VALVES OF THE VEINS. 309 



traced into their folds. There exists, also, a fibrous ring fol- 

 lowing the line of attachment of the valvular curtains to the 

 vein, which renders the vessel much stronger and less dilata- 

 ble here than in the spaces between the valves. The valves 

 are by far the most numerous in the veins of the lower ex- 

 tremities. They are generally situated just below the point 

 where a small vein empties into one of larger size, so that the 

 blood, as it passes in, finds an immediate obstacle to passasre 



7 JL -L O 



iii the wrong direction. The situation of the valves may be 

 readily observed in any of the superficial veins. If the flow 

 of blood be obstructed, little knots will be formed in the con- 

 gested vessels, which indicate the position and action of the 

 valves. The simple experiment of Harvey, already referred 

 to, presents a striking illustration of the action of the valves. 

 When the vein is thus congested and knotted, if the finger be 

 pressed along the vessel in the direction of the blood current, a 

 portion situated between two valves may be emptied of blood ; 

 but it is impossible to empty any portion of the vessel by 

 pressing the blood in the opposite direction. On slitting 

 open a vein, we observe the shape, attachment, and extreme 

 delicacy of structure of the valves. When the vessel is 

 empty, or when fluid moves toward the heart, they are closely 

 applied to the walls ; but if liquid or air be forced in the 

 opposite direction, they project into its caliber, and by the 

 application of their -free edges to each other, effectually pre- 

 vent any backward current. Fabricius noted the following 

 peculiarity in the arrangement of the valves. When closed, 

 the application of their free edges forms a line which runs 

 across the vessel ; it is found that in successive sets of valves 

 these lines are at right angles to each other, so that if in one 

 set, this line has a direction from before backwards, in the 

 sets above and below the lines run from side to side. 



There are certain exceptions to the general proposition 

 that the veins of the great cavities are not provided with 

 valves. Yalves are found in the portal system of some of 

 the inferior animals, as the horse. They do not exist, how- 



