THE VELOCITY OF THE CIRCULATION 235 



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

 were not for the numerous communications, or venous anastomoses. 

 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 hin- 

 drance by the closure of the valves and by 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 intra-ventricular 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 pres- 

 sure 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 depend- 

 ing 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 as 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 

 total 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 it may be concluded that half a minute represents the average 

 rate. Stewart estimated that the circulation time in man is probably not 

 less than twelve nor more than fifteen seconds. 



Satisfactory data for these estimates are afforded by the results of. expe- 

 riments to ascertain the rapidity with which chemicals introduced into the 

 blood are transmitted from one part of the vascular system to another. The 



