CHAPTER XIII 



THE THIRD POSITION IS CONFIRMED : AND THE CIRCU- 

 LATION OF THE BLOOD IS DEMONSTRATED FROM IT 



THUS far have we spoken of the quantity of blood 

 passing through the heart and the lungs in the centre 

 of the body, and in like manner from the arteries into 

 the veins in the peripheral parts and the body at large. 

 We have yet to explain, however, in what manner the 

 blood finds its way back to the heart from the ex- 

 tremities by the veins, and how and in what way these 

 are the only vessels that convey the blood from the 

 external to the central parts ; which done, I conceive 

 that the three fundamental propositions laid down for 

 the circulation of the blood will be so plain, so well 

 established, so obviously true, that they may claim 

 general credence. Now the remaining position will be 

 made sufficiently clear from the valves which are 

 found in the cavities of the veins themselves, from the 

 uses of these, and from experiments cognizable by the 

 senses. 



The celebrated Hieronymus Fabricius of Aquapen- 

 dente, a most skilful anatomist, and venerable old man, 

 or, as the learned Riolan will have it, Jacobus Silvius, 

 first gave representations of the valves in the veins, 

 which consist of raised or loose portions of the inner 

 membranes of these vessels, of extreme delicacy, and 

 a sigmoid or semilunar shape. They are situated at 

 different distances from one another, and diversely in 

 different individuals ; they are connate at the sides of 

 the veins ; they are directed upwards or towards the 



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