220 THE CIRCULATION OF THE BLOOD 



times inordinately great. The presence of a muscular element, however, 

 provides for a certain uniformity in the amount of pressure exercised: the 

 muscles are in constant action or tone, and it is by this adaptive, uniform, 

 gentle muscular contraction that the normal tone of the blood vessels is 

 maintained. Deficiency of this tone is the cause of the soft and yielding 

 arterial pulse, and the sluggish blood flow through the arterioles. 



Incidentally it may be mentioned that the elastic and muscular contrac- 

 tion of an artery may also be regarded as fulfilling a natural purpose when, 

 the artery being cut, the sudden contraction at first limits, and then, in con- 

 junction with the coagulated fibrin, completely arrests, the flow of blood. 

 It is only in consequence of such contraction and coagulation that we are 

 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 the united sectional area at that point. If the 

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

 as the area 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 volume of 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 about eight hundred times as great as that of the truncated apex rep- 

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

 and the capillaries is about one-eight-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 



