HARVEY'S WORK 61 



weight, from all flowing into inferior parts." They do 

 not, in fact, always do this, " for the valves in the jugular 

 veins [in the neck] hang downwards, and are so arranged 

 that they prevent the blood from rising upwards ; in 

 a word, the valves do not invariably look upwards, but 

 always towards the trunks of the veins, invariably towards 

 the seat of the heart" 



1 6. Now comes the real explanation of the action of 

 the valves of the veins, on which much of Harvey's 

 theory turns. These valves are solely lest the blood 

 should pass from the greater into the lesser veins . . . 

 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." 



* And this I have frequently experienced in dissec- 

 tions of the veins. If I attempted to pass a probe from 

 the trunk of the veins into one of the smaller branches, 

 I found it impossible to introduce it far by reason of the 

 valves ; whilst, on the contrary, it was easy to push it 

 along in the opposite direction, from the branches to- 

 wards the trunk." 



That all this may be made the more apparent, let an 

 arm be tied up above the elbow at A, A (Plate VII. Fig. i). 

 In the course of the veins, especially in labouring men and 

 those whose veins are large, are certain knots or elevations 

 as at B, C, D, E, and F, which will now be seen. These 

 knots are not only at the places where the veins branch, as 

 at E and F, but also where they do not, as at C and D. 

 These knots are formed by valves, which thus show them- 

 selves externally." 



' If you now press blood from the space above one of 

 the valves, as from H to O (Plate VII. Fig. 2), and keep the 

 point of the finger upon the vein below, you will see no 

 influx of blood from above. The portion of the vein be- 

 tween the point of the finger and the valve O will remain 



