CH. XXII.] THE VENOUS FLOW 299 



the lost tissue is replaced by a regeneration of the surrounding 

 tissues.* 



The circulation through the capillaries must, of necessity, be 

 largely influenced by that which occurs in the vessels on either 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 pres- 

 sure of the arterial, and more especially of the venous blood-stream. 

 The apparent contraction of the capillaries, on the application of 

 certain irritating substances, and during fear, and their dilatation in 

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



The Venous Plow. 



The blood-current in the veins is maintained 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 assist- 

 ance to the flow is afforded by the action of the muscles capable of 

 pressing on the veins with valves, as well as by the suction action 

 of the heart, and the aspiratory action of the thorax (vis a 

 fronte). 



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. 202, B, p. 220). 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 an action, 

 if it were not for the numerous 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, the onward course of 

 which 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 circula- 

 tion. 



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

 and possess the remarkable property of rhythmical contraction and 

 dilatation, whereby the current of blood within them is distinctly 

 accelerated (Wharton Jones). The contraction occurs, on an average, 



* This question is closely related to that of immunity, which is discussed in the 

 chapter on the Blood (Chapter XXIX.). 



