Niels Stensen

Born 1638
Died 1686

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Danish anatomist, natural scientist, and theologist, born January 10/11, 1638, Copenhagen; died November 25/December 5, 1686, Schwerin, Germany. Name also written as Steensen. Latin: Nicolaus Stenonius. Italian: Stenone; French Stenon.

Biography of Niels Stensen

Niels Stensen - anatomist, geologist, palaeontologist, bishop and saint.

Niels Stensen grew up in affluent circumstances. He was the son of Sten Pedersen, a goldsmith and court jeweller, who came from a family of preachers in Skåne (now in Sweden), on the other side of the Öresund. Of the mother, Anne Nielsdatter, we know very little; Steensen never mentions her. Both parents had been married before, and Anne would do so twice more to goldsmiths before being finally laid to rest in St. Nicolai church. They lived in an imposing two-storey town house at the corner of Købmagergade and Klareboderne, not far from the Rundetårn, the Round Tower that had been built for the stody and observations of the stars.

A quiet child, from his third to his sixth year he suffered from a troublesome complaint which kept him from playing with his contemporaries and led him instead to listen to the conversation of grown-ups, especially when the subject was religion. Stensen was afflicted with poor health all his life, and was described when grown up as small and slight, frail in stature. It was in his father's workshop that he had his first experience with tools. As an anatomist, he became great not only as a scientist but also as a craftsman.

His father's workshop would be his first experimental laboratory, the place where his interest in natural and technical science would be encouraged until, age twenty-one, he left home. His own notes show that when a boy he measured the weight and cubic content of gold and he also described hydraulic machines and determined the cubic content of expelled air. He constructed a microscope and studied the refractions of light by means of two lenses fixed to a rod. He made chemical studies of sulphur and saltpetre, and produced red dye from vitriol. No interesting possibility of probing the mysteries of nature escaped his ever alert mind.

One of his close childhood friends, Peder, lived at the opposite corner of Købmagergade and Løvstræde. He was the son of Joachim Schumacher, a Rhenish wine merchant and wine bar proprietor who was an immigrant German. Peder, two years or so older than himself, would, as Peder Griffenfeld, become the country's greatest statesman of the century, and the two would have dealings with each other later. What the playwright and historian Ludvig Holberg said of Griffenfeld rather more than a century after may be applied to both: he can be "reckoned among the singular men Denmark can exhibit as a sample of what this Northern climate can produce".

Niels Stensen's father died as early as in 1644. As required by the law, a guardian was appointed for the child. In this case it was a treasury official called Jørgen Carstensen, who was married to Stensen’s half sister, who oversaw the investment of the child’s 1402 Rigsdaler. A year or two later, his mother married the goldsmith Johann Stichmann, who had probably come in from Germany (his bills are written in German), and who now took over the business.

A student in the time of plague and war
At the age of ten, Niels enrolled in the country's most exclusive school, the grammar school Vor Frue. Here, in the old humanistic grammar school, was laid the foundation of his knowledge of mathematics and languages. In his short life, Stensen acquired a knowledge, besides Danish, of German, Dutch, French and Italian, plus Latin of course, and he also learnt Greek, Hebrew and Arabic. During the dreadful plague year of 1654-1655 more than 8,000 people died – a third of the population of Copenhagen. At Vor Frue School 240 died - about every other pupil - and sixty were buried on one day alone.

Stensen began the study of medicine at the University of Copenhagen (Københavns Universitet) in 1656. During the war with Sweden and the siege of Copenhagen 1658-1660 teaching at the university was interrupted, and, like the other students, he had to take part in the defence of the city. In their attempt to storm the city in the night of 10-11 February 1659 the Swedes suffered a decisive defeat, to which, in the words of a contemporary report, "most of the Swedes brought their shrouds".

Still, under Thomas Bartholin (1616-1680), Bartholin’s brother Erasmus (1625-1698), and the anatomist and botanist Simon Paulli (1603-1680), he eagerly conducted studies of natural science and anatomy. Stensen's studies in these disastrous years must have involved a period of intense intellectual activity. The best evidence that we have for this is a notebook dating from then. Containing nearly a hundred closely written, double-column foolscap pages, it is titled - perhaps because of its mixed contents – Chaos. Discovered in 1946, it is now in the National Library in Florence. It reveals that he read about a hundred scientific works by about eighty authors.

The Great Anatomist
Anatomical studies in Copenhagen, however, had fallen into decline, and as soon as peace had "broken out" he followed a tradition going back to the middle ages and went on the customary study journey. His first destination was Rostock in Germany, where his fatherly friend Simon Paulli was able to get him every possible contact, but soon he went on to Holland which had just assisted Denmark at a time of need, and which was then in the midst of its golden age. Stensen arrived in Amsterdam – at that time three or four times as large as Copenhagen - in March 1660, and it was clear three weeks after his arrival that his apprenticeship, so to speak, had ended, and that among scholars he was an equal.

The connection between Denmark and the political and cultural great power Holland at that time was a very lively one. In the period 1575-1700, nearly a thousand Danes studied at Leyden. Owing to their abilities many Dutchmen obtained good positions in Denmark, and not infrequently Dutch money was a necessity, especially for financing wars. The might of this small nation is illustrated by the fact that at the end of the seventeenth century, 16.000 of Europe’s total of 25.000 sailing vessels were Dutch.

In Amsterdam Stensen perfected his knowledge of anatomy under professor Gerhard Bläes (Blasius), who was later to make unjust claims to some of Stensen’s discoveries. Here, on April 7, 1661, Stensen dissected the head of a sheep, making his first discovery by finding the excretory duct of the salivary gland. At once modest and full of a beginner's joy in research, he tells of it in a letter to Thomas Bartholin: "Having been allowed to dissect on my own, I succeeded in the first sheep's head which I purchased and dissected by myself in the study hall on 7 April in finding a duct which - as far as I know - has not previously been described. It was my intention after removing the ordinary outer parts to do a section of the brain when I happened to decide first to examine the vessels running through the mouth. Examining with that intention the course of the veins and arteries, by inserting a probe I observed that the point is no longer enclosed in the narrow sheath but moves freely in a spacious cavity; and pushing the instrument further forward, I at once heard it clink against the teeth themselves".

Elsewhere in his letter to Bartholin, Stensen modestly calls his discovery an inventiuncula - a small observation. In a way, it was this inventiuncula that would make him most famous, because, the duct of the parotid gland was named, by one of his Dutch friends, the Ductus stenonianua.

Conflict
This duct, however, became the cause of his conflict with Blassius, a conflict that moved the world of anatomy. Stensen had of course summoned his professor - and landlord/host - Gerard Bläes (Blasius) to show him his discovery. It was at once dismissed as a badly performed dissection. But on further reflection, Bläes claimed the duct as his own discovery, and in a brief publication entitled General Medicine in the spring of 1661 laid public claim to it. The prolonged dispute about the discovery had only one result, which was to lead the young Dane on to a succession of further discoveries. One of the lasting acquaintances he made in Amsterdam was his contemporary fellow student Jan Swammerdam, one of the truly great zoologists, and one of the first to use a microscope in scientific studies. The two would meet one another frequently in the future. Stensen concluded his studies in Amsterdam with one of the most popular disputations of the time when, with Arnold Senguerd, professor of philosophy, presiding, he defended a short thesis on hot springs.

The conflict with Blasius occasioned Stensen to move to the University of Leiden, where he was matriculated on July 27, 1661. The three years there were to be his most fruitful ones in the field of anatomy, making his name known far beyond Holland. He published ten papers in this period. Among the professors he particularly attached himself to were Johannes van Horne (1621-1670), whose work was chiefly done on the reproductive organs, and Frans de la Böe (Franciscus Sylvius), who had not only distinguished himself with his contributions to the knowledge of glands and the brain but was also an admired and inspiring academic teacher. Among natural scientists besides Swammerdam whom Stensen also met here was Reiner de Graaf. Dissecting was carried out avidly in Leyden's famous anatomical theatre as well as at the hospital.

Stensen appeared before the public as an independent research worker with his dissertation: "On the glands of the mouth and recently discovered ducts", of 6-9 July 1661, with Johannes van Horne presiding.

Quietly, he now began to assemble his discoveries in a publication, Anatomical Observations, which appeared early in 1662. This little book of four papers (including his dissertation) was respectfully dedicated to his teachers in Copenhagen and Leyden, among them the mathematician Jakob Golius (1596-1667). Gerard Bläes made a few last desperate efforts to assert his claim, and was briefly refuted by Stensen in his Precursor to an Apology of 1663, in which he demonstrated the difference between his duct and the one described by Bläes - which, other than the "discoverer", was known to no-one but the man in the moon.

Late in the autumn of 1663, Stensen received news from home of the death of his stepfather Johann Stichmann. He was forced to break off his brilliant studies, and such were the circumstances, he says, that they "robbed me of all hope of ever returning to them". In March 1664 he was back in Copenhagen, where he published his famous work on glands and muscles. De musculis et glandulis observationum specimen. In it he clearly recognized that the heart was a muscle, which was a great advance in the knowledge of this organ.

Now a world famous anatomist, Stensen dedicated the book to Frederick III of denmark, expecting to receive the vacated seat in Copenhagen’s Domus anatomica.
This had been instituted in 1645 by Simon Paulli, the royal physician, who is also known for the publication of the great botanical work Flora Danica. His old teacher, Thomas Bartholin, however, became fiercely jealous of Stensen and used any intriguant means available to prevent him from achieving a prominent position, hiring his "own", in this case Bartholin’s nephew Mathias Jacobsen.

At this time Stensen lost his mother, and the young scientist must soon after have realized that now, more than ever, he had to look around for a living, and that at any rate he could not reckon on a university appointment in his native city. Now when nothing tied him to the home any longer, he decided to spend the 300 rigsdaler left him by his mother on continuing his studies abroad, and there perhaps obtaining the post he was unable to get in Denmark.

In the learned circles of Paris
Stensen did not return to Holland, but went straight to his friend Jan Swammerdam in Paris, staying in the house of the French patron of the arts Melchisedec Thévenot, Louis XIV's Chamberlain and librarian. At Thévenot's house in Paris or on his farm near Issy were held the learned meetings that a year later would lead to the establishment of the Académie royale des Sciences. From the University of Leyden news soon came that for his "uncommon learning" he had been made a doctor of medicine in absentia, without having to write a special treatise, on December 4, 1664.

In Paris in 1665, in a meeting of the Thévenot circle, he presented – in French – his elegant and legendary lecture on the anatomy of the brain – Discours sur l’anatomie du cerveau. Later neurologists agree that this lecture is a breakthrough in the understanding of the brain. It was reprinted in Jacob Benignus Winsløw's (1669-1760) work Exposition anatomique. “Le seul discours de feu M. Stenon,” Winslow said, “a été la source primitive et le modèle genéral de toute ma conduite dans les travaux anatomiques.

He began by saying how ignorant he was of the brain, this "the most beautiful masterpiece of nature", that is the principal organ of our mind. "Our mind thinks that nothing can set a limit to its knowledge, but when it withdraws to its own habitation it is unable to give a description of it, and no longer knows itself". With good reason, he criticized earlier theories that the brain was connected with the mind through an egg-shaped gland. That sort of thing was no more than ingenious speculation, Stensen declared, and he compared the brain with a machine that one could understand only by taking it to pieces down to its smallest parts.

One of the most valuable aspects of the lecture - and the short paper written later - is the demonstration of the principles and methods that future research should follow in order to arrive at certain knowledge. Even at this stage, Stensen was far in advance of his learned contemporaries, who were captives of the current systems. Stensen does not draw his inspiration from classical and later literature but starts out right from the beginning with basic and unprejudiced first-hand observations, in which he is able to a rare degree to pick on the essential. From there he develops his own theories link by link, accompanied all the time by critical reflections. He also anticipates the complementarily theory of today by constantly pointing out the influence of the observation on what is observed.

Together with later works, the lecture established Stensen as one of the pioneers of comparative anatomy.

New country – new sciences
After a fruitful year in Paris, Stensen in the autumn of 1665 left Paris in order to travel around France. Going through the Loire valley, he proceeded via Angers and Bordeaux to the old university town of Montpellier, where he made the acquaintance of a number of eminent British natural scientists. From Montpellier in March 1666 he went to Pisa, winter residence of the grand duke of Tuscany. When the Medici court moved back to Florence before Easter, he, keeping to his original itinerary, went south, and in April we find him in Rome. At a banquet in the Villa Ludovisi, Stensen made friends with Marcello Malpighi (1628-1694). Among other important people he met in that Roman spring were the greatest Italian opticians of the time, Eustachio Divini (1610-1685) and Giuseppe Campani (1635-1715), and the later cardinal Michelangelo Ricci (1619-1682), who had just published a short but valuable paper on the calculation of tangents. Stensen also visited the Jesuits at the distinguished Collegium Romanum, including the famous German-born polyhistor Athanasius Kircher, with his celebrated museum.

From Rome, Stensen went to Tuscany, where he remained until 1668, mostly at the court of Grand Duke Ferdinand II (1610-1670) in Florence, under whom the Medici family experienced its last cultural flowering. Stensen received an apartment in the Palazzo Vecchio. Ferdinand was succeeded in 1670 by Cosimo III, Cosimo de’ Medici (1642-1723). Ferdinand was known for his support of art and science, a patronage necessary for Stensen, who lacked private means or a regular post. Stensen had been given letters of recommendation in Paris, and they got him such a cordial reception by the grand duke that he remembered it movingly many years after. Florence was to be the place where he would always feel at home, and would reach the climax of his scientific career.

In Florence, the grand duke Ferdinand granted the Danish scientist a monthly stipend and appointed him anatomist of S. Maria Nuova, with the right to use the great hospital's special departments for wounded and patients with stone, and not least its student college with anatomy hall. The grand duke himself was interested in natural history, and had his own laboratories in the palace where he made his own instruments. In the company of his scientific friends, he preferred to be a private citizen, the equal of the rest. The same may be said of his brother Leopold, who in his youth had had Galileo as a teacher. His greatest service, perhaps, was the institution and direction of the Accademia del Cimento, the experimental academy which, under the motto Provando e riprovando (test and re-test), had engaged in experimental science in the Palazzo Pitti since 1657. Cimento had been founded by Grand Duke Ferdinand II’s brother Cardinal Leopoldo.

A Catholic on the Rocks
Although he came from a deeply religious Lutheran family, Stensen converted to the catholic faith through two women. One of them was an elderly nun of high birth who had charge of the small pharmacy of the Annalena nunnery. Her name was Maria Flavia, and Stensen bought his medical requirements from her. The other was one of the city's noblest ladies, Lavinia Cenami Arnolfini, wife of the minister of Lucca at the Medici.

On All Souls’ Day – November 7 – 1667 during a period of research in anatomy and geology, Stensen converted to Catholicism. Stensen, described as sympathetic and noble personality, for some years found a pleasant life in the house of the Medicis and among the learned circles. He travelled extensively and in 1869 published his geological observations in De solido intra solidum naturaliter contento dissertationis prodromus. In this work, which is a milestone in geology, he laid the foundation of the science of crystallography. he wrote that, despite the fact that quartz crystals are physically very varying in appearance, they share the same angles between corresponding sides.

In December 1667, Stensen received a summons from King Frederik III to return home, with the promise of an annual salary of 400 rigsdaler. But the recall seems to have been suspended, apparently because news had reached Copenhagen of his conversion. Only when Griffenfeld four years later obtained assurances of his freedom of religious practice in Denmark did he return to Copenhagen.

The great anatomist
Before he left Florence, Stensen promised Cosimo that he would return after a few years and become tutor to the grand duke’s son, Ferdinando. Stensen arrived at Copenhagen on July 3, 1672 and assumed the position of anatomicus regius, a position that was probably created for Stensen, as non-Lutherans could not hold faculty positions.

He lodged with his sister Anne in their old home, where her husband Jacob Kitserow had carried on the workshop with great proficiency. There were children in the house, and they had bought a garden in the neighbourhood with a gatehouse and six dwellings where Stensen several times performed dissections. Many a thing had changed in his native city and the fortifications the young Christian V caused to built attracted the geologist’s attention at once by the piece of amber which had been found and from which Stensen deduced marine deposits beneath Copenhagen. Mostly for a circle of interested friends, he held a series on anatomical demonstrations, and in 1672 described the tetralogy of Fallot. In the Protestant Denmark it was not possible to give him a position at the University of Copenhagen, as a Catholic could not hold office.

With great enthusiasm the Royal Anatomist, as he was called for want of another title, immersed himself in his dissections to which the chancellor himself, Peder Griffenfeld, supplied rare material like reindeer and bears. He was no longer at ease with medicine, however, and on 5 June, he submitted his resignation to Griffenfeld, revealing, among other things, that there had been no opportunity at all for dissection in the second year. He had already thanked the grand duke on 29 May for the renewed invitation to become his heir's tutor. His passport is dated 14 July 1674.

Scientist turned tutor
On his way back to Italy, Stensen continued to work on anatomy, when, at the invitation of Duke Johann Friedrich of Hannover, he demonstrated the circulation of the blood, as well as the structure of the heart, in several dissections at the court.

At the duke's dinner table, however, he was at least as much interested in religious discussions with the courtiers and town preachers; and when Johann Friedrich presented him with his portrait on a costly gold medal with gold chain, his guest begged permission to convert the ornament into money for the poor.

At the end of 1674, Stensen was back in Florence, beginning at once to teach and educate the 12-year-old heir, the later Ferdinand III, who was to learn philosophic christiana; that is to say, about both natural science and the moral-religious duties of a ruler.

Tutor turned priest
At about the same time as his return to Florence, Stensen must have made the definitive decision to conclude his work as a scientist and devote the rest of his life in thanksgiving to God, to whom he always felt that he owed everything. His theological attainments were so highly esteemed by ecclesiastics that an apostolic dispensation exempted him from the obligatory examination. The conditions imposed on himself by Stensen were stricter, as indicated, inter alla, in his ascetic notes from that time. On Easter Day in April 1675, he conducted his first mass in SS Annunziata, the church that was so dear to him from the days of his conversion.

Soon, however, in 1676, Duke Johann Friedrich of Hanover, the brother of the Danish queen, Sophie Amalie, wanted Stensen, as the bishop in his city. The duke had converted to the Catholic faith after a visit to Assisi, and there was a small congregation in Hanover, an almost purely Protestant country. The year before, Johann Friedrich had appointed another great man of the period, the famous philosopher Gottfried Wilhelm (1646-1716) Leibniz, as his librarian, and it is highly likely that it was Leibniz who, knowing Stensen personally, had suggested him as bishop of "the Northern missions".

Bishhop in Münster
Upon the invitation of Duke Johann Friedrich, Stensen went to Rome, where he was appointed apostolic vicar of northern missions by Pope Innocent XI on August 21, 1677, and was consecrated titular bishop of Titiopolis on September 19. He left Rome on 27 September, going first on horseback to Florence in order to take leave of his friends there. Then, crossing the Alps by mail coach, he took ship from Frankfurt for Cologne, arriving in Hanover in November 1677, with the gigantic plan to recapture northern Europe for Catholicism. Until the end of June 1678, he ministered the scattered remnants of Catholicism in northern Germany, Denmark, and Norway.

At this very time, the prince-bishop of Paderborn, Ferdinand of Fürstenberg, was looking round for a suffragan bishop – Weihbishof – for the diocese of Münster in Westphalia, whose rule he had just taken over after the death bishop Ferdinand von Fürstenburg. Happening to read a letter from Stensen to his secretary, he was so struck by its zeal that he applied to Rome for the writer as an assistant. The papal bull of consent is dated 7 October 1680, but Stensen had already taken up his appointment in this see ravaged by the wars of religion in July. He was to serve there, both as bishop and dean of the collegiate church St.Ludgeri, from 1680 to 1683, and was assigned a wealth of functions, especially since the prince-bishop was ailing.

Stensen made many visitations in the parishes, many of which had not seen a bishop for decades, and altogether managed to visit 200 of the 250 of them. He would often walk for 5-6 hours a day, and would frequently preach three times a day. His curate has recorded that "his words convinced not only by their reasoning but also by their temperance", and "he displayed great dignity and extreme modesty in all his conduct". Besides being bishop, Stensen was also the parish priest of a congregation of some 2,000 people, and the social suffering he found in many places made a deep impression on him. His first winter in Münster was so severe that he personally became an outspoken beggar for the poor at the prince-bishop's. Writing in a letter of three small boys who had to sleep in the open every night, he says: "I cannot take my eyes off these people who thus live in the severest and most extreme distress; and although I have incurred debt out of charity for the poor, I have instructed the curate by no means to allow them to sleep out in future".

Last rites
On September 1, 1683, he left the city in protest against the simoniac election of the bishop’s successor. After the death of the bishop he went to Hamburg and, late in 1685, upon the wish of the duke of Mecklenburg, went to Schwerin, where he founded a Catholic congregation and died here lonely and in acute pain from gallstones on November 25, 1686.

The parotid duct
It was in Leiden, then the most important university on the Rhine, Stensen sought contact with two leading medical professors: Johannes van Horne, who independently of Jean Pecquet (1622-1674) had discovered the chief thoracic lymph passage, and Franciscus Sylvius (1614-1672), famous as an iatrochemist and for his studies on the brain. A warm and stimulating friendship with Jan Swammerdam also began in Leiden. Soon after Stensen’s arrival, van Horne demonstrated on humans the course of the parotid duct and declared it to be Stensen’s discovery, although Blasius, in his Medicina generalis (1661), not only claimed it for himself but incited his friends to slander Stensen, his former student. There ensued a long quarrel that Stensen settled both objectively and devastatingly in his Apologiae prodromus (1663).

The controversy spurred Stensen to the further investigation of the glands. He wrote: “I owe much to the famous man Blasius because he not only gave med cause to assert my property rights, but also to discover new things.”

Glands and lymph vessels
The glands and lymph vessels were then a new and exciting subject for investigation. In 1622 Gasparo Aselli (1581-1626) had demonstrated the lacteal vessels in the mesentery of a dog; in 1642 Johann Georg Wirsung (1600-1643) had shown the excreting duct of the pancreas; and in 1651 Pecquet had demonstrated the cisterna chyli and its continuation, the thoracic lymph duct; he also had realized that the latter poured its contents into the veins. In 1653 Thomas Bartholin demonstrated the thoracic lymph passage and the lymphatic system in humans. He also showed that the lymph vessels connecting the liver to the thoracic duct carried lymph away from the liver, thereby throwing doubt on the Galenic doctrine that blood originated in the liver. When Thomas Wharton (1614-1673) published his systematic presentation of the contemporary theory of glands in his Adenographia (1656), he announced the discovery of the duct of the submaxillary salivary gland; he also designated the brain and tongue as glands.

Stensen soon advanced to a basic understanding of the whole glandular lymphatic system, which he counted among the most sublime artifices of the Creator. Without changing the names of the conglomerate and conglobate glands, the terms by which Sylvius had already distinguished the anatomical form of the real glands from that of the lymph nodes, Stensen distinguished them according to their function. Arguing against such contemporaries as Louis De Bils (1624-1670), Anton Deusing 1612-1666, and Anton Everaertz (died 1679), on the basis of his observations Stensen stated in his Leiden dissertation (1661):

    “I gather from this that the saliva consists of the fluid secreted in the oral glands from the arterial blood which is carried through the lymph ducts with the aid of the Spiritus animales [a term then used for the nerves] into the mouth and the closely adjoining muscles, but that the round or conglobate glands in the proximity carry the lymphs received from the outer parts back to the veins so that it becomes mixed with the blood streaming back to the heart.
Heartbeat, why do you miss when my baby kisses me? (Buddy Holly, 1958)
In a letter of August 26, 1662, Stensen told Thomas Bartholin how fascinated he was by the independent motions of the vena cava, which continued even after the stopping of the heartbeat; this stimulated Stensen to make many investigations of the heart and respiratory organs. On March 5, he had spoken of a careful investigation of the heart musculature. He had bought and cooked the heart of an ox, and while loosening fibre after fibre from the well-prepared organ, he established its muscular structure. On April 30, he stated: “As to the substance of the heart, I think I am able to prove that there exists nothing in the heart that is not found also in a muscle, and that there is nothing missing in the heart which one finds in a muscle."

The controversy over his views caused Stensen, during his first year in Italy (1666/1667) to publish his Elementorum myologiae specimen, which dealt chiefly with the questions: Does the muscle increase in size during contraction? Are the hardness and the swelling of the muscle signs of an increase in volume? These were acute questions at the time when even Giovanni Alfonso Borelli (1608-1679), one of the leading members of the Accademia del Cimento, still believed that swelling was caused by the influx of nerve fluid.

Stensen described the muscles as being parallel strips of bundles of structural units and argued that the total response of the muscle was the summation of tensile force developed by each unit, opposing the view of other anatomists of the time that the apparent increase in size of a muscle during contraction was due to the inflow of hypothetical fluid. This matter was not finally settled until the 1990 – by the use of computer animation.

William Croone (1633-1684) revised his De ratione motus musculorum (1664) according to Stensen’s findings in the second edition (1670).

Stensen also stated that the heart is neither the seat of joy nor the source of the blood or of the spiritus vitales. The automatic movement, independent of the will, is shared by the heart with other muscles.

Reproduction
In 1667 he postulated that the ovary in the female was not, as had been previously thought, a female testis, but was in fact an organ which corresponded to the egg-producing organs of birds and reptiles. He named the ovary and his views were proven by Regnier de Graaf (1641-1693).

On the Anatomy of the Ray, Stensen describes the dissection of two rays which he had performed three days before in the house of his fatherly friend Professor Paulli, and which led to a number of new findings on the fish's mucous duct, reproductive organs and respiration. Johannes Peter Müller (1801-1858) credited Stensen with a discovery made by Aristotle but then forgotten; that in the so-called smooth sharks (Mustelus laevis) the eggs are not deposited, but the embryos remain connected to the uterus by a placenta, similar to that of mammals. In On the Yolk Duct to the Intestines of the Chick, Stensen reports this sensational discovery, stating that the chick's nourishment is drawn direct from the yolk through a connection duct to the intestine.

Stensen’s embryological observations were not published until 1675, but he had communicated them to De Graaf and Swammerdam. Therefore, the Royal Society of London, in the priority discussion of the procreative organs, assigned the credit to Stensen.

In the short paper Dissection of a Monstrous Foetus in Paris was found, quite unexpectedly, the first description of tetralogy, the discolouring of the skin known as "blue baby" due to congenital heart disease that is often called after the Fallot, who made his observation 200 years after Stensen.

Mathematics and natural science
From an early age Stensen was interested in technical-mathematical questions, such as minerals and metals, lenses and light refraction, telescopes, microscopes, and thermoscopes. The Danish mathematician Thomas Rasmussen Walgensten, inventor of the laterna magica, belonged to the circle of his acquaintances. Stensen’s physical-mathematical interest are indicated by his reading as a young man not only of the works Galileo Galilei (1564-1642) (Siderus nuncius – 1610) and Johannes Kepler (1571–1630) (De nive sexangula– 1611) but also those of Pierre Gassendi (1592-1655), the Jesuit astronomer Christopher Clavius (1537-1612), Gaspar Schott, Snel van Roijen (1546-1613), and the geographer Bernhardus Varenius (1622-1650/51).

As a young man, he knew Kepler's paper on hexagonal snow; it is characteristic of him that he was not satisfied until he had seen it for himself. He went out into the snow in the inner city, avid for knowledge. Allowing the delicate snow crystals to drift on to his hand, he watched them melt and evaporate, leaving no trace of their beautiful forms; but he copied them on a scrap of paper in order to study them later. He also read Galileo and Copernicus; and became convinced of the correctness of the Copernican system. Tycho Brahe (1546-1601) for his part could not accept it, as it was incapable of proof with the imperfect instruments of that time, but the 20-year-old Stensen was inclined to agree with Copernicus. He certainly subscribed to the new scientific method. "In natural science", he wrote "we know only experiments and observation, together with what we can deduce from them with the help of metaphysical and mechanical principles".

Stensen had a precocious desire for methodically founded knowledge, and he was critical of analogies and purely authoritarian statements. A letter of 26 August 1662 reveals a wish, already at the beginning of that year, to exchange the blooded dissection scalpel for the geometrical compass. This revival of his love of a mathematically exact understanding of nature is not surprising, for Stensen encountered it everywhere in the Netherlands. His friend Swammerdam, the great biologist, was then occupied, as he still was on his deathbed, with a planned work on bees, "in which the wisdom and omnipotence of God were demonstrated most mathematically". And for the ever open-minded Stensen it was an experience during vacations to travel round the Netherlands with his Danish friends, headed by Ole Borch. There can be no doubt that his sense of the country's scenery, and especially its geological structure, was sharpened on these tours, and everywhere the travellers encountered high technical development based on Stensen's favourite subject, mathematics.

Jaws and rocks
Stensen was led to his study of geology in Florence at the end of October 1666, when some French fishermen had observed, off the coast of Livorno, an immense shark - a Lamia or Carcarodon Rondeletti, as it was called at that time - and had managed to haul the creature ashore and secure it to a tree in order to kill it. The shark's weight was estimated at about 1,700 kilograms; but after removing the liver, which weighed 150 kilograms, and cutting the head off, the fishermen threw the rest back into the sea. At Ferdinand's command, the head was sent to Florence for examination by Stensen. In the course of a month, the latter had written his report, Dissection of a Shark.

He made acute observations of its skin, its canals, the brain and nerves, the Lorenzinin ampullae, and the eyes. The rows of pointed teeth in the mouth, however, led him to a thorough study of their number and substance. Stensen was surprised at their large number, and failed to understand why the inmost rows were curved inwards and partly concealed. He realized later that these were reserve teeth, and that the shark does not use its teeth for chewing, but for catching its prey. The transition from anatomy to palaeontology occurred when it struck him how much the shark's teeth resembled Glossopterae, which could be found in large numbers in Malta especially. Stensen concluded that they were fossil shark’s teeth, contradicting the fantastic explanations of the phenomenon given in his time.

At first, Stensen, too, was very cautious about comparing the shark's teeth and Glossopterae. He wrote: "I have not yet arrived at a firm conviction as regards this for me to submit my own judgment". All he would do was indicate a few points suggesting that the fossils derived from animals, and he concludes by saying: "Those who proclaim the large Glossopterae to be sharks' teeth do not seem to me to be very far from the truth". But eventually - in De Solido - he is no longer in any doubt that Glossopterae are fossilized sharks' teeth, and that all manner of animals and plants, or parts thereof, from the earth's successive ages are to be found as fossils.
As Stensen himself suggested, only a few men before him had the right idea, among them - one might almost say of course - Leonardo da Vinci. But few before Stensen had made systematic studies and undertaken the laborious demonstration.

The geologist
Now Stensen embarked upon an entirely new science, geology, which has thus naturally grown out of palaeontology. The most important result he should take account of was that the earth's strata were sediments of a liquid, "because the finely distributed matter of which the strata consist can only have been introduced into it by the whole being absorbed in a liquid and depositing itself by its own weight, thereby being smoothed by the motion of the above liquid".

Stensen thus came to realize that the earth was for long periods in the past covered by sea, which deposited the strata, which later became hardened. The fact that the strata are no longer always horizontal is due to the operation of currents of water and volcanic action. In this connection Stensen also speaks of the formation of mountains, which can originate in various ways, among others from volcanoes, and "by heavy rain and violent operation of mountain streams".

A third subject of discussion was the study of crystals. He did not know himself how crystals originated, he said, but he realized how they grew: "A rock crystal grows by new crystalline matter being deposited on the surface of the crystal already formed". His greatest contribution in this field certainly is his establishment of the law of angle constancy. The underlying idea of this is that although individual crystal surfaces of the same matter can vary in size, the angles between the surfaces are constant. It is the discovery of this law which makes Stensen the real founder of crystallography, for which reason it is now generally known as Steno's law.

De Solido concludes with a chapter on the Tuscan landscape. Stensen speaks of six different periods in the region's geological development, and says, finally: "This I prove about Tuscany.. . and confirm it for the whole earth". The quotation indicates that he was actually writing a general geology of every part of the earth.

Through his revolutionary idea that fossils are remnants of earlier organisms and that many rocks are the result of sedimentation, he became one of the founders of palaeontology. Steno was one of the first to recognise that earth’s crust is a chronological history of geological events and that this history can be read through careful studies of layers of fossils. He rejected the idea that mountains grew like trees, and in stead suggested that they be made up by changes in the earth’s crust. Despite the religious intolerance of his time (those seem to have been rather ordinary times), he was forced to locate all of this within a time span of 6.000 years.

Dead and buried
Stensen's body was buried in the dome of Schwerin, but soon afterwards, upon the wish of grand duke Cosimo (de Medici) of Tuscany, was transferred to Florence, where it is buried in the princely tomb of the Medicis in San Lorenzo. Since August 1883 his sarcophagus has been decorated with a memorial plaque, instigated by the geologists of the world, in which he is honoured as “Vir inter geologos et anatomicos praestantissimus” and as founder of scientific geology.

Already in his lifetime, Stensen earned himself a reputations for "holiness" and after his death attempts were made to have him declared a saint. This happened 302 years after his death, on October 23, 1988, when he was beatified by Pope John Paul II. His day of celebration in the Catholic church is November 25.

    "Like the flash of lightning in the night, shedding a flood of light suddenly and clearly on the wild host of clouds, so does Stensen rise above his contemporaries".
    Max Neuburger, medical historian, on Stensen's brain anatomy.

    The young geologist K. Mieleitner compares Stensen to the mythical King Midas of Phrygia, who turned everything that he touched into gold.

    It is the simple greatness of the apostolic age itself which here advances to meet us... he had the promise of Christianity: blessed be they that hunger and thirst after righteousness, for they shall be filled".
    A.D. Jørgensen, keeper of the Danish national records,
    at the end of his biography of Stensen,

    "One wonders whether this European personality, this tireless diaspora apostle, might not to a special degree be a pattern and patron for our century, characterized as it is by great advances in the natural sciences, serious endeavours toward European unity, and a deep longing for Una Sancta?"
    Cardinal Dr. Julius Döpfner

    "Skjønt er det vi ser, skjønnere er det vi erkjenner, men skjønnest er det vi ikke fatter."
    Beautiful is is that which we see, more beautiful is what we perceive, but most beautiful of all is that which we cannot comprehend.

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