264 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. xxi. 



estimates gives the velocity of the systemic capillary circulation al 

 about one inch (25 mm.) per minute. The velocity in the capil- 

 laries of warm-blooded animals is somewhat greater ; in the dog 

 it is Jjj- to j-j^j- inch (-5 to '75 mm.) a second. This may seem 

 inconsistent with the facts, which show that the whole circulation 

 may be accomplished in less than half a minute. But the whole 

 length of capillary vessels through which any given portion of 

 blood has to pass, probably does not exceed from -^j-th to -g^th 

 of an inch ('5 mm.); and therefore the time required for each 

 quantity of blood to traverse its own appointed portion of the 

 general capillary system will scarcely amount to a second. 



In the Veins. The velocity of the blood is greater in the veins 

 than in the capillaries, but less than in the arteries : this fact 

 depends upon the relative capacities of the arterial and venous 

 systems. If an accurate estimate of the proportionate areas of 

 arteries and the veins corresponding to them could be made, we 

 might, from the velocity of the arterial current, calculate that of 

 the venous. A usual estimate is, that the capacity of the veins 

 is about twice or three times as great as that of the arteries, and 

 that the velocity of the blood's motion is, therefore, about twice 

 or three times as great in the arteries as in the veins, 8 "inches 

 (200 mm.) a second. The rate at which the blood moves in the 

 veins gradually increases the nearer it approaches the heart, for 

 the sectional area of the venous trunks, compared with that of 

 the branches opening into them, becomes gradually less as the 

 trunks advance towards the heart. 



Of the Circulation as a whole. Among the earliest investigators 

 of the question how long an entire circulation takes was Hering. 

 He injected a solution of potassium ferrocyanide into the central 

 end of a divided jugular vein and collected the blood either from 

 the other end of the same vein, or from the corresponding vein 

 of the other side. The substance injected is one that can be 

 readily detected by a chemical test (the prussian blue reaction). 

 Vierordt improved this method by collecting the blood as it flowed 

 out, in a rotating disc divided into a number of compartments. 

 The blood was tested in each compartment, and the ferrocyanide 

 discovered in one which in the case of the horse received the 

 blood about half a minute after the injection had been made. 

 The experiment was performed in a large number of animals, and 

 the following were a few of the results obtained : 



In the horse . . . 31 seconds. 



dog . . . . 16 

 cat . . . 6-5 



fowl . . . . 5 



