THE CIRCULATION" OF THE BLOOD. 22 ~> 



pressure is prevented from being any serious hindrance to the circula- 

 tion, 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 mus- 

 cular 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 the anastomoses 

 of the veins. 



The venous flow is also assisted by the suction action of the heart, 

 since at some time during every cardiac cycle the pressure falls below 

 that of the atmosphere. 



The aspiration of the thorax 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 of the Flow. 



The velocity of the blood-current at any given point in the various 

 divisions of the circulatory system is inversely proportional to their sec- 

 tional area at that point. If the sectional area of all the branches of a 

 vessel united were always the same as that of the vessel from which they 

 arise, am if the aggregate sectional area of the capillary vessels were 

 equal to that of the aorta, the mean rapidity of the blood's motion in the 

 capillaries would be the same as in the aorta; and if a similar corre- 

 spondence of capacity existed in the veins and arteries, there would be 

 an equal correspondence in the rapidity of the circulation in them. But 

 the arterial and venous systems may be represented by two truncated 

 cones with their apices directed toward the heart; the area of their 

 united base (the sectional area of the capillaries) being 400800 times 

 as great as that of the truncated apex representing the aorta. Thus 

 the velocity of blood in the capillaries is not more than jiy- of that in 

 the aorta. 



In the Arteries. The velocity of the stream of blood is greater in 

 the arteries than in any other part of the circulatory system, and in them 

 it is greatest in the neighborhood of the heart, and during the ventricu- 

 lar systole. The rate of movement diminishes during the diastole of 

 the ventricles, and in the parts of the arterial system most distant from 

 the heart. Chauveau has estimated the rapidity of the blood -stream in 

 the carotid of the horse at over 20 inches per second during the heart's 

 systole, and nearly 6 inches during the diastole (520-150 mm.). 



