124 WILLIAM HARVEY 



springs from the puncture in the vein, and flows in a 

 tinuous stream. 



CHAPTER XIII 



THE THIRD POSITION is CONFIRMED: AND THE CIRCULATION 

 OF THE BLOOD is DEMONSTRATED FROM IT 



THUS far we have 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 extremities 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 suf- 

 ficiently 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 Aquapendente, 

 a most skilful anatomist, and venerable old man, or, as the 

 learned Riolan will have it, Jacobus Silvius, first gave rep- 

 resentations 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 up- 

 wards towards the trunks of the veins; the two for there 

 are for the most part two together regard each other, 

 mutually touch, and are so ready to come into contact by 

 their edges, that if anything attempts to pass from the 

 trunks into the branches of the veins, or from the greater 

 vessels into the less, they completely prevent it; they are 

 farther so arranged, that the horns of those that succeed are 

 opposite the middle of the convexity of those that precede, 

 and so on alternately. 





