THE CIRCULATION 219 



is not complete. A child grows up with a perforate inter- 

 auricular septum. If the aperture be very small it causes little 

 inconvenience. Shortness of breath and blueness of lips indi- 

 cate its existence if it be large enough to lead to deficient 

 aeration of the blood. 



The two sides of the heart being quite separate, it is clear 

 that all blood ejected by the right ventricle into the lungs must 

 return to the left auricle, to be driven by it round the body. 

 Yet it does not follow that the heart must at each stroke drive 

 exactly the same quantity of blood into the pulmonary artery 

 and into the aorta. On the average, each of the two sides ejects 

 the same amount about 3 ounces. Nor does it follow that 

 as much blood is lodged in the lungs as in the whole of the rest 

 of the body. The amount varies, but on the average the lungs 

 contain not more than one-fifteenth of the whole blood. The 

 heart may be likened to two cogwheels ; the blood-stream to a 

 chain, folded into a figure eight, against which the cogwheels 

 work. Synchronously each cogwheel lifts a link, the right one 

 of the smaller, the left one of the larger loop. Any given link 

 returns to its starting-place in half a minute. Such an illustra- 

 tion gives an idea of the arrangement of the circulation as 

 a whole, although the motion of a fluid is widely different 

 from the motion of a chain. 



If, the jugular vein of the neck being cut, a colouring matter 

 such, for example, as ferrocyanide of sodium or methylene blue 

 is injected into its central end towards the heart, it begins to 

 appear in the blood which issues from its distal end in half a 

 minute. In this short space of time it has passed through the 

 right heart, through the lungs, through the left heart, and 

 through the vessels, arteries, capillaries, and veins, of the head. 

 Half a minute is therefore the " circulation time." Not that 

 all the blood-corpuscles of the body make the circuit as rapidly 

 as this. The time taken depends upon the particular route 

 they follow in the greater or systemic circulation. Some 

 traverse the vessels which supply the walls of the heart itself 

 a short journey ; others go down to the foot and up again. 

 But the average circulation time does not exceed a minute or 

 a minute and a half. It is particularly in the veins of the liver 

 and other abdominal viscera that blood tends to linger. Usually 

 half the blood of the body, or even more, is lodged in these 



