THE VELOCITY IN THE VEINS 203 



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 intraauricular 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 estimate 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, for the sectional area of the 

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

 comes 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 experi- 

 ments to ascertain the rapidity with which chemicals introduced into the blood 

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

 required for the passage of solutions of potassium ferrocyanide, mixed with 

 the blood, from one jugular vein, through the right side of the heart, the 

 pulmonary vessels, the left cavities of the heart, and the general circulation, 

 to the jugular vein of the opposite side, varies from twenty to thirty seconds 

 in the dog. The same substance is transmitted from the jugular vein to the 

 great saphenous vein in twenty seconds; from the jugular vein to the mes- 

 enteric artery in between fifteen and thirty seconds; to the facial artery, 

 in one experiment, in between ten and fifteen seconds; in another experi- 

 ment, in between twenty and twenty-five seconds; in its transit from the 

 jugular vein to the metatarsal artery, it occupies between twenty and thirty 

 seconds. The result is said to be nearly the same whatever the rate of the 

 heart's action. In more recent methods some innocuous dye like methylene 



