182 



VELOCITY OF THE BLOOD IN THE BLOOD-VESSELS. 



moving parallel with the index b. The dromograph curve, III, shows 

 the primary elevation, P, and the dicrotic elevation, R. 



Dromograph A, B, tube inserted in artery; C, lateral tube connected with a 

 manometer; b, index moving in a caoutchouc membrane, a; G, handle. Ill, 

 curve obtained by dromograph. 



90. Velocity of the Blood in Arteries, Capillaries, 



and Veins, 



(1.) Division of Vessels. In estimating the velocity of the blood, it is 

 important to remember that the sectional area of all the branches of 

 the aorta becomes greater as we proceed from the aorta towards the 

 capillaries, so that the capillary area is 700 times greater than the 

 sectional area of the aorta (Vierordt). As the veins join and form 

 larger trunks, the venous area gradually becomes smaller, but the 

 sectional area of the venous orifices at the heart is greater than that 

 of the corresponding arterial orifices. 



The common iliacs are an exception ; the sum of their sectional areas is less 

 than that of the aorta ; the sections of the four pulmonary veins are together less 

 than that of the pulmonary artery. 



(2.) Sectional Area. An equal quantity of blood must pass through 

 every section of the circulatory system, through the pulmonic as well 

 as through the systemic circulation, so that the same amount of blood 

 must pass through the pulmonary artery and aorta, notwithstanding 

 the very unequal blood-pressure in these two vessels. 



