126 WILLIAM HARVEY 



from the centre to the extremities; but the delicate valves, 

 while they readily open in the right direction, entirely pre- 

 vent all such contrary motion, being so situated and ar- 

 ranged, that if anything escapes, or is less perfectly ob- 

 structed by the cornua of the one above, the fluid passing, 

 as it were, by the chinks between the cornua, it is im- 

 mediately received on the convexity of the one beneath, 

 which is placed transversely with reference to the former, 

 and so is effectually hindered from getting any farther. 



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. In many places two valves are so placed and 

 fitted, that when raised they come exactly together in the 

 middle of the vein, and are there united by the contact of 

 their margins ; and so accurate is the adaptation, that neither 

 by the eye nor by any other means of examination, can the 

 slightest chink along the line of contact be perceived. But 

 if the probe be now introduced from the extreme towards 

 the more central parts, the valves, like the floodgates of 

 a river, give way, and are most readily pushed aside. The 

 effect of this arrangement plainly is to prevent all motion of 

 the blood from the heart and vena cava, whether it be up- 

 wards towards the head, or downwards towards the feet, 

 or to either side towards the arms, not a drop can pass; all 

 motion of the blood, beginning in the larger and tending 

 towards the smaller veins, is opposed and resisted by them; 

 whilst the motion that proceeds from the lesser to end in the 

 larger branches is favoured, or, at all events, a free and 

 open passage is left for it 



But that this truth may be made the more apparent, 

 let an arm be tied up above the elbow as if for phlebotomy 

 (A, A, fig. i). At intervals in the course of the veins, 

 especially in labouring people and those whose veins are 

 large, certain knots or elevations (B, C, D, E, F) will be 

 perceived, and this not only at the places where a branch 





