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 proposition 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 cognisable by the senses." Harvey returns again to his anatomical demonstrations to prove his point. He explains the true uses of the valves in the veins, whose existence, he says, were known to his old teacher "Hieronymus Fabricius, of Aquapendente, a most skilful anatomist and venerable old man.... The discoverer of these valves did not rightly understand their use, nor have succeeding anatomists added anything to our knowledge; for their office is by no means explained when we are told that it is to hinder the blood by its weight from all flowing into the inferior part; for the edges of the valves in the jugular veins hang downwards, and are so contrived that they prevent the blood from rising upwards; the valves, in a word, do not invariably look upwards, but always towards the trunks of the veins, invariably towards the seat of the heart. Let it be added that there are no valves in the arteries, and that dogs, oxen, &c., have invariably valves at the divisions of their crural veins, in the veins that meet towards the top of the os sacrum, and in those branches which come from the haunches, in which no such effect of gravity from the erect position was to be apprehended." "The valves are solely made and instituted lest the blood should pass from the greater into the lesser veins, and either rupture them or cause them to become varicose.... The delicate valves, whilst they readily open in the right direction, entirely prevent all contrary movement.... And this I have frequently experienced in my dissections of the veins: if I attempted to pass a probe from the trunk of the veins into one of the smaller branches, whatever care I took, I found it impossible to introduce it far any way, by reason of the valves; whilst, on the contrary, it was most easy to push it along in the opposite direction from without inwards, or from the branches towards the trunks and roots." He concludes his argument by again pointing out that the uses of the valves can be clearly shown in an arm which has been tied up for phlebotomy, and that the valves are best seen in labouring people. The fourteenth chapter is devoted to the "Conclusion of the Demonstration of the Circulation." It runs thus:-- "And now I may be allowed to give in brief my view of the circulation of the blood, and to propose it for general adoption.