Published in 1628 in the city of Frankfurt (host to an annual book fair that Harvey knew would allow immediate dispersion of his work), the 72-page "Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus" contains the mature account of the circulation of the blood. Opening with a dedication to King Charles I, the quarto has 17 chapters which give a clear and connected account of the action of the heart and the consequent movement of the blood around the body in a circuit. Having only a tiny lens at his disposal, Harvey was not able to reach the adequate pictures that were attained through such microscopes used by Antonie van Leeuwenhoek; thus he had to resort to theory and not practical evidence in certain parts of his book. After the first chapter, which simply outlines past ideas and accepted rules regarding the heart and lungs, Harvey moves on to a fundamental premise to his treatise, stating that it was important to study the heart when it was active in order to truly comprehend its true movement; a task which even he found of great difficulty, as he says: