HARVEY. 



THE importance of Harvey's discovery of the circulation 

 of the blood can only be properly estimated by bearing 

 in mind what was done by his predecessors in the same 

 field of inquiry. Aristotle had taught that in man and 

 in the higher brutes the blood was elaborated from the 

 food in the liver, conveyed to the heart, and thence dis- 

 tributed by it through the veins to the whole body. 

 Erasistratus and Herophilus held that, while the veins 

 carried blood from the heart to the members, the arteries 

 carried a subtle kind of air or spirit. Galen discovered 

 that the arteries were not merely air-pipes, but that they 

 contained blood as well as vital air or spirit. Sylvius, 

 the teacher of Vesalius, was aware of the presence of 

 valves in the veins ; and Fabricius, Harvey's teacher at 

 Padua, described them much more accurately than 

 Sylvius had done ; but neither of these men had a true 

 idea of the significance of the structures of which they 

 wrote. Servetus, the friend and contemporary of Vesalius, 

 writing in 1533, correctly described the course of the 



