, and was so charmed with his conversation, that he passed whole days with him; and when the hour of departure came, he left him with regret. He set so high a value on Ruysech's cabinet of curiosities, that when he returned to Holland in 1717, he purchased it for thirty thousand florins, and sent it to St Petersburg.

In 1685 he was made professor of medicine, an office which he discharged with great ability. In 1728 he got his thigh-bone broken by a fall in his chamber. The year before this misfortune happened, he had been deprived of his son Henry, a youth of talents, and well skilled in anatomy and botany, who had been created a doctor of physic, and was supposed to have assisted his father in his discoveries and publications. Ruysech's family now consisted only of his youngest daughter. This lady had been early insensible with a passion for anatomy, the favourite science of her father and brother, and had studied it with success. She was therefore well qualified to assist her father in forming a second collection of curiosities in natural history and anatomy, which he began to make after the emperor of Russia had purchased the first. Ruysech is said to have been of so healthy a constitution, that though he lived to the age of ninety-three, yet during that long period he did not labour above a month under the infirmities of disease. From the time he broke his thigh he was indeed disabled from walking without a support; yet he retained his vigour both of body and mind without any sensible alteration, until in 1731 his strength at once deserted him. He died on the 22d of February the same year. His anatomical works are printed in four vols. 4to.

The style of his writings is simple and concise, but sometimes inaccurate. Instruction, and not ostentation, seems to be his only aim. In anatomy he undoubtedly made many discoveries; but from not being sufficiently conversant with the writings of other anatomists, he published as discoveries what had been known before. The Academy of Sciences at Paris in 1727 elected him an honorary member, in place of Sir Isaac Newton, who had lately deceased. He was also a member of the Royal Society of London.