Note 25 — Chapter IV · OF THE MOTION OF THE HEART AND ITS AURICLES, AS SEEN IN THE BODIES OF LIVING ANIMALS
Robert Willis translation
[At the period Harvey indicates, a rudimentary auricle and ventricle exist, but are so transparent that unless with certain precautions their parietes cannot be seen. The filling and emptying of them, therefore, give the appearance of a speck of blood alternately appearing and disappearing.]