296 THE CIRCULATION IN THE BLOOD-VESSELS [CII. XXI. 



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 trans- 

 mitted from the heart and arteries ; but very effectual assistance 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. 230, 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 circulation. 



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, 

 about ten times in a minute ; the existence of valves prevents regur- 

 gitation, so the entire effect of the contractions is auxiliary to the 

 onward current of blood. Analogous phenomena have been observed 

 in other animals. 



A venous pulse is observed under the conditions previously 

 described (p. 294), when the arterioles are dilated so that the arterial 

 pulse passes through the capillaries to the veins. 



A venous pulse is also seen in the superior and inferior vena 

 cava near to their entrance into the heart ; this corresponds to varia- 

 tions of the pressure in the right auricle. When the ventricle is con- 



