THE VELOCITY OF THE ARTERIAL BLOOD FLOW 199 



free from danger through even very slight wounds; for it is only when the 

 artery is closed that the processes for the more permanent and secure pre- 

 vention of bleeding are established. 



The Velocity of the Arterial Blood Flow. The velocity of the 

 blood current at any given point in the various divisions of the circulatory 

 system is inversely proportional to their united sectional area at that point. 

 If the united sectional area of all the branches of a vessel were always the same 

 as that of the vessel from which they arise, and 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 small arteries and in the capillaries would be the 

 same as in the aorta. If a similar correspondence of capacity existed in the 

 veins there would be an equal correspondence in the rapidity of the circula- 

 tion 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 bases, the sectional area of the capillaries, being four hundred 

 to eight hundred times as great as that of the truncated apex representing 

 the aorta. Thus the velocity of blood in the smallest arterioles and the 

 capillaries is not more than one-four-hundredth of that in the aorta. 



The velocity of the stream of blood is greatest in the neighborhood of 

 the heart. The rate of movement is greatest during the ventricular systole 

 and diminishes during the diastole. The rate of flow also decreases along 

 the arterial system, becoming least in the parts of the 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.), see figure 191. 



The Capillary Flow. It is in the capillaries that the chief resistance 

 is offered to the progress of the blood; for in them the friction of the blood 

 is greatly increased by the enormous multiplication of the surface with which 

 it is brought in contact. 



When the capillary circulation is examined in any transparent part of a 

 full-grown living animal by means of the microscope, figures 193, 194, the 

 blood is seen to flow with a constant equable motion; the red blood-corpus- 

 cles moving along, mostly in single file, and bending in various ways to ac- 

 commodate themselves to the tortuous course of the capillary, but instantly 

 recovering their normal outline on reaching a wider vessel, 



At the circumference of the stream and adhering to the walls of the larger 

 capillaries, but especially well marked in the small arteries and veins, there 

 is a layer of plasma which appears to be motionless. The existence of this 

 still layer, as it is termed, is inferred both from the general fact that such a 

 one exists in all fine tubes traversed by fluid, and from what can be seen in 

 watching the movements of the blood-corpuscles. The red corpuscles occupy 

 the middle of the stream and move with comparative rapidity; the color- 

 less corpuscles run much more slowly by the walls of the vessels; while next 



