Function and structure in early modern muscular mechanics. Four episodes and a dialogue between Stensen and Borelli on two chief muscular systems.

The dispute on the movement of skeletal muscles in 1667 between Giovanni Alfonso Borelli, who maintained the ancient movement caused by inflation theory, and Niels Stensen (Nicolaus Steno), who proposed the first recorded theory of fibre contraction, had far reaching implications for understanding the relation between muscle morphology and function. A dialogue is reconstructed from citations from the two authors' main works. They had a similar dispute on the movement of the heart along the lines of the debate in the 1630s between William Harvey favouring contraction and René Descartes favouring swelling. Evidence is provided for the delayed general acceptance of fibre contraction in both heart and skeletal muscles. It is shown that the inflation interpretation of muscular mechanics elaborated by Borelli, Johann Bernoulli, his son Daniel, and by others, was maintained from ancient authors and Descartes in part due to a conceptual block resulting from the mechanical philosophy that denied any force of attraction in nature. The alternative theory, that of fibre contraction, was thought of as self-motion, which violated an accepted mechanical principle and therefore was rejected. In the mid-18th century, Albrecht von Haller recorded no microscopic structures in support of inflation. He adopted the view that contraction in fibres of muscles is generated through an 'irritability'. Research on this entity has taken place ever since with a clear preponderance of studies on single fibre properties and subcellular structures. Haller did not, however, refer to the original contribution of Stensen on fibre contraction. Haller even rejected Stensen's functional architecture of skeletal muscle. This structure, now called the unipennate, or semipennate, actuator, was overlooked and had to await confirmation by anatomical rediscovery and pragmatic demonstration through successful applications in computer models of muscular contraction in the 1980s.