144 



THE CIRCULATION. 



toraoses 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 hin- 

 drance 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. 55. 



FIG. 54. Vein with valves open (Dalton). 



FIG. 55. Vein with valves closed ; stream of blood passing off by lateral channel 

 (Dalton). 



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 being 

 a hindrance by the closure of the valves and the anastomoses 

 of the veins. 



The effects of such muscular pressure are well shown by 

 the acceleration of the stream of blood when, in venesection, 

 the muscles of the forearm are put in action, and by the 

 general acceleration of the circulation during active exercise ; 

 and the numerous movements which are continually taking 

 place in the body while awake, though their single effects may 

 be less striking, must be an important auxiliary to the venous 

 circulation. Yet they are not essential ; for the venous circu- 

 lation continues unimpaired in parts at rest, in paralyzed 

 limbs, and in parts in which the veins are not subject to any 

 muscular pressure. 



