STENSEN, NIELS, also known as Nicolaus Steno (b. Copenhagen, Denmark, 1%6111 January 1638; d. Schwerin, Germany, 25 November/5 December 1686), anatomy, geology, mineralogy.

    of the muscle fluid. Later, Stensen (before the theory of irritability proclaimed by Haller) discovered that a muscle contraction can also result from direct stimulation of the muscle.

Stensen's muscle research was also a symptom of his philosophical-religious struggle. At the turn of 1662%611663 he had wanted to abandon anatomy for mathematics and physics, a wish probably fostered by the spirit of the times and by his desire for certainty. The need for quantitative knowledge was indicated by Stensen's early enthusiasm for Descartes's method of attaining certainty. During his years in Leiden, Stensen became friendly with Spinoza, whose rationalism may have influenced Stensen so strongly that his Christian belief was endangered. His discovery of the muscle structure of the heart showed him the fallacy of the Cartesian view of the heart as a hearth of fire and made him skeptical of their whole seemingly firmly anchored geometrical philosophy. As he admitted latter to Leibniz: “If these gentlemen have been so mistaken with material things which are accessible to the senses, what warranty can they offer that they are not mistaken when they talk about God and the soul?”

The third period of research extended from 1665 to 1667, the last great anatomical-physiological period of his life. The period can be divided into three parts: brain anatomy, embryology, and comparative anatomy.

Stensen's study of brain anatomy was confined almost entirely to his Discours . . . sur l'anatomie du cerveau (1669), which he presented in Paris to Thévenot's circle, among whom were many Cartesians. Stensen was stimulated to undertake his brain studies not only by his teacher, the brain anatomist Sylvius, but also by Descartes's Traité de l'homme. Vigorously but tactfully, Stensen opposed Descartes's mechanical theory and revealed his anatomical errors, refuting especially his theory concerning the epiphysis. According to Stensen, the epiphysis could not possibly carry out the slightest motion and thereby contribute to one's actions; whereas the Cartesian view was that it inclined itself to one side and then to the other side. In the Discours Stensen calls for a sober terminology and proposes new methods of dissection and the preparation of specimens. Stensen demands that the investigation trace the course of the brain fibers and that the investigator strive for a secure knowledge of the anatomical parts before interpreting their functions. In recent times his drawings of the brain have shown that he had a very rich knowledge of its anatomy.

From 1667 to 1669 Stensen contributed two concepts about the ovum and the ovary, oviduct, and uterus. The ovaries were considered to be testes muliebres, a type of semen producer. In a recently hatched chick Stensen discovered the oviduct and recognized that it was a canal destined to conduct the yolk directly into the intestine. In the Elementorum Stensen says that since he had found the true ova in the female testes, he concluded that they were really ovaries. Johannes Peter Müller 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. 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 dispute concerning the procreative organs, assigned the credit to Stensen.

Among Stensen's unpublished observations are those on rays and sharks. He established the mucous canal system of these fishes and recognized the significance of the spiral fold in the intestine as a substitute for its greater length in other creatures. He also observed the eyeball stalk, the optical nerve, and the crossing of the optic nerves.

The technical side of Stensen's research was highly developed. He employed simple but effective means, such as the induction of bristles into the gland ducts or the expansion of vessels by inflation. It is not known to what extent he used the microscope, but he knew the optical experts of his time and such microscopists as Swammerdam and Malpighi. He was also a skilled draftsman. His diagram of the blood circulation, which for the first time revealed the heart as two relatively independent hearts or pumps, enabled his pupil Caspar Bartholin, Jr., to develop further the concept of circulation.

The fourth and greatest period of Stensen's research began in Florence at the end of October 1666, when he received the head of a gigantic Carcharodon rondeletii that had been caught near Leghorn. He made acute observations of its skin, its canals, the brain and nerves, the Lorenzinian ampullae, and the eyes. The rows of pointed teeth in the mouth, however, led him to a thorough study of their number and substance and also 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