Depending on whether you date according to the Julian or the Gregorian calendar, Niels Stensen was born in Copenhagen January 1 (Julian) or January 11 (Gregorian) 1638. (The Gregorian calendar only began to be used in Denmark itself after 1700.) He was a second child of the goldsmith Sten Pedersen; his mother's name was Anne.
In 1656 he matriculated under the name Nicolaus Stenonis at Copenhagen University, and it is under variants of this name that he is most widely known. While he was attending University, Denmark and Sweden became involved in a war, and King Karl X Gustav of Sweden invaded. Because of winter ice in 1658, Karl Gustav was able to cross over to Zealand, the island on which Copenhagen was located. King Frederik III of Denmark had to cede territory to stop the advance. Karl Gustav invaded again in 1659 in an attempt to take all of Denmark; Copenhagen repelled the main attack, but remained under a landside siege until 1660. We know that Steno spent some time in a student company manning the ramparts, but not much more; most of what has survived of Steno's life is found in a text called the Chaos-manuscript (discovered in 1946 in the Biblioteca Nazionale Centrale of Florence, Italy, by Father Gustav Scherz): 92 folio pages of closely written observations, experiments, reflections, & excerpts.
In 1659 Steno seems to have sailed to Amsterdam, perhaps with an extended stop in Rostock, where he attended lectures by Gerard Blaes (Blasius), the City Physician. He was given leave at the time to do his own dissections, and entered the first major controversy of his life. During dissection of a sheep's head, he discovered the parotid excretory duct, and showed it to Blaes, who was inclined to dismiss it as either an artifact of dissection or a freak of nature. While it had been discovered before, this was not known at the time. Several days later he found the parotid excretory duct in a dog's head, and showed it to Blaes. After defending his thesis (on hot springs), he left Amsterdam for Leiden. At the University of Leiden, he showed his discovery to to several professors, one of whom (Van Horne) began demonstrating it in his anatomical lectures as the ductus Stenonianus (Stensen's duct, which is its name still). At about the same time, however, Blasius was demonstrating in his lectures - as his own discovery - and by 1631 had published the discovery. Niels found himself attacked as a plagiarizer by Blaes and his supporters. The dispute, quite fierce, lasted for some time, and did not entirely die out until it became more generally known just how brilliant an anatomist Steno actually was. Spurred on by the dispute, Steno plunged into his investigation of glands and ducts, and discovered (among many others) the lateral nasal gland, which is still called Steno's gland. Steno published his work, which was very well received. At this point he wanted to give anatomy a rest, but for various reasons soon returned to it. One of those reasons was the posthumous publication in 1662 of Descartes's Treatise on Man. Niels began to study the major subjects of that work: the heart, the muscles, and the brain. In 1662 he discovered sino-atrial and atrio-ventricular dissociation. He proved that the heart was entirely a muscle (which has been affirmed, without explanation, by Harvey, but was not the common view at that time); he also discovered, pace Harvey, that the muscle was arranged spirally rather than circularly.
While in Leiden he made a number of acquaintances, including Swammerdam, de Graat, and Spinoza, but he didn't stay long; in early 1664, he returned to Copenhagen. There he published De musculis et glandulis observationum specimen, one of the most important works in the history of cardiology, which contemporaries said had turned medicine upside down. At this time he was 26.
In autumn of 1664, Steno left Copenhagen for Paris, and at some point in winter of 1665 he delivered a lecture on the brain (published in 1669 as Discours sur l'anatomie du cerveau). In the lecture he criticized Descartes's view of the brain, and, in particular, the appeal to animal spirits. After traveling in the south of France, Stensen arrived in Tuscany. In 1667 he published his major work on muscles, the Elementorum myologiae specimen, one of the distinctive features of which is that in it Steno develops a theory of muscle contractions that did not appeal to animal spirits. His alternative theory was attacked again and again, so that it was no longer held by anyone by the end of the 18th century. Work on the subject since 1980 has shown that he was largely right, and, on this point at least, myology has now caught up to where Steno was at age 29 in 1669.
The Elementorum myologiae specimen is also significant in that appended to it were two works describing shark dissections. One of these works, called the Canis carchariae dissectum caput, noted the resemblance between shark's teeth and certain fossils; Steno agreed with those who had suggested that the latter were somehow versions of the former, and began to develop an argument that this was possible. The second treatise, the Historia dissecti piscis ex canum genere, showed that the 'testes mulierum' of the non-oviparous dogfish were sufficiently ovary-like to be considered ovaries. This is commonplace now, but at the time it was unclear whether the females of many species had ovaries. After the publication of this work, Steno continued his study of female reproductive organs, and is credited with being the first discoverer of the mammalian ovarian follicle.
In November 1667, Steno became Catholic. He was confirmed December 8, and on the same day he received a letter from Frederik III ordering him to return to Copenhagen; he replied with a letter asking if the order still stood given that he was no longer Lutheran. In the meantime, he studied geological formations, publishing a preliminary report on them in 1669: the De solido intra solidum naturaliter contento dissertationis prodromus (also here in Latin). In it he gives the first systematic classification by common origin for solids within solids, and in so doing laid down the principles of reconstruction of geological history. The Prodromus is the founding text of paleontology and dynamic geology; it is, with the work of Erasmus Bartholin on Iceland spar, one of the founding works of crystallography. One of its many important contributions was the recognition that the faces of quartz crystals are related to each other by a constant angle, which is perhaps the fundamental insight of crystallography. Here and there it is referred to as "Steno's rule".
From late 1668 to early 1670, Steno traveled through Europe confirming his geological theories and giving anatomical demonstrations. At one such demonstration (at Innsbruck in June) he dissected the head of a hydrocephalic calf, showing that the deformity was caused by a disease, and thus providing a strong argument against the view that it was caused by maternal fantasies, a view that was still being used in the early nineteenth century. When he returned to Florence in 1670, he was made court geologist by the Grand Duke, Cosimo III. Steno became more involved in theological discussions, and on the publication of Spinoza's Tractatus Theologico-Politicus in 1670, he wrote a letter to his old acquaintance urging him to become Catholic.
In April 1675 he was ordained a priest in Florence and became tutor and moral preceptor to the Crown Prince. In 1677 he was appointed by Innocent IX apostolic vicar of the northern missions and was consecrated the titular bishop of Titiopolis. Steno went to Hanover at the invitationof Duke Johann Friedrich of Brunswick-Lüneburg. When Johann Friedrich died, Steno became auxiliary bishop to Prince Bishop Ferdinand von Fürstenberg of Münster. Catholicism there seems to have been rather lax; Steno spent much of his time there advocating pastoral reform against strong opposition, and eventually left in protest. He began to live an ascetic life of poverty at Hamburg, during which he began, but never completed, an essay reviewing confirmed knowledge of the nervous system.
Steno died November 25 (Julian, 5 December Gregorian). He was 48. His last words are said to have been Jesus sis mihi Jesus et misericordiam tuam, Domine, in aeternum cantabo. Cosimo III had Steno's body brough back to Florence, where it can be found in the Church of San Lorenzo. on 23 October 1988, John Paul II beatified him. His feast, officially celebrated in certain areas of Europe, is celebrated (as they often are) on the day of his death, December 5.
It's difficult to find good works on Steno in English. Here are two recommendations.
Troels Kardel. Steno: Life - Science - Philosophy. Acta Historica Scientarum Naturalium et Medicinalium, vol 42. Munksgaard (Copenhagen) 1994.
Hans Kermit. Niels Stensen: The Scientist who was Beatified. Michael Drake, tr. Gracewing (Leominster, Herefordshire) 2003. (This is good as a purely introductory work.)