RATE OF BLOOD-FLOW IN THE VESSELS. 325 



greatly reduced. The velocity in the large arteries may be con- 

 sidered as approximately 3 dcm. a second. 



Rate of Flow in the Capillaries. The sectional area of 

 the capillaries is 700 times greater than that of the aorta, where 

 the velocity is, perhaps, 500 mm. per second ; when this portion 

 of the circulatory apparatus is reached the rate of flow is greatly 

 reduced, being in the dog from 0.5 mm. to 0.75 mm. per second. 



The rate of flow through the capillaries of the retina has been 

 ascertained by Vierordt in his own eye to be from 0.6 mm. to 0.9 

 mm. per second. 



The length of any given capillary through which a specified 

 portion of the blood passes is only 0.5 mm., so that the length of 

 time such portion would remain in the capillary system would be 

 less than a second, and yet during this brief time important 

 interchanges take place between the blood and the tissues. It is 

 here that the tissues receive from the blood the materials they 

 require for their nutrition, and in the case of glands for their 

 secretion ; and it is likewise here that the blood receives from the 

 tissues their waste products. The thin walls of the capillaries are 

 admirably adapted for this interchange. In fact, it is within 

 bounds to say that the heart, the arteries, and the veins are simply 

 subsidiary to the capillaries, the arteries carrying to these vessels 

 the blood which the heart pumps into them, while the veins return 

 the blood to the heart. 



Rate of Flow in the Veins. It is estimated that the 

 sectional area of the veins is at least twice as great as that of the 

 arteries, and therefore the velocity would be twice as great in the 

 arteries as in the veins. Some estimate the area of the veins as 

 three times that of the arteries. Inasmuch as the sectional area 

 of the veins decreases as the heart is approached, the rate of flow 

 in these vessels gradually increases. 



The Circulation-time. The length of time which the 

 blood takes to make the entire round of the body was first ascer- 

 tained by dividing the jugular vein and injecting a solution of 

 potassium ferrocyanid into the end nearest the heart ; the blood 

 was collected from the vein of the other side and tested by adding 

 a solution of ferric chlorid to the serum ; as soon as the Prussian- 

 blue reaction appeared it was demonstrated that the blood had 

 completed the circulation of the body. These observations con- 

 ducted on different animals gave the following results : In the 

 horse, 31.5 seconds; dog, 16.7 seconds; cat, 6.69 seconds; and 

 goose, 10.89 seconds. 



It was also found that this represented the time occupied by 

 27 heart-beats in each animal. If 72 times a minute are considered 

 as representing the average number of beats in man, it will be seen 

 that the blood requires 22.5 seconds to complete the entire circula- 

 tion. 



Stewart has devised a method for ascertaining the circulation- 



