First edition of Niels Stensen’s definitive treatise on the structure of muscles and his first presentation of his revolutionary theory of the formation of the fossils. Written during his two-year stay at the court of Grand Duke Ferdinand in Florence and published shortly before his conversion to Catholicism, the first part of the work builds upon the discoveries that he had spelled out in his De musculis et glandulis. At the time, the swelling and hardening of muscles during their contraction was still thought to be due to the influx of fluid from the nerves. Stensen showed here that the true cause is the tension of the individual muscles fibers. This geometrical analysis of muscle structure, which lay the groundwork for the science of muscular mechanics, permitted him to show that muscles do not increase in volume during contraction. In the second part Niels Stensen presented for the first time his theory of the formation of fossils. The study of teeth in a shark’s head that he had dissected in 1666 had ‘placed immediately before him the question of the relation of these teeth to the so-called glossopetra or tongue-stones, which were common on Malta and were considered lapides sui generis. Stensen concluded that they were fossil shark’s teeth’ (DSB). This discovery led him to develop theories of the formation of fossils and geological structures, enunciated here for the first time, in "the first outline of a scientific theory of the development of the earth’ (Norman). This was translated by A. Garboe as The Earliest Geological Treatise (1667), London 1958. Collation: Pp. (8), 123, (1) blank, 3 folding woodcut plates and 4 engraved plates, numbered tab. I-VII, and 25 woodcut diagrams in the text. Woodcut arms of Medici on the title. Woodcut initials and tailpieces, type-ornament headpieces. Binding: Later half calf, gilt (c. 1820), marbled boards. Provenance: Bears the signatures of three members of the Retzius family on the title page: Nils Retzius (1712–1757), Anders Jahan Retzius (1742–1821), and Anders Retzius (1796–1860). References: Garrison-Morton 577; Norman 2012; Heirs of Hippocrates 609; Osler 4021; DSB XIII, pp. 30-35; Lilly Library, Notable Medical Books, p. 79.Christies’ Norman Sale 808. Waller 9223.