Anatomy During his stay in Amsterdam, Steensen discovered a previously undescribed structure, the "
ductus Stenonis" (the duct of the
parotid salivary gland) in sheep, dog and rabbit heads. A dispute with Blasius over credit for the discovery arose, but Steensen's name remained associated with this structure known today as the
Stensen's duct. In Leiden, Steensen studied the boiled
heart of a cow, and determined that it was an ordinary
muscle. and not the center of warmth as
Galenus and Descartes believed. In
Florence, Steensen focused on the
muscular system and the nature of
muscle contraction. He became a member of
Accademia del Cimento and had long discussions with
Francesco Redi. Like
Vincenzo Viviani, Steensen proposed a geometrical model of muscles to show that a contracting muscle changes its shape but not its
volume. Steensen was the first to describe the
lateral line system in fish.
Paleontology In October 1666, two fishermen caught a huge female
shark near the town of
Livorno, and
Ferdinando II de' Medici, Grand Duke of Tuscany, ordered its head to be sent to Steensen. Steensen
dissected the head and published his findings in 1667. He noted that the
shark's teeth bore a striking resemblance to certain stony objects, found embedded within rock formations, that his learned contemporaries were calling
glossopetrae or "tongue stones". Ancient authorities, such as the
Roman author
Pliny the Elder, in his
Naturalis Historia, had suggested that these stones fell from the sky or from the
Moon. Others were of the opinion, also following ancient authors, that
fossils naturally grew in the rocks. Steensen's contemporary
Athanasius Kircher, for example, attributed fossils to a "lapidifying virtue diffused through the whole body of the geocosm", considered an inherent characteristic of the earth – an
Aristotelian approach.
Fabio Colonna, however, had already shown by burning the material to show that
glossopetrae were organic matter (limestone) rather than soil minerals, in his treatise
De glossopetris dissertatio published in 1616. Steensen added to Colonna's theory a discussion on the differences in composition between glossopetrae and living sharks' teeth, arguing that the chemical composition of fossils could be altered without changing their form, using the contemporary
corpuscular theory of matter. Steensen's work on shark teeth led him to the question of how any solid object could come to be found inside another solid object, such as a rock or a layer of rock. The "solid bodies within solids" that attracted Steensen's interest included not only fossils, as we would define them today, but minerals, crystals, encrustations, veins, and even entire rock layers or
strata. He published his geologic studies in
De solido intra solidum naturaliter contento dissertationis prodromus, or
Preliminary discourse to a dissertation on a solid body naturally contained within a solid in 1669. This book was his last scientific work of note. Steensen was not the first to identify fossils as being from living organisms; his contemporary
Robert Hooke also argued that fossils were the remains of once-living organisms.
Geology and stratigraphy Steensen, in his
Dissertationis prodromus of 1669 is credited with four of the defining principles of the science of
stratigraphy. His words were: • the
law of superposition: "At the time when a given stratum was being formed, there was beneath it another substance which prevented the further descent of the comminuted matter and so at the time when the lowest stratum was being formed either another solid substance was beneath it, or if some fluid existed there, then it was not only of a different character from the upper fluid, but also heavier than the solid sediment of the upper fluid." • the
principle of original horizontality: "At the time when one of the upper strata was being formed, the lower stratum had already gained the consistency of a solid." • the
principle of lateral continuity: "At the time when any given stratum was being formed it was either encompassed on its sides by another solid substance, or it covered the entire spherical surface of the earth. Hence it follows that in whatever place the bared sides of the strata are seen, either a continuation of the same strata must be sought, or another solid substance must be found which kept the matter of the strata from dispersion." • the
principle of cross-cutting relationships: "If a body or discontinuity cuts across a stratum, it must have formed after that stratum." These principles were applied and extended in 1772 by
Jean-Baptiste L. Romé de l'Isle. Steensen's ideas still form the basis of stratigraphy and were key in the development of
James Hutton's theory of
infinitely repeating cycles of seabed deposition, uplifting, erosion, and submersion.
Crystallography Steensen gave the first accurate observations on a type of crystal in his 1669 book
De solido intra solidum naturaliter contento (the
Dissertationis prodromus). The principle in
crystallography, known simply as ''Steensen's law
, or the law of constancy of interfacial angles or the first law of crystallography'', states that the angles between corresponding faces on crystals are the same for all specimens of the same mineral. Steensen's seminal work paved the way for the
law of rational indices of French mineralogist
René-Just Haüy in 1801. This fundamental breakthrough formed the basis of all subsequent inquiries into
crystal structure. == Conversion and priesthood ==