SECONDARY AIDS TO THE CIRCULATION. 243 



more rapid current. Actual measurements 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 getting from 

 the aorta through the carotid and its branches, and the 

 capillaries, and rhen through veins to the right auricle, 

 that is in going round the systemic circulation, is about 

 23 seconds of which time about one second is spent in 

 the capillaries; 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. (-f^ inch). 

 The rate of flow in the great veins is about 100 mm. (4 

 inches) in a second, but is subject to considerable varia- 

 tions dependent on the respiratory and other movements 

 of the Body (see below). 



Secondary Causes of 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 whipped 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 venee cavae. Not unfrequently the statement 

 is made in books that, probably, the systemic capillaries 

 have an attractive force for arterial blood and the pulmonary 

 capillaries for venous blood, but there is not the slightest 

 evidence of the correctness of such a supposition, nor any 

 necessity for making it. 



The Influence of Gravity. Under ordinary circum- 

 stances this ma) 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 possible be 



