162 HAND-BOOK OF PHYSIOLOGY. 



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 

 semiluiiar 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 distension, but are drawn out toward the axis of the canal. Then, if 

 the pressure continues on the vein, the compressed blood, tending to move 

 equally in all directions, presses the valves down into contact 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 onward in the proper 

 course of the circulation, and partly in pressing it backward and closing 

 the valves behind (Fig. 128, 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 pressure 

 of the blood onward is all advantageous, and the pressure of the blood back- 

 ward 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 circulation 

 during active exercise: and the numerous movements which are continu- 

 ally taking place in the body while awake, though their single effects may 

 be less striking, must be an important auxiliary to the venous circulation. 

 Yet they are not essential; for the venous circulation continues unim- 

 paired 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 contraction occurs, 

 on an average, about ten times in a minute; the existence of valves pre- 

 venting regurgitation, the entire effect of the contractions was auxiliary 

 to the onward current of blood. Analogous phenomena have been fre- 

 qu3iitly observed in other animals. 



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

 as we proceed from the heart to the arteries, from these to the capillaries, 

 .and thence along the veins to the right auricle. The blood-pressure in 



