PRESSURE IX VEINS. 



I/I 



themselves and at their margins of attachment, do not 

 follow the vein in its distension, but are drawn out towards 

 the axis of the canal. Then, if the pressure continues on 

 the vein, the compressed blood, tending to move equally in 

 all directions, presses the valves down into contact at their 

 free edges, and they close the vein and prevent regurgita- 

 tion of the blood. Thus, whatever force is exercised by 

 the pressure of the muscles on the veins, is distributed partly 

 in pressing the blood onwards in the proper course of the 

 circulation, and partly in pressing it backwards and closing 

 the valves behind. 



The circulation might lose as much as it gains by such 

 compression of the veins, if it were not for the numerous 

 anastomoses by which they communicate, one with another ; 



Fig. 55-t 



for through these, the closing up of the venous channel by 

 the backward pressure is prevented from being any serious 



* Fig. 54. Vein with valves open (Dalton). 



f Fig. 55. Vein with valves closed ; stream of blood passing off by 

 lateral channel (Dalton). 



