VELOCITY OF THE BLOOD. 1 35 



artery and aorta, notwithstanding the very unequal blood-pressure in these two 



ve 



(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 considerably 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, so that the total sectional area of all the capillaries must be 

 500 times greater than that of the aorta. Donders found the velocity of the stream 

 in the small afferent arteries to be 10 times faster than in the capillaries. 



Veins. The current becomes accelerated in the veins, but in the larger trunks it 

 is 0*5 to 0*75 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 ansemic parts 

 (Volkmann, Hexing). 



(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 com- 

 mencement and at the end of that particular section of a blood-vessel"; it depends, 

 therefore, on (1) the vis atergo (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). 



Corresponding to the smaller difference in the arterial and venous pressure in the foetus ( 85), 

 the velocity of the blood is less in this case (Cohnstein and Zuntz). 



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

 of the blood-current (as w^ell as of the blood-pressure) takes place in the arteries 

 (pp. 126, 133). In large vessels, Vierordt found the increase of the velocity during 

 the systole to be greater by \ to ^ than the velocity during the diastole. The 

 variations in the velocity caused by the heart-beat are recorded in fig. 113, obtained 

 by Chauveau's dromograph from the carotid of a horse. The velocity curve corre- 

 sponds with a sphygmogram P represents the primary elevation and R the 

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

 A pulsatory acceleration, more rapid during its first phase, is observable in the 

 small arteries, although the arteries themselves are not distended thereby. 



(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 on the con- 

 traction and dilatation of the heart and on the blood-stream ( 60), it is clear that respiration 

 favours the blood-stream, and so does artificial respiration. When artificial respiration is inter- 

 rupted, the blood-stream becomes slower (Dogiel). If the suspension of respiration lasts some- 

 what longer, the current is again accelerated on account of the dyspnceic stimulation of the 

 vaso-motor centre (Reidenhain) ( 371, I.). 



(9) Modifying Conditions. Many circumstances affect the velocity of the blood 

 in the veins. (1) There are regular variations in the large veins near the heart 

 due to the respiration and the movements of the heart ( 50 and 60). (2) Irregular 

 variations due to pressure, e.g., from contracting muscles ( 87), friction on the 

 skin in the direction or against the direction of 4 the venous current ; the position 

 of a limb or of the body. The pump-like action of the veins of the groin on 

 moving the leg has been referred to ( 87). When the lower limb is extended and 

 rotated outwards, the femoral vein in the iliac fossa collapses, owing to an internal 

 negative pressure ; when the thigh is flexed and raised, it fills under a positive 

 pressure (Braune). A similar condition obtains in walking. 



91. CAPACITY OF THE VENTRICLES. Vierordt calculated the capacity of the left 

 ventricle from the velocity of the blood-stream, and the amount of blood discharged per second 



