William Harvey大概是第一个真正的physiologist, 用逻辑推理而非表观来分析问题。他提出血液循环理论时根本不知道有毛细血管来连通动静脉。这正是他的过人之处。而直接论证他的理论的人也过不了多久出现了,他就是Marcello Malpighi (1628-1694). 他在研究青蛙心脏时看到一些东西,在他给朋友的信里写道:
Observations by means of a microscope reveals more wonderful things than those viewed in regard to mere structure and connection: for while the heart is still beating the contrary movement of the blood is observed in the vessels - though with difficulty - so that the circulation of the blood is clearly exposed....
Thus by this impulse the blood is driven in very small arteries like a flood into the several cells, one or other branch clearly passing through or ending there. Thus the blood, much divided, puts off its red colour, and, carried round in a winding way, is poured out on all sides till at length it may reach the walls, the angles, and the absorbing branches of the veins....