224 THE CIRCULATION OF THE BLOOD 



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 circulation; 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. 



The venous flow is also assisted by the aspiration of the thorax and to 

 some extent by that of the heart, since at some time during every cardiac 

 cycle the intra- auricular and intraventricular pressure falls below that of 

 the atmosphere. This activity will be considered more fully in the chapter 

 on Respiration. In this connection it may be said, however, that the pressure 

 in the great veins falls during inspiration and rises during expiration. 



The Velocity in the Veins. The velocity of the blood is greater 

 in the veins than in the capillaries, but less than in the arteries; this fact 

 depending upon the relative capacities of the arterial and venous systems. 

 If an accurate estimate of the proportionate areas of arteries and the veins 

 corresponding to them could be made, we might, from the velocity of the 

 arterial current, calculate that of the venous. The usual estimation is that 

 the capacity of the veins is about two or three times a* great as that of the 

 arteries, and that the velocity of the blood's motion is, therefore, about one- 

 half or one- third as great in the veins as in the arteries, i.e., 200 mm. a second. 

 The rate at which the blood moves in the smallest venules is only slightly 

 greater than that in the capillaries, but the speed of flow gradually increases 

 the nearer the vessel approaches to the heart. The sectional area of the 

 venous trunks, compared with that of the branches opening into them, 

 becomes gradually smaller as the trunks advance toward the heart, 

 figure 191. 



The Velocity of the Circulation as a Whole. It would appear that 

 a portion of blood can traverse the entire course of the circulation, in the 

 horse, in half a minute. Of course it would require longer to traverse 

 the vessels of the most distant part of the extremities than to go through 

 those of the neck, but taking an average length of the vessels to be traversed 



