De musculis et glandulis observationum specimen.

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.