2/6 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. xxi. 



side of them in the arteries or the veins ; their intermediate 

 position causes them to feel at once any alteration in the size, 

 rate, or pressure of the arterial or venous blood-stream. Thus, the 

 apparent contraction of the capillaries, on the application of certain 

 irritating substances, and during fear, and their dilatation in blush- 

 ing, may be referred primarily to the action of the small arteries. 



The Venous Flow. 



The blood-current in the veins is maintained (a) primarily by 

 the vis a tergo, that is, the force behind, which is the blood 

 pressure transmitted from the heart and arteries; but very 

 effectual assistance to the flow is afforded (b) by the action of the 

 muscles capable of pressing on the veins with valves, as well as 

 (c) by the suction action of the heart, and the aspiratory action 

 of the thorax (vis a f route). 



The effect of muscular pressure upon the circulation may be 

 thus explained. When pressure is applied to any part of a vein 

 and the current of blood in it is obstructed, the portion behind 

 the seat of pressure becomes swollen and distended as far back 

 as the next pair of valves, which are in consequence closed 

 (fig. 221, B, p. 218). Thus, whatever force is exercised by the 

 pressure of the muscles on the veins, is distributed partly in press- 

 ing 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 an 

 action, if it were not for the mimerous communications which 

 the veins make with one another ; 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. Thus, 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 back- 

 wards is prevented from being a hindrance by the closure of the 

 valves and the anastomoses of the veins. 



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

 valves, and possess the remarkable property of rhythmical con- 

 traction and dilatation, whereby the current of blood within 

 them is distinctly accelerated (Wharton Jones). The contraction 

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



