182 



THE CIRCULATION. 



ever 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 ; 

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

 the backward pressure is prevented from being any serious 

 hindrance to the circulation, since the blood, of which the 

 onward course is arrested by the closed valves, can at once 

 pass through some anastomosing channel, and proceed on its 

 way by another vein (figs. 54 and 55). Thus, therefore, the 



Fig. 54." 



Fig. 55-t 



effect of muscular pressure upon veins which have valves, is 

 turned almost entirely to the advantage of the circulation 

 the pressure of the blood onwards is all advantageous, and 

 the pressure of the blood backwards is prevented from 



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



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

 lateral channel (Dalton). 



