THE CIRCULATION OF THE BLOOD 369 



of which is to make the water flow in a narrower channel and so 

 with a more rapid current in that part of the river. Actual meas- 

 urements as to the rate of flow in the arteries cannot be made on 

 man, but from experiments on lower animals it is calculated that 

 in the human carotid the blood flows about 400 millimeters 

 (16 inches) in a second. In the capillaries the current travels only 

 from 0.5 to 0.75 mm. (-^ to ^ inch) in a second. The total time 

 taken by a portion of blood in making a complete circulation has 

 been measured by injecting some easily detected substance into 

 an artery on one side of the body and noting the time which 

 elapses before it can be found in a corresponding , vein on the 

 opposite side. In dogs this time is 15 seconds, and it is calcu- 

 lated for man at about 23 seconds. Of this total about 

 a second is spent in the systemic and another second in the 

 pulmonary capillaries, as each portion of blood on its course 

 from the last artery to the first vein passes through a length of 

 capillary which on the average is 0.5 mm. (^ inch). The rate of 

 flow in the great veins is about 100 mm. (4 inches) in a second, 

 but is subject to considerable variations dependent on the respira- 

 tory and other movements of the Body; in the small veins it is 

 much slower. 



Secondary Factors Affecting the Circulation. While the heart's 

 beat is the great driving force of the circulation, certain other 

 things help more or less viz., gravity, compression of the veins, 

 and aspiration of the thorax. All of them are, however, quite 

 subsidiary; experiment on the dead Body shows that the injection 

 of defibrinated blood into the aorta under a less force than that 

 exerted by the left ventricle during life is more than sufficient 

 to drive it round and back by the vena cavse. 



The Influence of Gravity. Under ordinary circumstances this 

 may be neglected, since in parts of the Body below the level of 

 the heart it will assist the flow in the arteries and impede it equally 

 in the veins, while the reverse is the case in the upper parts of the 

 Body. In certain cases, however, it is well to bear these points in 

 mind. A part " congested " or gorged with blood should if possi- 

 ble be raised so as to make the back-flow in its veins easier; and 

 sometimes when the heart is acting feebly it may be able to drive 

 blood along arteries in which gravity helps, but not otherwise. 

 Accordingly in a tendency to fainting it is best to lie down, and 



