308 MANUAL OF PHYSIOLOGY. 



4. The time of day. It gains in rapidity in the morning till 9 

 o'clock, and in the evening till 6 o'clock, and falls in the day- 

 time, being at its minimum at midnight. 



5. Muscular exercise quickens it. 



6. It is quicker during inspiration than expiration. 



7. It increases with increase of temperature. 



8. It is variously affected by emotions. 



VELOCITY OF THE BLOOD CURRENT. 



The velocity of the blood must not be confounded with the 

 velocity of the pulse wave, which bears to it the same relation as 

 the surface waves on a river do to the rate of the stream of water. 



It has already been mentioned that the general bed of the 

 blood increases from the aorta to the capillaries, and decreases 

 from the capillaries to the vena cava, on account of the branches 

 or tributaries of nearly every artery or vein being collectively 

 of larger area than the vessel from which they spring or to which 

 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. 283. 



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. 



