Heart and Blood 81 



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 centra) 

 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 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 is received (E, F), but also where 

 none enters (c, D) : these knots or risings are all formed 

 by valves, which thus show themselves externally. And 

 now if you press the blood from the space above one of 

 the valves, from H to o, (fig. 2,) and keep the point of 

 a finger upon the vein inferiorly, you will see no influx 

 of blood from above ; the portion of the vein between the 

 point of the finger and the valve o will be obliterated ; 

 yet will the vessel continue sufficiently distended above 

 that valve (o, G). The blood being thus pressed out, 

 and the vein emptied, if you now apply a finger of the 

 other hand upon the distended part of the vein above 

 the valve o, (fig. 3,) and press downwards, you will find 

 that you cannot force the blood through or beyond the 

 valve ; but the greater effort you use, you will only see 

 the portion of vein that is between the finger and the 

 valve become more distended, that portion of the vein 



G 



