THE CIRCULATION OF THE BLOOD. 925 
The methods of determining the velocity of the blood-stream 
have not entirely surmounted the difficulties, but they do give 
results in harmony with the above-noted anticipations. 
The area of the great aortic trunk being so much less than 
that of the capillaries, the flow in that vessel we should expect to 
be very much swifter than in the arterioles or the capillaries. 
Moreover, there must be a great differeuce in the velocity during 
cardiac systole and diastole, and according as the beat of the 
heart is forcible or otherwise. But, apart from these more ob- 
vious differences, there are variations depending on complex 
changes in the peripheral circulation, owing to the frequent 
variations in the diameter of the arterioles in different parts, 
as well as differences in the resistance offered by the capillaries, 
the causes of which are but ill understood, though less obscure, 
we think, than they are often represented to be. Since, for the 
maintenance of the circulation, the quantity of blood enter- 
ing and leaving the heart must be equal, in consequence of the 
sectional area of the great veins that enter the heart being 
greater than that of the aorta, it follows that the venous flow 
even at its quickest is necessarily slower than the arterial. 
Comparative-—There must be great variations in velocity in 
different animals, as such measurements as have been made 
demonstrate. Thus, in the carotid of the horse, the speed of 
the blood-current is calculated as about 306 mm., in the dog at 
from 205 to 357 mm. These results can not be considered as 
more than fair approximations. 
Highly important is it to note that the rate of flow in the 
capillaries of all animals is very slow indeed, not being as much 
as 1 mm. in a second in the larger mammals. The time occu- 
pied by the circulation is also, of course, variable, being as a 
rule shorter the smaller the animal. As the result of a num- 
ber of calculations, though by methods that are more or less 
faulty, the following law may be laid down as meeting approxi- 
mately the facts so far as warm-blooded animals are concerned : 
The circulation is effected by 27 heart-beats; thus, for a 
man with a pulse of 81, the time occupied in the completion of 
the course of the blood from and to the heart would be $4 = 38; 
i.e, the circulation is completed three times in one minute, or 
its period is twenty seconds; and it is to be well borne in mind 
that by far the greater part of this time is occupied in travers- 
ing the capillaries. 
15 
