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.

    the eyelids on the same principle that applied to the saliva and the mucous membrane of the intestinal canals. He grouped the afferent and efferent lachrymal ducts around the tear gland proper and what was then called glandula lacrimalis, in the inner eye corner. The moisture of the nose led him, in this connection, to the discovery of the nasal glands. Stensen considered the possibility that the moisture necessary for the nose could come from the ears through the eustachian tubes, and from the eyes through the nasal duct, but decided that the principal source was the nasal glands. He assumed that this fluid disappeared again through the nostrils and also through an opening to the gullet, the ductus nasopalatini, also called ductus Stenoniani.

In his survey De musculis et glandulis (1664) Stensen enumerated all his new observations and individual discoveries, especially those he had made during the first half of his stay in Leiden: earwax duct, ducts of the cheek glands, the smaller gland ducts under the tongue, the glandular ducts of the palate, the glandular ducts of the epiglottis, the nasal gland, the nasal gland peculiar to sheep, the passage from the eyelids to the nose, the lachrymal ducts, and the gland ducts that lubricate the exterior surface of the ray. In 1673 Stensen found, independently of Peyer, the accumulations of lymph follicles in the small intestine (named for Peyer), but published his findings four years later.

The second period of research began with a challenge to the traditional overestimation of the heart: “One has glorified the heart as the sun, even as the king while upon closer examination, one finds nothing but a muscle,” which was directed not only against Aristotle, who saw the heart as the seat of the soul, the source of life, and the central organ of all sensation and motion, but also against Galen, who, following the authority of Plato, assigned the life forces of blood motion and heat distribution to the left ventricle. Even Harvey, who first recognized the purely muscle activity of the heart in maintaining the circulation of the blood, did not abandon the idea of a vital warmth within the heart, or did so only very late.

In a letter of 26 August 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 5 March he had spoken of a careful investigation of the heart musculature, and on 30 April he had 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.”

De musculis et glandis (1664) shows an abundance of new observations and discoveries concerning the anatomy and physiology of individual muscle, and the triangularis, which leads from function of the intercostal muscles, the sacrolumbar muscle, and the triangulararis, which leads from the bony end of the true ribs to the central line of the sternum. He described the role of the diaphragm and several other muscles during respiration; classified the tongue as a muscle; and also described the temporal muscle, and the muscle layer of the esophagus, which has its fibers arranged spirally.

From this research Stensen drew comprehensive conclusions concerning the structure of the muscles: that in each muscle there are arteries, veins, fibers and fibrils, nerves, and membranes; that each muscle fiber ends in a tendon on both sides; that no muscle tissue is a parenchyma (caro) but consists instead of closely woven fibers; and that the contractility lies in the muscle substance proper. He then applied all his finding to the heart and proved its muscle structure from both positive and negative evidence. He stated that the heart possesses all the characteristics of a muscle structure and that it 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. The findings were new, and even ten years later Bartholin, in a new edition of his Institutiones anatomicae (1611), did not accept them. Croone revised his De ratione motus musculorum (1664) according to Stensen's findings in the second edition (1670).

The controversy over his views caused Stensen, during his first year in Italy (1666%611667) to publish his Elementorum myologiae specimen, which dealt chiefly with the questions: Does the muscle increase in size during contraction? Are hardness and swelling of the muscle signs of an increase in volume? These were acute questions at the time, when even Borelli, one of the leading members of the Accademia del Cimento, still believed that swelling was caused by the influx of nerve fluid. Stensen first provided clear concepts and a clear-cut terminology of the parts of the muscle. Then he characterized the individual muscle fiber and the muscle itself as a parallepiped bordered by six parallelograms. In the second part of the Elementorum he dealt with objections against the new knowledge about muscles, and lamented the insufficient knowledge