314 MANUAL OF PHYSIOLOGY. 



they may lead ; or, in other words, if we imagined the whole 

 vascular system fused together into one tube it would form two 

 somewhat irregular cones, one corresponding to the arteries and 

 the other to the veins, with their bases placed at the capillaries 

 and their apices at the heart. Between the two a still wider 

 aggregate would represent the capillaries. Compare Fig. 128, 

 p. 289. 



Since the same quantity of blood must pass through each sec- 

 tion of these cones in a given time, the rate at which it flows must 

 vary greatly in the different parts, being faster in proportion as 

 the diameter of the part is narrower, in accordance with the well- 

 known physical law that with the same amount of liquid flowing 

 its velocity changes inversely with the diameter of the tube. 

 Thus, the mean velocity of the flow in the arteries becomes slower 

 and slower as the capillaries are approached, and in the wide bed 

 of the latter the rate of the current is reduced to a minimum. In 

 the small veins the rate is slower than in the larger trunks, but 

 on the venous side its rapidity never reaches that of the aorta, 

 where it may be said to move at least twice as quickly as in the 

 vena cava. 



The following table may be useful in giving a general idea of 

 the average velocity in different parts of the circulation : 



Near valves of aorta while the ventricles are contracting it 

 reaches 1200 mm. per sec. 



Descending aorta, 300-600 " 



Carotid, . . . 205-357 



Radial, 100 



Metatarsal, 57 * 



Arterioles, 50 



Capillaries, .5 " 



Venous radicles, 25 



Small veins on dorsum of hand, . . 50 



Venae cavae, 200 



In the aorta, near the valves, the blood current varies in 

 rapidity, because the flow through the aortic orifice is intermit- 

 tent, and this variation must be more or less communicated to 

 the neighboring arteries in the form of an increase of rapidity 

 coincident with the beat of the arterial pulse. The variation in 



