Looking back on the years spent in writing this book, I feel sometimes that it was done primarily for my own enjoyment. I wanted to visualise in a single perspective the path of man's knowledge about the function of muscles, progressing so slowly for so many centuries but then during the last seventy years reaching speedily towards the goal in a rush of great discoveries. Professor A. V. Hill, whose fundamental work constantly appears in these pages, tells me that he never took theories of contraction seriously; but perhaps he would not disagree with Dr William Croone who in the seventeenth century reckoned ‘such speculations amongst the best entertainments of our mind’. Be this as it may, I hope that, in spite of its defects and omissions, this book will be useful to some of those for whom the fearful and wonderful phenomena of muscular movement retain all their fascination. We can respond to the words of Sir Thomas Browne (Religio Medici 1. 13):