Stensen described the structure of muscles, the fibra motrix, confirming that contraction actually occurs in the muscle fibres, not in the tendon as Galen had thought. He attempted a geometrical description of muscle contraction. He described the anatomy of the heart and its function as a muscle, and described the anatomy and function of the respiratory muscles including the diaphragm. English translation of the section on the muscles and the tongue in J.E. Poulson & E. Snorrason, Nicolaus Steno 1638-1686, A re-consideration by Danish scientists. Gentofte, Denmark, 1986.