THE CIKCULATION OF THE *BLOOD. 157 



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 to the next pair of valves. These, acting 

 like the semilunar valves of the heart, and being, like them, inextensible 

 both in themselves and at their margins of attachment, do not follow the 

 vein in its distention, but are drawn out towards the axis of the canal. 

 Then, if the pressure continues on the vein, the compressed blood, tend- 

 ing to move equally in all directions, presses the valves down into con- 

 tact at their free edges, and they close the vein and prevent regurgitation 

 of the blood. 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 back- 

 wards and closing the valves behind (Fig. 109, A and B). 



The circulation might lose as much as it gains by such compression 

 of the veins, if it were not for the numerous anastomoses 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 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, 

 therefore, the effect of muscular pressure upon veins which have valves, 

 is turned almost entirely to the advantage of the circulation; the pres- 

 sure 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 accelera- 

 tion of the stream of blood, when, in venesection, the muscles of the 

 fore-arm are put in action, and by the general acceleration of the circu- 

 lation during active exercise: and the numerous movements which are 

 continually taking place in the body while awake, though their single 

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

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

 tinues unimpaired in parts at rest, in paralyzed limbs, and in parts in 

 which the veins are not subject to any muscular pressure/ 



Rhythmical Contraction of Veins. 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 contrac- 

 tion occurs, 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 frequently observed in other animals. 



Blood-Pressure in the Veins. The blood -pressure gradually les- 

 sens as we proceed from arteries near the heart to those more remote, and 



