VELOCITY OF THE CURRENT. 175 



THE VELOCITY OF THE CURRENT IN THE ARTERIES, 

 CAPILLARIES, AND VEINS. 



In analyzing the results of observation on the velocity of the blood 

 it must be constantly borne in mind that the sectional area of the 

 arterial system beginning with the trunk of the aorta increases pro- 

 gressively by subdivision of the branches, so that in the capillary 

 system the sectional area of the blood-channel is increased yoo-fold 

 and more. From this point, owing to the reunion of the venous trunks, 

 the sectional area again diminishes, but it is still greater than at the 

 beginning of the arterial system. 



Exceptions are found in the common iliac arteries, which, taken together, 

 are narrower than the trunk of the aorta. The cross-section of the four pulmo- 

 nary veins, taken together, is also somewhat smaller than that of the pulmonary 

 artery. 



An equal quantity of blood must pass through each successive trans- 

 verse section of both the greater and the lesser circulation. Therefore, 

 the same quantity of blood must flow through the aorta and the pul- 

 monary artery in spite of the great difference between the pressure in 

 the two vessels. 



The velocity of the blood-current in the individual transverse sections 

 of the blood-channel must, thus, be inversely proportional to the 

 lumen or their sectional area. 



Hence, there is a marked progressive diminution in the velocity 

 from the root of the aorta and pulmonary artery to the capillaries; 

 so that in mammals it is only 0.8 mm. a second (in the frog 0.53 mm.), 

 and in man from 0.6 to 0.9 mm. According to A. W. Volkmann the 

 velocity of the blood in mammals is 500 times less in the capillaries 

 than in the aorta. Therefore, the total cross-section of all the capil- 

 laries must be 500 times greater than that of the aorta. In the small 

 afferent arteries Bonders found that the velocity was still 10 times 

 greater than in the capillary vessels. 



In the venous trunks the velocity again becomes accelerated, being, 

 in the large trunks, from 0.5 to 0.75 times less than in the correspond- 

 ing arteries. 



The velocity of the blood-current does not depend on the height of 

 the mean blood-pressure, and it may accordingly remain the same both 

 in anemic and in plethoric vessels. 



On the other hand, the velocity in a given section of the circulation 

 is determined by the difference between the pressure in the cross-section 

 at the beginning and that at the end of the section. It will, therefore, 

 depend on, i, the vis a tergo (heart's action) and, 2, the amount of 

 resistance at the periphery (dilatation or narrowing of the smaller 

 vessels) to the arterial current. 



In accordance with the slight difference in pressure in the arterial and 

 venous systems in the fetus the velocity here is low. 



In the arteries every pulse-beat causes an acceleration in the move- 

 ment of the current (as well as an increase in the blood-pressure) corre- 

 sponding to the form of the pulse-curve. In large vascular trunks 

 C. Vierordt found the pulsatory increase of velocity to be from i to ^ of the 

 velocity during the diastole. These pulsatory variations in the veloc- 

 ity of the current have been recorded by Chauveau by means of his 



