12 Vesalius : [lect. 



only safe vantage ground from which to fight against error. 

 When he asserted that such a structure was not as Galen had 

 described it but different, he could appeal to the direct visible 

 proof laid bare by the scalpel. Even then he found it 

 difficult to convince his hearers, so ready were men still to 

 trust Galen rather than their own eyes. Much harder was the 

 task when, in dealing with function, he had to leave the solid 

 ground of visible fact, and to have recourse to arguments and 

 reasoning. He seems to have said to himself, " Here am I a 

 " young man fighting against the world ; it will be enough if I 

 " in the first instance secure a general acceptance of the truths 

 " of structure. When I have done this, I may later on take up 

 " the truths of function ; but for the present I will not risk the 

 " further burden which the introduction of these more debateable 

 " matters would entail, matters moreover about which I clearly 

 " see that if I go very far I shall come into dangerous conflict 

 " with the Church. I will expose, without any shrinking, Galen's 

 " wrong statements about structure ; but, when I have to quote 

 " his views about function, I will expound them without attack- 

 " ing them. I will content myself with letting drop here and 

 " there a hint of my distrust and doubt." 



Let me here briefly describe the main outlines of the 

 Galenic physiology or rather of the central parts of that 

 physiology, the features and uses of the heart and blood 

 vessels. 



The parts of the food absorbed from the alimentary canal 

 are carried by the portal vein to the liver, and by the influence 

 of that great organ are converted into blood. The blood thus 

 enriched by the food is by the same great organ endued with 

 the nutritive properties summed up in the phrase 'natural 

 spirits.' But blood thus endowed with natural spirits is still 

 crude blood, unfitted for the higher purposes of the blood in the 

 body. Carried from the liver by the vena cava to the right side 

 of the heart, some of it passes from the right ventricle through 

 innumerable invisible pores in the septum to the left ventricle. 

 As the heart expands it draws from the lungs through the vein- 

 like artery (or as we now call it pulmonary vein) air into the left 

 ventricle. And in that left cavity the blood which has come 



