“With only their eyes for tools, through no fault of their own, naturalists were stuck at the surface level of biology for thousands of years. Although dissection allowed some progress in understanding large-scale internal anatomy, it too was often misleading. For example, arteries and veins could be seen in dissected animal bodies. Yet the fact that they connected to each other through tiny capillaries in a closed circulatory system escaped even the great Roman surgeon Galen, who thought blood was pumped out by the heart to sink into the tissues, much as water in irrigation canals in his day sank into the ground. His mistaken ideas were taught for thirteen hundred years.”