202 THE CIRCULATION OF THE BLOOD 



the veins, their intermediate position causing them to respond at once to any 

 alteration in the size or rate of the arterial or venous blood stream. Thus, the 

 apparent contraction of the capillaries, on the application of certain irritating 

 substances or during certain mental states, and their dilatation in blushing 

 may be referred primarily to the corresponding action of the small arteries. 



The Measurement of Velocity in the Capillaries. The observation of 

 Hales, E. H. Weber, and Valentin agree very closely as to the rate of the 

 blood current in the capillaries of the frog; and the mean of their estimates 

 gives the velocity of the systemic capillary circulation at about 0.5 mm. per 

 second. The velocity in the capillaries of warm-blooded animals is greater, 

 in the dog 0.5 to 0.75 mm. per second. This may seem inconsistent with the 

 facts, which show that the whole circulation is accomplished in about half 

 a minute. But the whole length of capillary vessels, through which any 

 given portion of blood has to pass, probably does not exceed 0.5 mm. There- 

 fore the time required for each quantity of blood to traverse its own appointed 

 portion of the general capillary system will scarcely amount to more than a 

 second. This comparatively slow velocity is evidently favorable to the 

 nutritive interchanges that go on through these thin-walled vessels between 

 the blood within the capillaries and the outside active tissues. 



The Venous Flow. The blood current in the veins is maintained, 

 a, primarily by the contractions of the left ventricle ; but very effectual assist- 

 ance to the flow is afforded, b, by the action of the muscles capable of pressing 

 on the veins with valves, and c, by the aspiration of the thorax and possibly, 

 d, by the aspiration of the heart itself. 



The effect of muscular pressure upon the circulation may be thus ex- 

 plained: 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. Thus, whatever force is exercised by the external 

 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. 



The circulation might lose as much as it gains by such an action, if it 

 were not for the numerous communications, or venous anastomoses; for owing 

 to these anastomoses the closing up of the venous channel by the backward 

 pressure is prevented from being any serious hindrance to the circulation, 

 since the blood which is arrested in its onward course 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; the pressure of the blood onward is all advantageous, and the pressure 

 of the blood backward is prevented from being a hindrance by the closure 

 of the valves and of the anastomoses of the veins. 



