64 MOTION OF THE 



as it would have to pass from larger into continually smaller 

 vessels, being separated from the mass and fountain head, and 

 attaining from warmer into colder places. 



But 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 ; lest, instead 

 of advancing from the extreme to the central parts of the body, 

 the blood should rather proceed along the veins from the centre to 

 the extremities ; but the delicate valves, while they readily open 

 in the right direction, entirely prevent all such contrary motion, 

 being so situated and arranged, that if anything escapes, or is 

 less perfectly obstructed by the cornua of the one above, the 

 fluid passing, as it were, by the chinks between the cornua, it 

 is immediately 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 ex- 

 actly together in the middle of the vein, and are there united 

 by the contact of their margins ; and so accurate is the adapta- 

 tion, that neither by the eye nor by any other means of exami- 

 nation 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 upwards towards 

 the head, or downwards towards the feet, or to either side to- 

 wards 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 pro- 

 ceeds from the lesser to end in the larger branches is favoured, 

 or, at all events, a free and open passage is left for it. 



