VELOCITY OF THE BLOOD IN THE BLOOD-VESSELS. 183 



(3.) Lumen. The velocity of the current, therefore, in various 

 sections of the vessels must be inversely as their lumen. 



(4.) Capillaries. Hence, the velocity must diminish very consider- 

 ably as we pass from the root of the aorta and the pulmonary artery 

 towards the capillaries, so that the velocity in the capillaries of mammals 

 0'8 millimetre per sec.; frog=0'53 mm. (E.H.Weber); man = 0'6 

 to 0'9 (C. Vierordt). According to A. W. Volkmann the blood in 

 mammalian capillaries flows 500 times slower than the blood in the 

 aorta. Hence the total sectional area of all the capillaries must be 

 500 times greater than that of the aorta.. Bonders found the velocity 

 of the stream in the small afferent arteries, to be 10 times faster than 

 in the capillaries. A pulsatory acceleration, more rapid during its first 

 phase, is observable in the small arteries, although these are not 

 themselves distended thereby. 



Veins. The current becomes accelerated in the veins, but in the 

 larger trunks it is 0'5 to 075 times less than in the corresponding 

 arteries. 



(5.) Mean Blood-Pressure. The velocity of the blood does not 

 depend upon the mean blood-pressure, so that it may be the same in 

 congested and in anaemic parts (Volkmann, Hering). 



(6.) Difference of Pressure. On the other hand, the velocity in any 

 section of a vessel is dependent on the difference of the pressure which 

 exists at the commencement and at the end of that particular section 

 of a blood-vessel; it depends, therefore, on (1) the vis a tercjo (i.e., the 

 action of the heart), and (2) on the amount of the resistance at the 

 periphery (dilatation or contraction of the small vessels C. Ludwig 

 and Dogiel). 



(7.) Pulsatory Acceleration. With every pulse-beat a corresponding 

 acceleration of the blood-current (as well as of the blood-pressure) takes 

 place in the arteries, so that every ascent of the sphygmogram corre- 

 sponds to an acceleration, and every descent to a diminished velocity of 

 the blood-stream. The variations in the velocity caused by the heart- 

 beat are recorded in Fig. 84, obtained by Chauveau's dromograph from 

 the carotid of a horse. The velocity-curve corresponds with a sphyg- 

 mogram P represents the primary elevation and B, the dicrotic wave. 

 This acceleration, as well as the pulse, disappears in the capillaries. 

 In large vessels, Vierordt found the increase of the velocity during 

 the systole to be greater by to -^ than the velocity during the 

 diastole. 



(8.) Respiratory Effect. Every inspiration retards the velocity in the 

 arteries, every expiration aids it somewhat; but the value of these 

 agencies is very small. 



If we compare what has already been said regarding the effect of the respiration 



