VELOCITY OF THE BLOOD CURRENT. 313 



spondiug to the closed tube), a positive wave centripetal is re- 

 flected, and is added to the pulse wave so as to diminish the di- 

 crotic notch, and give the curve known as characteristic of " high- 

 tension" pulse, as in Bright's disease. (Fig. 141, II.) On the 

 other hand, when the arterioles are widely dilated (corresponding 

 to the open condition of the tube), a negative wave is deflected, 

 and is subtracted from the force of the pulse wave so as to exag- 

 gerate the dicrotic notch, and give the tracing characteristic of 

 the " low-tension " pulse seen in fever, etc. (Fig. 141, III.) 



The mean rate of the pulse varies in different individuals, 

 seventy-two per minute being a fair average for a middle-aged 

 adult. It varies also with many circumstances, which must be 

 borne in mind in taking the pulse as a clinical guide. 



1. Age. At birth it is about 140 per minute, and is, generally 

 speaking, quicker in young than in old people, commonly falling 

 to 60 in aged persons. 



2. Sex. It is more rapid in females than in males. 



3. Position. It is quicker standing than lying, particularly if a 

 patient who has been lying down, stand or sit up, the pulse be- 

 comes more rapid. 



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. 



G. 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 

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