172 THE CIRCULATION. 



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 

 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 venesec- 

 tion, the muscles of the fore-arm are put in action, and by 

 the general acceleration of the circulation during active 

 exercise; and the numerous movements which are con- 

 tinually taking place in the body while awake, though 

 their single effects may be less striking, must be an im- 

 portant auxiliary to the venous circulation. Yet they 

 are not essential ; for the venous circulation continues 

 unimpaired in parts at rest, in paralysed limbs, and in 

 parts in which the veins are not subject to any muscular 

 pressure. 



Besides the assistance thus afforded by muscular pressure 

 to the movement of blood along veins possessed of valves, 

 it has been discovered by Mr. Wharton Jones that, in the 

 web of the bat's wing, the veins are furnished with valves, 

 and possess the remarkable property of rhythmical contrac- 

 tion and a dilatation, whereby the current of blood within 

 them is distinctly accelerated. The contraction occurred, 

 on an average, about ten times in a minute ; the existence 

 of valves preventing regurgitation, the entire effect of the 

 contractions was auxiliary to the onward current of blood. 

 Analogous phenomena have been now frequently observed 

 in other animals. 



